Review ARticle Journal of Agricultural and Marine Sciences 2022, 27(1): 38–61 DOI: 10.53541/jams.vol27iss1pp38-61 Received 02 March 2021 Accepted 01 Sep 2021 إستخدام احملسنات العضوية للرتبة يف عمان خالل الــ۲٥ عاما املاضية: مراجعة روندا جانكي, دانيل بالكبرين, ممتاز حان, أمحد البوسعيدي, وليد البوسعيدي, علي اهلنائي, مشتاق أمحد A 25-Year History of the use of Organic Soil Amendments in Oman: A review Rhonda Janke*1, Daniel Blackburn2, Mumtaz Khan1, Ahmed Al Busaidi2, Waleed Al Busaidi1, Ali Al Hinai1 and Mushtaque Ahmed2 Rhonda Janke*1( ) rhonda@squ.edu.om, 1Department of Plant Scienc- es, 2Department of Soil, Water and Agricultural Engineering, College of Agricultural and Marine Sciences, Sultan Qaboos University, PO Box 34, Al Koud, 123 Muscat, Oman. Introduction Since the beginning of agriculture, humans have found ways to enrich the soil, and thus benefit from healthier crops and better yields. (Price and Gebauer, 1996). With the advent of processed fertilizer production, the manufacture and mining and concen- tration of the minerals required by plants has become more convenient, but not without cost to soil quality, if soil carbon enrichment is neglected. Different farming systems around the world use var- ious methods and combinations of materials to add this soil carbon, including the addition of livestock manures, composts, green manure legume cover crops, adding Abstract. Organic soil amendments have been used in Oman since prehistoric agriculture began and are still being used today. Recently, interest in certified organic farming, and the use of organic soil amendments to enhance soil quality has motivated more research on traditional and new diverse soil amendment products. In addition, the arid climate of Oman combined with sandy soils benefit from non-traditional soil amendments and nutrient sources, such as treated human waste and wastewater. These two are not allowed in certified organic farming but offer sustainable solutions to building soil health for non-certified crops. This review will cover studies of soil quality in Oman related to the comparison of these various amendments, including manures, composts, organic mulch materials, biochar, ash, and others. In general, most of these amendments improve the soil by adding organic carbon, increasing the water holding capacity, improving infiltration rate, and stimulating or providing habitat and food sources for diverse soil microbiolog- ical communities. Some amendments can also help crops overcome some of the stresses of agriculture in Oman, such as soil salinity, heat and drought. Most also provide macro and micronutrients for crop growth. Some anti-quality factors may be present however, such as a high carbon to nitrogen ratio in some mulches, or high heavy metal content, human pathogens, and pharmaceutical residues in treated waste or wastewater. Biochar may have a positive or nega- tive effect on soil microbes, depending on the source material and temperature of combustion can result in byproducts that inhibit microbes. The value of soil microorganisms has been shown in organic cropping systems, and several new species have been discovered in Oman. Some of these provide possibilities for biocontrol of pathogens, and increased salt tolerance in crops like tomato. Though much valuable research has been done in Oman and the rest of the world, there is much left to be done to determine the effects of these organic amendments over the long term, and also the interactions among various amendments, soil conditions, soil microbes, and on crops grown with different irrigation methods and cropping systems. Keywords: Organic farming, soil quality, microbial diversity, biochar, soil fertility. امللخــص:مت اســتخدام احملســنات العضويــة للرتبــة يف عمــان منــذ أن بــدأت الزراعــة يف عصــور مــا قبــل التاريــخ ومــا زالــت تســتخدم حــى اليــوم. يف اآلونــة األخــرية، أدى اإلهتمــام ابلزراعــة العضويــة املعتمــدة وإســتخدام احملســنات العضويــة مــن أجــل حتســن جــودة الرتبــة إىل حتفيــز املزيــد مــن البحــث حــول املنتجــات احملســنة يف الرتبــة التقليديــة واجلديــدة املتنوعــة. ابإلضافــة إىل ذلــك ، فــإن املنــاخ اجلــاف يف عمــان مــع الرتبــة الرمليــة أدى اىل اإلســتفادة مــن حمســنات الرتبــة غــري التقليديــة ومصــادر املغــذايت ، مثــل النفــاايت البشــرية املعاجلــة وميــاه الصــرف الصحــي. ل ُيســمح هبذيــن الثنــن يف الزراعــة العضويــة املعتمــدة ولكنهمــا يقدمــان حلــوًل مســتدامة لصحــة الرتبــة للمحاصيــل الغــري معتمــدة يف الزراعــة العضويــة. ســتغطي هــذه املراجعــة دراســات جــودة الرتبــة يف ُعمــان فيمــا يتعلــق مبقارنــة هــذه احملســنات املختلفــة، مبــا يف ذلــك الســماد الطبيعــي، الســماد العضــوي، مــواد الغطــاء العضويــة، الفحــم احليــوي، الرمــاد، وغريهــا. بشــكل عــام، تعمــل معظــم هــذه احملســنات علــى حتســن الرتبــة عــن طريــق إضافــة الكربــون العضــوي، زايدة القــدرة علــى الحتفــاظ ابمليــاه، حتســن معــدل الرتشــيح، حتفيــز أو توفــري مصــادر الغــذاء و بيئــة للمجتمعــات الكائنــات الدقيقــة للرتبــة املتنوعــة .كمــا ميكــن لبعــض احملســنات أن تســاعد احملاصيــل يف التغلــب علــى بعــض ضغــوط الزراعــة يف عمــان، مثــل ملوحــة الرتبــة واحلــرارة واجلفــاف. أيًضــا توفــر معظمهــا العناصــر الغذائيــة الكــرى والصغــرى لنمــو احملاصيــل. ومــع ذلــك، قــد توجــد بعــض العوامــل املؤثــرة علــى اجلــودة، مثــل ارتفــاع نســبة الكربــون إىل النيرتوجــن يف بعــض األغطيــة، أو احملتــوى العــايل مــن املعــادن الثقيلــة، مســببات األمــراض البشــرية، وخملفــات األدويــة يف النفــاايت املعاجلــة أو ميــاه الصــرف الصحــي. قــد يكــون للفحــم احليــوي أتثــري إجيــايب أو ســليب علــى ميكــروابت الرتبــة، اعتمــاًدا علــى مصــدر املــادة ودرجــة حــرارة الحــرتاق وعليــه ميكــن أن يــؤدي إىل تشــكل منتجــات جانبيــة متنــع امليكــروابت. مت توضيــح قيمــة الكائنــات احليــة الدقيقــة يف الرتبــة يف أنظمــة احملاصيــل العضويــة، كمــا مت اكتشــاف العديــد مــن األنــواع اجلديــدة يف عمــان. بعضهــا يكــون قــادر علــى مكافحــة مســببات األمــراض حيــواي، وزايدة حتمــل امللوحــة يف احملاصيــل مثــل الطماطــم. علــى الرغــم مــن إجــراء الكثــري مــن األحبــاث القيمــة يف ُعمــان وبقيــة العــامل، إل أنــه ل يــزال هنــاك الكثريمــن مــا يتعــن القيــام بــه لتحديــد آاثر هــذه احملســنات العضويــة علــى املــدى الطويــل، وكذلــك نتائــج التفاعــالت بــن احملســنات املختلفــة، وحــال الرتبــة، ومــا حتتويــه مــن ميكــروابت، وعلــى احملاصيــل املزروعــة بطــرق الــري وأنظمــة الزراعــة املختلفــة. الكلمات املفتاحية: الزراعة العضوية، جودة الرتبة، التنوع امليكرويب، الفحم احليوي، خصوبة الرتبة. 39Research Paper Janke, Blackburn, Khan, Al Busaidi, Al Busaidi, Al Hinai, Ahmed mulch to the surface of the soil, by leaving the residue from high biomass crops, and rotation between annual and perennial crops. Oman has a unique combination of climate, sandy soils, and limited fresh water for ag- riculture. Both perennial and annual crops are grown, in multi-layered mixtures on traditional farms, and monocultures with high tillage inputs on more recent contemporary farms (Gaube and Gangler, 2012). There are trade-offs among various cropping systems and methods; some enhance soil quality, and some deplete it, jeopardizing future agriculture on some sites. The purpose of this review is to summarize what we know about soil quality in Oman, both from research conducted within the country, and to combine it with what we know about similar research around the world. The review will start with a brief summary of the current situation in Oman, some historical aspects, and the re- lationship among farming practices, soil quality, water quality and quantity, and other contextual factors. Then we summarize commonly used soil fertili- ty amendments, and then move on to recent research on novel, less traditional amendments such as biochar. The role of microbial activity and diversity within these cropping systems are discussed, with special emphasis on studies for the future related to soil organic matter and soil quality, as well as the way that bacteria and fun- gi can protect plants from biotic and abiotic stresses. In addition, new ways of recycling human waste, as sludge, compost, and wastewater are considered as partial solu- tions to the need to improve soil quality and conserve water resources in an arid climate. Methodology The goal of this literature review was to summarize all the research that could be located in the topic areas just described for Oman. The 25 year time-frame corre- sponds to the 25 year history of the Journal of Agricul- tural and Marine Sciences at Sultan Qaboos University. The authors of this paper have all been publishing in their respective areas of expertise and used the multi- ple scholarly data-bases available through the library system, the internet, and also harder to locate published data in Ministry of Agriculture annual reports and sur- veys. In some cases, graduate student thesis and disser- tation data was cited if the work has not been yet sub- mitted to a journal. The Oman data was then put into context by also citing well-known, recent, or review pa- pers from world-wide refereed journals. This review is not a quantitative summary, using only certain key-words, but qualitative; sifting through what is known, and what is not yet known about soils in Oman. Through this iterative process of comparing what is known in Oman and studies in other countries, gaps in our understanding and fruitful areas for future research could be identified. History of Farming, the Soil Survey, and Irriga- tion Systems in Oman - Context Oman has a long history and tradition of agriculture, with some of the irrigation systems, or aflaj, dating back 3000 years. “This oasis culture, formed thousands of years ago, represents a symbiotic unity of lifestyle and environment” (Gaube and Gangler, 2012). Traditional farming systems are often planted with multiple species of crops in the same area, and contain more biologi- cal diversity, including both weeds and also medicinal plants, than modern monoculture date palm plantations (Al Yahyai and Al Hashmi, 2017). Date palm and pomegranate are traditional perenni- al fruit crops (Al Said et al., 2013; Al-Yahyai and Khan, 2015), along with annual crops like wheat, barley and garlic, and forages such as alfalfa. At least 30 different crops have landraces and local varieties that have been recognized and are being conserved in Oman (Al-La- wati et al., 2017). In an example traditional oasis farm- ing communities, 66 different crop species are under cultivation in the mountain village, Balad Seet (Gaube and Gangler, 2012), including traditional crops, as well as more recently introduced species. About half of the cropped area was planted to alfalfa and barley, to be used as forage crops for ruminants. The manure from the ap- proximately 200 goats or sheep, and up to 30 cattle were applied to the crops, both with and without additional chemical fertilizers, or sometimes fertilizer alone was applied. Rates of up to 380 kg N, 30 kg P, and 400 kg K per ha, were applied and varied across crops and seasons (Gaube and Gangler, 2012). A soil survey of Oman was completed in 1990 (MAP, 1990) and classified the soils according to international soil taxonomy standards into 61 different mapping units, belonging to 3 soil orders: Entisols, Inceptisols, and Aridisols (Cookson, 1996). Entisols are soils that show little profile development or diagnostic horizons, and are basically unaltered parent material. Inceptisols are recently formed soils, show more horizon development than Entisols, but do not have any accumulation of clays, iron oxide, aluminum oxide, or organic matter. Aridisols form in arid and semi-arid climates, and make up about one third of the earth’s land surface. In addition to the presence of rocks and steep slopes, the soils of Oman have other limitations for agriculture such as gypsum accumulation, calcium carbonate accumulation, and soil and water salinity. About 2,000,000 ha of the total land area is categorized as Class 1 or 2, which is highly/mod- erately, or marginally suitable for agriculture, which is about 6.5% of all soils. Only 800,000 ha are in the Class 1 category, or only 2.59% of the soils of Oman (Cookson, 1996; MAF, 1990). Between 1988 and 1997, more detailed soil surveys of the agricultural regions were carried out in the Salalah region/Governate of Dhofar (MAF, 1992), in South Al Batinah (MAF, 1993) and in North Al Batinah, in 3 ar- 40 SQU Journal of Agricultural and Marine Sciences, 2022, Volume 27, Issue 1 A 25 Year History of the use of Organic Soil Amendments in Oman: A review eas; Sohar, Liwa and Shinas (MAF, 1997a; MAF 1997b; MAF 1997c). The Salalah detailed soil survey covered an area of about 4,200 ha (about 0.04% of the total land area of Dhofar). From this study area about 60 percent of the topsoil was coarse loamy (loam or sandy loam) and about 7 percent was fine loamy (heavy loam or sandy clay loam). Available water holding capacity was conducted and found that about 56 percent was high or moderate, and about 42 percent were low and very low. Infiltration rate of the study area was generally low to moderately low (71%), moderate (13%) and moderately high to very high (14%). Soil pH was generally slightly to moderately alkaline. Most soils of the study area were none to slight- ly saline (83%), moderately saline (14%) and strongly sa- line (2.5%) as measured by electrical conductivity (EC). The average topsoil cations exchange capacity (CEC) was low. Calcium followed by magnesium were the main cations while potassium content was moderate. Organic matter was higher than in other areas in the Sultanate but it was classified as low by international standards. The average of available phosphorus was moderate to high (9.6 ppm) (MAF, 1992). The soil survey of South Al-Batinah was conducted between 1988 and 1992. It covered an area of 65,900 ha of Willayats of Barka, Masanaa, and Suwayq. About 3,200 soil samples were collected for general chemical and physical analysis. Infiltration and permeability tests were also conducted by using of double rings infiltrom- eters. The soils of the study area developed mostly on alluvium derived from the Hajar mountains. In the study area the topsoil, it was mostly coarse textured; about 80% sandy or coarse loamy, and 20% fine textured. The infiltration rate for 60% of soils was moderately to very rapid, and only about 9 % had slow permeability. The av- erage topsoil calcium carbonate content was higher in Barka compared to Masanaa and Suwayq. The soil pH was generally moderately to strongly alkaline. In addi- tion, about 50 % of the soils of the area were not saline. However, 30 % were strongly affected by salinity, due to inadequate salt leaching and saline irrigation water. The average topsoil cation exchange capacity was generally low. Calcium followed by magnesium were the main cations in the soil while potassium content was moder- ate. In addition, soil organic matter and nitrogen con- tent were low while available phosphorus was generally moderate (MAF, 1993). In the North Al-Batinah soil survey, the soil surveys for Sohar, Shinas and Liwas were conducted between 1995 and 1997. Soil samples were analyzed for pH, EC, and carbonate content, CEC and extractable bases, or- ganic carbon and available phosphorus contents on the topsoil samples. In addition, triplicate double-ring infil- tration tests was carried out (MAF, 1997a; MAF, 1997b; MAF, 1997c). Results were similar to the South Al Bati- nah study. Salinity, partly due to seawater intrusion, is a limitation to much of the coastal farmlands in both North and South Al Batinah, with an estimate that over- all, 52% of the soils in these Governates are affected to some degree by salinity, with an EC level greater than 3.0 dSM-1 (Hussain, 2005). The most recent agricultural survey (MAF 2015) shows that at present, only 0.48% of the land of Oman is in agriculture, and that only 0.23% is cropped (Table 1). This is quite a bit lower than the 6.5%, or even the 2.59% of soils that are suitable for farming according to the soil survey. The difference between cropped and agricultur- al lands is that some land is used for buildings, grazing, and other livestock holdings or fallow. The other limita- Table 1. Land area in agriculture, cropped land, and percent ag land treated with pesticide, chemical fertilizer, non-processed organic fertilizer and processed organic fertilizer (MAF 2015). Governate total area in ha total agricul- tural land area (ha) per- cent of total area in agri- cul- tural land Cropped Area (ha) percent of total area in cropped land percent of ag area that is cropped # hold- ings (with land) per- cent of total ag area treated with pesti- cide percent of total ag area treated with chem- ical fertil- izer percent of total ag area treated with non-pro- cessed organic fertilizer (ma- nures) per- cent of total ag area treated with pro- cessed organic fertil- izer (com- post) Al Batina North Al Batinah South Ad Dakhiliyah Dhofar Ash Sharqiyah North Adh Dahirah Ash Sharquiyah South Al Buraimi Muscat Musandam Al Wusta Total 790,000 532,000 3,180,000 9,910,000 2,110,000 3,590,000 1,200,000 746,000 380,000 162,000 8,250,000 30,850,000 35,750 20,574 19,208 27,687 11,560 13,984 6,387 6,772 4,853 1,362 969 149,105 4.53% 3.87% 0.60% 0.28% 0.55% 0.39% 0.53% 0.91% 1.28% 0.84% 0.01% 0.48% 24,875 10,838 7,547 7,467 5,687 4,963 3,280 3,154 2,601 911 163 71,488 3.15% 2.04% 0.24% 0.08% 0.27% 0.14% 0.27% 0.42% 0.68% 0.56% 0.00% 0.23% 69.6% 52.7% 39.3% 27.0% 49.2% 35.5% 51.4% 46.6% 53.6% 66.9% 16.9% 47.9% 21,777 19,532 31,293 11,498 24,284 15,406 12,861 4,504 8,354 3,871 630 154,010 49.5% 47.9% 70.5% 80.7% 49.0% 56.9% 43.9% 66.6% 55.0% 41.8% 85.6% 59.3% 71.9% 63.8% 64.7% 53.2% 49.4% 56.7% 40.7% 50.5% 50.2% 18.4% 27.6% 59.4% 83.9% 83.6% 92.2% 92.3% 93.0% 88.8% 92.5% 92.5% 92.8% 68.9% 93.1% 88.6% 47.9% 56.0% 39.2% 49.1% 46.4% 38.1% 36.8% 23.7% 38.3% 11.4% 37.2% 44.8% 41Research Paper Janke, Blackburn, Khan, Al Busaidi, Al Busaidi, Al Hinai, Ahmed tion to agriculture in Oman besides soil suitability is the availability of water, which is why these numbers are fall below the potential based on soil suitability. Oman is categorized as a country of “extreme water scarcity” by the FAO, and yet it has a thriving agricul- tural sector, with an estimated 50% self-sufficiency rate for vegetable crops, and a 70% self-sufficiency rate for fruit (Mbaga, 2014). However, this production comes at the expense of water use, which was estimated in a 1999 study to exceed sustainable re-charge by 25% (National Water Resources Master Plan, 1999). A follow-up study in 2013 also confirmed that Oman continues to have a net water deficit of 315 Mm3 per year (Ministry of Re- gional Municipalities and Water Resource, 2013). Both traditional aflaj methods of irrigation (usually flood) and drip irrigation from wells are used, but in dif- ferent ratios in different parts of Oman (Figure 1). Of the total water used for agriculture of 1546 Mm3, about two-thirds, or 1060 Mm3 are groundwater extracted by wells/pumping, and 486 Mm3 flow through the aflaj sys- tems (Ministry of Regional Municipalities and Water Resource, 2013). Wells are the primary irrigation sourc- es for both North and South Al Batina, which account for 50% of all of the cropped land in Oman. In these two Governates, agriculture makes up 2 to 3% of the total land area. Ad Dakhiliyah and Dhofar are 3rd and 4th place in number of cropped ha, but only have 0.24 and 0.08% of their total land area in crops respectively (Table 1). Farms in the Governate of Ad Dakhiliyah have access to a mix of aflaj and groundwater/wells (Figure 1), while in Dhofar, the only source is groundwater. Al Wusta has the lowest amount of land in agricultural production (Table 1), even though some soils are suitable, due to the lack of any easily accessible water source (Figure 1). Moving towards drip irrigation on many vegetable farms has improved water use efficiency (WUE), but much of Oman farmland continues to be watered by flood/alfaj system. Flood irrigation is the most used sys- tem for fruit/date farms, while vegetables are primarily grown using drip/modern irrigation (Figure 2). Drip ir- rigation has the advantage of only irrigating in the crop row, but a disadvantage of possible salt accumulation on the surface. Flood irrigation is less efficient for a single crop, but is used in Oman to irrigate multiple crops in the same field, and has the advantage of leaching salts. Carbon, nitrogen, and phosphorus cycling dynamics are likely to be quite different in these two systems, and both types of farming systems have been studied in Oman, but to our knowledge, there are no studies with direct com- parison of carbon or nutrient cycling. In both irrigation systems, due to the coarse/sandy texture of most soils of Oman, leaching of nutrients is an issue, and virtually all cropped land in Oman receives irrigation in one form or another, due to the low annual rainfall. Perennial forag- es are grown using both systems, while field crops are more often planted in the traditional aflaj irrigated areas of Oman, and may be planted as monoculture, or as un- derstory crops within a planting of perennial fruit trees. The tradition of integrated crop and livestock farm- ing has carried into the present, in terms of recommend- ed fertilization practices, and actual use of fertilizers and manures on cropped soils. Though chemical fertilizers are used on an average of 59% of agricultural land (Ta- ble 1, statistics from MAF 2015), there is an estimated 88% of land receiving non-processed organic fertilizer/ manure. In addition, about 45% of land also receives processed organic fertilizer (compost). The percentage of land receiving fertilizer is slightly higher in North and South Al Batina, which are heavily cropped with vege- tables and forages (72 and 64% respectively), but these lands still receive non-processed organic fertilizer on about 84% of the land. The percentage of land receiv- ing organic fertilizer goes up in Governates such as Ad Dakhiliya and Dhofar to 92%. Pesticides are used on ap- proximately 59% of agricultural land, on average. In addition to tradition and the presence of livestock on farms in most regions of Oman, the Ministry of Ag- riculture specialists recommend annual fertilization with non-processed manures at rates ranging from 10 to 60 kg per tree for crops such as mango and coconut (Hameed and Habees, 2014; Al Jabri, 2003), depending on the tree age and size. Organic manure is also recom- mended at fairly high rates for crops like sorghum, al- falfa and Rhodes grass (Table 2) (Ghabeshi, 2005), but at lower rates for most vegetable crops (Table 3)(Anon, 2011). The application of mineral fertilizers is also rec- ommended, in addition to the nutrients that will be re- leased from organic matter. Table 2. Recommended rates of organic manures and fertilizer products for example field crops in Oman (Ghabishi 2005)1 and rates applied according to a survey of small, medium and larger vegetable farms in Oman (Al Salmi, 2020)2. Recommended Rates (kg/ha per season)1 Rates applied by small, medium and large farms in Oman (kg/ha per season)2 Recommended for sorghum Recom- mended for alfalfa Recommended for Rhodes grass small farm (less than 5 feddan) medium farm (5 to 20 feddan) large farm (more than 20 feddan) organic manure NPK (balanced) Urea fertilizer (46% N) Other fertilizer Triple super phosphate (46% P2O5) Potassium sulfate (48% K2O) 14,286 357 238 238 14,286 60 238 119 14,286 714 238 476 1,919 369 221 76 4,302 567 888 55 5,588 490 329 174 42 SQU Journal of Agricultural and Marine Sciences, 2022, Volume 27, Issue 1 A 25 Year History of the use of Organic Soil Amendments in Oman: A review A recent survey of 198 farms in the Al Batinah Gover- nates, North Sharqiyah and Ad Dakhiliyah, conduct- ed between February 2018 and April 2019 allows us to compare recommendations and actual farm practices (Al Salmi, 2020). The applied rates of urea fertilizer are in the range of the recommended rates (Table 2, rec- ommended rates from Ghabishi, 2005) and will depend on the crop grown. For example, lower rates would be recommended for nitrogen fixing crops like alfalfa. The rates of P2O5 and K2O are difficult to compare, because the survey only recorded the use of balanced fertilizer, which would likely be a 10-10-10 or 20-20-20 mix of N, P and K. The organic manure applied by farmers is lower than the recommended rates for forage crops, but is still substantial, at between 2 and 5.6 metric tons per ha for small, medium and large farms. The rate of organic manure recommended to farmers is much lower for vegetable crops such as eggplant, tomato and okra (Table 3, recommendations from Anon, 2011), at only 110 kg per ha, as compared to 2.6 to 3 tons per ha reported as applied by the surveyed farms (Al Salmi, 2020). When the estimated nutrient values in this ma- nure are converted to N, P and K equivalent values, the rates applied by farmers are similar to recommendations for both Oman, and for sandy soils in Florida, USA. In general, we see the farmers might be over-applying ni- trogen to all 3 crops. This may be due to the slow release nature of nitrogen from the organic matter source, or to compensate for leached nitrogen due to constant irriga- tion, whether it is drip or flood. Table 3. Recommended rates of organic manures and fertilizers for selected vegetable crops in Oman (Anon. 2011)1, rates applied according to a survey of vegetable farms in Oman (Al Salmi 2020)2 and recommended for sandy soils in Florida (ref: Maynard and Hochmuth. 2007)3 Rate applied (kg/HA per season) Recom- mended for Eggplant (Oman)1 Applied to egg- plant2 Recom- mended for Eggplant (U of FL)3 Recom- mended for tomato (Oman) Applied to tomato (Oman) Recom- mended for tomato (U of FL) Recom- mended for okra Applied to okra Recom- mended for okra (U of FL) organic manure N P2O5 K2O 1071 192 82 143 2,8572 319 124 168 224 1683 179 107 274 82 171 3,095 332 148 196 224 168 168 107 219 82 114 2,619 327 110 151 135 168 168 Notes: 1the amount of NPK in the Oman organic manure was small (less than 3 kg/ha) and so was not added to the NPK totals. 2The estimated NPK value for the organic manure was added into the total nutrient values by using the average manure values of NPK from sheep, horse and feedlot (from Table 4.3 pg 151 in Maynard and Hochmuth, 2007, which was 2.53% N, 1.20 % P2O5, and 2.73% K2O). 3The recommended rates for the University of Florida are for sandy soils with very low soil P tests. If there is more soil P, the recommended rates would decrease from these values (Source: Table 4.32 pg 225 in Maynard and Hochmuth, 2007) Figure 1. Agricultural abstraction of water from groundwater and falaj flow by city/region from Northern Oman to South- ern Oman (reference: Ministry of Regional Municipalities and Water Resource, 2013). 43Research Paper Janke, Blackburn, Khan, Al Busaidi, Al Busaidi, Al Hinai, Ahmed Trials with Fertilizers and Organic Amendments Though manures and composts are widely used in Oman, fertilizer is also used, both alone, and in combi- nation with organic inputs. The Ministry of Agriculture & Fisheries (MAF) has conducted some field trials to de- termine optimal nutrient needs of many crops. There is some risk of under-fertilization, and under-yielding of crops, but also the risk of over-fertilization, especially in fields with a history of manure use, and also in the now widely used and intensively cropped greenhouse produc- tion systems. In addition, different types and qualities of organic amendments are sold by suppliers or by plant nursery stores with a lack of knowledge in terms of their behavior in soil and plant systems (Al-Ismaily, 2011). In Oman, there are now also 3590 greenhouses with a total area on 344 feddan, and 238 shade-houses on 188 feddan (MAF, 2015). These tend to be intensively cropped and fertilized, and are at risk of over-fertilization. For exam- ple, one grower in Oman was instructed by their input supplier to put 72 bags of compost for a new planting of tomatoes in one greenhouse (pers. comm. one of the authors). If one does the math, based on the average size of bag and % nitrogen in compost, that is equivalent to 41,143 kg of compost per ha, or 823 kg/ha of nitrogen. This is far above the recommended levels (Table 3), and could result in high soil EC levels, reduced plant growth, and reduced fruiting of tomato due to too much N. It also makes the plants more susceptible to disease. There are also hydroponic greenhouses in Oman, which have some sustainability characteristics such as water sav- ings. However, since they have no soil, and are not al- lowed under European organic certification guidelines, they are not covered in this review. Some of the experiments described in this section only look at the rate of mineral fertilizer applied, but most use some combination of mineral fertilizers and compost or manure, and some also include a salini- ty treatment. Examples of fertilizer response trials in Oman include those on bananas (Al Harthi and Al-Ya- hyai, 2009; Al Busaidi, 2013). In a trial conducted at the Agricultural Research Center in Rumais, Oman, banana yield after 18 months was lower in the high salinity plots as compared to the low salinity treatment. However, there were no significant differences among the other treatments which included two fertilizer treatments, three compost+mulch treatments and a dairy manure treatment (Al Busaidi, 2013). The fertilizer was applied at a rate of 400 g/plant nitrogen as urea, split over 7 ap- plication times, and the manure and compost applied at rates of 39 and 22.2 kg/plant at the time of planting, respectively. Phosphorus, potassium and micronutri- ents were also applied to the fertilizer treatments. An experiment with in Salalah compared zero, 300, 600 and 900 g/plant split into 5 applications over 12 months (Al Harthi and Al Yahyai, 2009). In addition, 20 kg of organic compost was added to each plant twice, and all plants received micronutrient foliar spray 3 times. Results showed that the optimal treatment of 600 g N/plant was significantly higher than the other treatments. Yields were reduced at the 900 g N/plant level. There was no Figure 2. Comparison of area supplied by drip vs. flood irrigation systems by type of crop in Oman (MAF, 2015). 44 SQU Journal of Agricultural and Marine Sciences, 2022, Volume 27, Issue 1 A 25 Year History of the use of Organic Soil Amendments in Oman: A review significant difference between the control and the 300 g N/plant treatment. An experiment in Salalah with forage sorghum over two years compared 4 levels of urea nitrogen (0, 17, 34 and 69 kg/feddan), three levels of super phosphate and three levels of potassium sulfate (Anon, 2013c). There were no significant differences between the potassium and phosphorus level treatments, but the nitrogen ap- plications were significantly different, with the highest yield at the highest level of urea application. The plots were watered with treated municipal wastewater, which would have resulted in the equivalent of 200 kg of nitro- gen and 100 kg of phosphorus application per year in addition to the fertilizers. In an on-going experiment with date palm seedlings, variety ‘Khalas Aldahirah,’ transplants from tissue cul- ture propagation, planted in 2011 are receiving annual rates of 0, 250, 350, 500 and 650 g N/plant in the form of organic amendments. So far results show that the rate of 500 g N is optimal, with the lowest yield for the 0 N, and the second lowest from 650 g N (Anon, 2018). Repli- cated experiments in farmer’s fields with garlic at 3 rates of fertilization found that the lowest rate gave the best yield (Anon, 2013a). There was no “control” check plot, and fertilizers were chemical formulations at rates of 97- 76-83, 129-101-110 and 161-127-138 kg/Feddan NPK blends. It was not reported if organic amendments were also applied during or prior to the experiment. Nutrient studies on tomatoes with fertilizers as com- pared to organic manure/compost vs. a combination found that the combination was the optimal treatment (Al-Ismaily et al., 2014), especially when subjected to sa- linity treatments of 6 or 9 dSm-1. Fertilizer was applied at rates of 238 kg/ha for N, P and K, but the NPK value in the manure treatment was 333, 52, and 338 kg/ha for NPK respectively. This experiment was conducted on land that had previously been fallow for 15 years and started with low levels of all nutrients. Generally, the N in manure is slowly available, and should have been applied at a 2x rate, assuming only half would be avail- able in the first year, but in this experiment was only 1.4x the fertilizer N rate. The half-fertilizer and half-manure rates combined the best properties of both materials, with some N quickly available at the beginning of the experiment, but some also released later from the ma- nure source. Organic farming promotes the use of manures, com- posts, cover crops, and mulches, but in moderation (Brinton 2001; Barker 2010). The benefits of compost are well known, and may be especially helpful on saline soils (Oo et al., 2015; Ouni et al., 2013). Compost quality is an area deserving of more research however, as dif- ferent composts from different feedstocks have different properties (Al-Ismaily, 2011). International standards for compost have been proposed (Brinton, 2001), but have had limited implementation in most parts of the world, including Oman. In a greenhouse experiment at Sultan Qaboos University with broccoli and okra, two commercial composts were compared. One was a plant- based compost, and other included livestock waste. There was no significant difference in the performance of the composts on plant growth or soil properties, but there was a significant benefit of compost as compared to the no-compost control (Al Busaidi, 2017). The com- post significantly increased the soil organic matter lev- els, but also raised the EC, so again, should be used in moderation and in conjunction with soil testing. Previous nitrogen and carbon studies conducted in Oman found that though significant amounts of nitro- gen are lost through volatilization, both carbon and ni- trogen can accumulate in vegetable crop rotations with organic amendments instead of fertilizer (Siegfried et al., 2011; Jordan et al., 2015). Studies in the US have docu- mented that carbon and nitrogen cycling are different in organic cropping systems, especially during the transi- tion years (Liebhardt et al., 1989) and the use of organ- ic amendments can reduce carbon and nitrogen losses from cropping systems as compared to fertilizer (Drink- water et al., 1998). Cover crops like Sesbania can also add significant carbon and nitrogen to soils, especially at the beginning of a long term crop rotation (Al-Maskri et al., 2018)., even though in the context of Omani agricul- ture, the cost of the water to grow the cover crop would need to be considered. Salalah has the potential to grow more of Oman’s fruits and vegetables, based on the availability of land and water, and more moderate climate as compared to the Al Batinah region (Prathapar et al., 2014). Much of the agricultural land in Dhofar is devoted to animal based agriculture at this time, and only 27% of the 27,687 ha of agricultural land in Dhofar is currently cropped. A study of the nutrient status of soils and plant nutri- ent status for some horticultural crops in Salalah found that in general, phosphorus concentrations were near- ly sufficient in all locations, potassium was deficient in about 50% of the crops, and nitrogen was deficient in most of the crops, as measured by leaf tissue samples (El Fouly and Shaban, 1999). A range of crops were sampled including the fruit crops banana, papaya, grapes, lime, other citrus and coconut, and the vegetables tomato and pepper. Soil samples showed that pH is generally high (average 8.4), EC is medium (average 2.37 mmhos/cm) and organic matter with one exception was low (average 1.81%). The soil texture was classified as a sandy loam, with 52% sand, 30% silt and 19% clay content. The cal- cium carbonate levels of the soil were very high, which agrees with the soil survey data for Salalah (MAF, 1992). Soil levels of phosphorus varied from medium to high, and soil potassium levels were low to very low. When farmers were asked about the fertilizers they used, it was difficult to get accurate information, but in general, ni- trogen and phosphorus fertilizers were being used, but very little potassium or micronutrients. High amounts of organic manure were being used on the vegetables, 45Research Paper Janke, Blackburn, Khan, Al Busaidi, Al Busaidi, Al Hinai, Ahmed along with intensive use of pesticides. In summary, the fertility needs of each type of crop may be different, and the cropping and fertilization his- tory for each site is different. There is a need to do these trials based on site specific conditions, and to report both the current and previous additions of organic mat- ter that could make a difference in the nutrient release during the experiment, and also the recommended rates based on the experiments. Organic amendment sourc- es and compost quality may also be different, and there may also be interactions between irrigation practices and soil fertility which should be studied. Experiments with Municipal Waste and Waste- water Though not allowed for certified organic production systems, treated municipal (human) waste, waste-based compost, and treated wastewater offer other strategies for soil improvement and nutrient application in Oman. Recent reviews suggest that all of these can be used safe- ly, though attention needs to be paid to government stan- dards, regulations, and policies for use (Hussain et al., 2019; Khaliq et al., 2017a). In Oman, there are currently 400 wastewater treatment facilities in the country (Hu- sain et al., 2019), and much of the wastewater, especially in the municipality of Muscat, the largest city, it is being used to water landscape plants for city beautification, parks, and ornamental gardens. Research is being done to evaluate the safety for use on crops on some test farms. One of the risks of using treated wastewater is the presence and accumulation of heavy metals, which can be avoided by keeping them out of the seawater source stream, or by growing plants that do not take up heavy metals, and monitoring accumulation in the soil. An- other risk factor is the presence of human pathogens. These can be reduced or eliminated by using tertiary water treatment systems, or by growing crops not des- tined for human consumption. However, two new risks have emerged with the use of treated wastewater; phar- maceuticals and personal care products (Al Farsi et al., 2018a; Al Farsi et al., 2018b). In an experiment with both low levels and spiked samples with pharmaceuticals, two were not detected in soil or plant samples, one antibiotic accumulated in radish roots, and a second antibiotic was translocated into the radish plant (Al Farsi et al., 2018a). A review of the literature for the uptake of personal care products from wastewater found that more than 100 have been studied in several countries, and that the up- take depends on the chemical nature of the product and also the plant (Al Farsi et al., 2018b). Many of these have the potential to reach and accumulate in plant tissue. The research on risk and its possible impact on human health is still unknown. A study on the use of grey water for home gardening, collected from a newly built home in Oman, combined with kitchen effluent supplied nutrients to plants and did not show any harmful chemical or biological contami- nation (Al Ismaili et al., 2017). In the same study, cost of the greywater treatment system was calculated and found to be technically, environmentally and economi- cally feasible. The system installed included a collection tank, a sand filter, and final treatment with chlorine prior to use in the garden. The water was tested on tomatoes and eggplant in a garden setting. Heavy metals in the treated greywater were negligible, and the Coliform and E. Coli levels were zero due to the chlorine treatment. In contrast, a survey of urban residents in Al Seeb, Oman, found that many were not aware of the risks of using untreated waste or wastewater from septic tanks on their gardens (Shaharoona et al., 2019). In addition to the survey of 75 households, 7 soil pedons were ex- cavated; 5 near garbage sites and 2 away from garbage sites, and were analyzed for heavy metals and microbial analysis. The questionnaire found that many gardeners were irrigating with untreated wastewater (25% un- treated black water, 13% untreated greywater). Water samples were then collected from three wells and one septic tank. The soil samples showed that of the heavy metals, only cadmium was at an elevated level for 2 samples, but it was below the US and EU environmen- tal standards. However, they found that garden soils ir- rigated with untreated black water were contaminated with a large number of pathogenic bacteria, including E. Coli, Staphylococcus, Salmonella, Shiglla, and fecal coliforms. In contrast, no pathogenic bacteria were detected in control soils only a few meters away. Only fecal coliform were found in soils irrigated with untreat- ed greywater. None of the urban growers surveyed had ever tested their soil for nutrient levels or contamina- tion. Some of the gardeners were using farm yard ma- nure, some were using chemical fertilizers, and 19% were not using any fertilizer at all. One of the reasons gardeners gave for using septic tank waste to water the garden, in addition to a source of nutrients, was to save money on purchasing water from the municipality, but also to save money on having the septic tank pumped, which can be expensive if done on a monthly basis. When treated wastewater is used, some of these risks can be avoided. Date palms leaves, fruits and soil were sampled from areas irrigated with treated wastewater for at least 7 years and compared to dates watered with groundwater. Results showed that heavy metals in the groundwater and treated wastewater were within interna- tional standards, and that heavy metals were at acceptable levels in the soil, plants and fruits (Al Busaidi et al., 2015). Wastewater only has the potential to add nutrients, and to replace some or all of the irrigation water that would have been required. Municipal sludge and/or compost has the additional benefit of improving the soil through the addition of organic matter (Khaliq et al., 2017a). In one experiment white radish and green beans were grown with either chemical (NPK) fertiliz- er or Kala compost (from municipal waste) and watered with either groundwater or wastewater (Khaliq, 2017b). 46 SQU Journal of Agricultural and Marine Sciences, 2022, Volume 27, Issue 1 A 25 Year History of the use of Organic Soil Amendments in Oman: A review Results showed that the yield and chlorophyll contents of both were higher with Kala compost as compared to the NPK fertilizer. Analysis of the soil and plants did not show any risk of heavy metal accumulation. The treated wastewater in this experiment did not show any statistically significant differences from watering with groundwater. The Kala compost resulted in higher soil total organic carbon levels as compared to the fertilizer. Experiments in Barka and Hamra looked at the ef- fect of Kala compost on cucumbers in commercial greenhouse production systems (Al Busaidi et al., 2017). High levels of nutrients were found in the soils after application, and also high EC levels at the start of the experiment, which were lowered during the course of the experiment through leaching by irriga- tion water. Most heavy metals were low in both soil and plants, though nickel was high in some of the plant samples, but not in the soil. It was concluded that the Kala compost is safe to use, and one of the greenhous- es had applied Kala compost for 5 consecutive years. In a third experiment in Oman, Kala compost was compared to inorganic NPK fertilizer or a mix of the two in open field plots with the crops cucumber, toma- to, cabbage, lettuce, carrot and potato (Al Busaidi and Ahmed, 2016). The soil had sufficient amount of nu- trients for good crop growth in all treatments, though yields were generally higher with the Kala compost as compared to the NKP treatment. No adverse chemical or physical plant or soil problems were noted. Chem- ical analysis of plant samples did not show any accu- mulation of heavy metals, and all measured elements were within the safe limit for human consumption. Kala compost is manufactured in a highly controlled process that combines the treated sludge from the Muscat municipal waste treatment plant with chipped wood and yard waste. The nutrient value of the Kala compost used in the trial was 3.17% N and 1.4% P, po- tassium was not measured, and fecal coliform levels were zero. In the experiment, the soil organic matter levels were highest with the Kala compost treatment. An experiment conducted on tomatoes comparing Kala compost and Al Mukassab (commercial, animal waste-based compost) at two rates found no significant difference between the rates used and the two products for tomato production (Anon, 2013b). The tomatoes were field grown during the 2012-2013 growing sea- son, the composts had identical nitrogen concentrations (2.5%), and the rates of 10 and 20 ton/ha were compared. Carbon Added to Soil as Mulch or Biochar Even though it is documented that manures and com- posts are used in farming systems in Oman, there is still much farm-generated biomass goes to waste through off-farm disposal or burning. Much of this waste could be a valuable resource and be used as mulch or made into biochar. Only weeds and diseased plants should be excluded. Date palm prunings and mango wood could be utilized at the farm level with minimal processing (Al Busaidi et al., 2017). The benefits of mulches from date palm leaves on saline soils in Oman has been doc- umented for tomatoes (Wahaibi et al., 2007) and also for sorghum (Al-Rawahy et al., 2011). The tomato ex- periment included date palm mulch, plastic mulch, and bare ground control, at 2 irrigation salinity levels; EC 3 and 6 dS/m. Date palm mulch resulted in higher tomato fruit yield, lower soil temperature and lower soil EC lev- els (Wahaibi et al., 2007). A similar result was found in the sorghum experiment, which also included date palm mulch, black plastic mulch, and control at the same two irrigation salinity levels (Al Rawahy et al., 2011). Again, the date palm mulch resulted in better yields, lower soil salinity levels. It was also noted that soil temperature was lower, and soil moisture was higher with the date palm mulch. Other experiments have noted benefits of organic mulches on saline soils. Municipal solid waste and date palm mulch both improve the soil carbon and nitro- gen levels, as well as microbial biomass and several soil enzymes that were measured under saline conditions (Ouni et al., 2013). Two recent reviews on soil salinity and organic wastes note that waste recycling of farm- yard manures, industrial byproducts, and other organic materials provide an effective way to restore soil quality in salt affected soils (Diacono and Montemurro, 2015; Ahmed et al., 2013). Farm generated waste or biomass could further en- hance soil quality if used as biochar, which is the burn- ing or pyrolysis of a carbon source without oxygen, to retain most of the carbon and not release it as CO2. Recent reviews have shown that biochar can improve crop productivity by increasing nutrient content, nutri- ent use efficiency and water holding capacity (Hussain et al., 2016; Aon et al., 2015). Generally, soil carbon levels increase with increasing amounts of added bio- char. The effect on crop yield will vary, depending on the quality and source material of the biochar, which affects the nutrient content and carbon/nitrogen ratio of the product. Application of biochar to fertile soil often results in less effect on plants than application to nutrient poor soils. The effect of biochar on microbial communities has shown both positive and negative ef- fects (Palansooriya et al., 2019). Biochar can increase the amount of carbon and increase the surface area/micro- pores for microbial growth in the soil, but the carbon is not always readily available to the microbes. Under some condition, the pyrolysis process can produce ar- omatic hydrocarbons, which can be toxic to microbes. In an experiment in Oman, activated charcoal and tannins were added to compost and to soil in an effort to reduce the leaching and volatilization of nutrients. Tannins applied directly to the soil reduced N2O emis- sions and NH3 volatilization (Jordan et al., 2015). How- ever, emissions of all gases increased in the compost and charcoal amended soils, but the organic soil carbon 47Research Paper Janke, Blackburn, Khan, Al Busaidi, Al Busaidi, Al Hinai, Ahmed levels also increased with these compost and charcoal as well. In a second experiment, tannins and charcoal were either added to the soil with goat manure, or fed to the goats, and then the enriched manure was applied to a sweet corn-radish crop rotation (Ingold et al., 2015). All of the treatments increased the soil organic carbon levels as compared to the NPK control plot, and also increased the soil microbial respiration. In this study, there were no additional measurable effects of the char- coal or the tannin addition to the manure, as compared to the manure alone. In two trials in Oman with cucumber seedlings un- der growth chamber conditions, the addition of biochar resulted in higher soil organic matter levels, higher lev- els of water soluble potassium and sodium bicarbonate (Olsen method) extraction of phosphorus, and in one of the two experiments, greater plant biomass (Al Toobi, 2018). The plants grown with biochar had decreased levels of sap NO3 and increased sap K, but showed no change in sap sodium (Na). A greenhouse experiment with corn and cucumber with biochar added at 5 or 10% by mass, found no effect on plant height, biomass, or chlorophyll content (Khan et al., 2018a). The soil pH did not change, but soil EC increased with increasing rates of biochar. In a second greenhouse experiment with okra and broccoli, EC levels were initially higher with biochar, but the same as the control at the end of the experiment (Al Busaidi, 2017). The pH was not affected by biochar, but the soil organic matter levels and water holding capacity were higher. In this experiment, plant height, chlorophyll levels and plant biomass were not af- fected. In all three experiments, biochar was made on site from mango wood prunings, using a home-made burning apparatus (Al Busaidi et al., 2017). In another greenhouse experiment, local wood bio- char, (Maerua crassifolia) was compared to wood ash, and also to biochar and ash made from local wild grass, Saccharum kajkaiense (Al Kindi et al., 2018; Khan et al., 2018b). Currently, many types of biomass from farms is burned and the ash applied to fields, and the biochar might be an alternative to ash. Both the wood and the grass biochar increased the soil organic matter levels, and the wood ash increased the levels of soil potassium. Soil EC levels were increased by wood ash, but not the biochar; soil sodium levels were unaffected by biochar or ash. Neither the biochar nor the wood ash resulted in an increase in plant biomass, but the grass ash resulted in higher okra plant dry weight. Experiments conducted in Saudi Arabia with biochar from date palm residue found that biochar produced at lower pyrolysis temperatures (300 and 400 C) were most effective at promoting wheat growth, but only when ap- plied with fertilizer (Alotaibi and Schoenau, 2019). Wa- ter retention in the soil was also improved with the low temperature biochar, especially when combined with incubation in the soil for up to 60 days. Soil bulk densi- ty was decreased, and soil porosity increased more with the higher temperature biochar (up to 600 C). Organic matter and CEC was increased as compared to the con- trol only for the biochar at 300C; pH and EC was not significantly different among all treatments. Biochar also has the potential to be of benefit in soils affected by abiotic stress such as salinity and drought. An investigation of biochar derived from rice straw and the effects on plant growth and certain metabolic ac- tivities of salt-stressed cowpea (Vigna unguiculata L.) showed significant plant growth reduction under the salinity treatment compared to control (Osman et al., 2019). Seed germination was enhanced with the appli- cation of rice straw-derived biochar. Further, biochar increased the photosynthetic pigments and ascorbic biosynthesis. Their results found the highest growth rate and plant physiological traits at 75% NPK combined with the biochar amendment. They concluded that the rice straw-derived biochar reduced the extra use of chemical fertilizer up to 25%. Biochar combined with microbial inoculation can also have benefits. The combined application of Bacil- lus amyloliquefaciens and biochar was more effective in mitigating the drought stress in wheat and enhanced the its productivity compared to the control. However, plant growth promoting rhizobacteria (PGPR) applica- tion alone did not show any significant increases in leaf chlorophyll contents (Zafar-ul-Hye et al., 2019). It has been shown that under drought conditions bio- char can increase the soil water holding capacity and its biological and physical properties. Biochar has shown decreased Na uptake under salt stress conditions and increased K uptake by the plants. Biochar may assist in lessening the drought and salt stress effects on plants (Ali et al., 2017). It has also been shown that biochar increases soil water holding capacity and plant pro- ductivity even under low water conditions (Zong et al., 2016). Moreover, it can reduce the trace element uptake in plants and enhance soil properties to reduce metal movement in the soil (Abbas et al., 2017). Effect of Organic Farming Methods and Carbon Amendments on Soil Microbiology Organic farming, in general, is a low external input farming system with a high reliance on healthy soil to sustain plant growth. High amounts of organic fertil- izers are used in substitution for synthetic industrial fertilizers to sustain crop production (Hartmann et al., 2015; Liao et al., 2018). In addition, synthetic pesticides (herbicides, insecticides, fungicides, etc) are not allowed in organic farming, but ‘alternative’ pesticides, usually plant or mineral based, with low biotoxicity are used in substitution, in addition to preventative measures such as crop rotation, crop diversity, cover crops (Lupatini et al., 2017; Zarb et al., 2005). These changes in inputs are associated with complex responses of the soil microbi- al community diversity and function (Hartmann et al., 2015), leading to what is often described as better soil 48 SQU Journal of Agricultural and Marine Sciences, 2022, Volume 27, Issue 1 A 25 Year History of the use of Organic Soil Amendments in Oman: A review health and quality (Doran and Parkin, 2015; Lori et al., 2017; Yadav et al., 2018). Although these terms are loosely described, organic farming systems aim at, and relies on, improving eco- system health and sustainability (Lupatini et al., 2017). It has been suggested that organic farming systems not only lead to higher microbial biomass, activity and func- tional diversity, but they are also dependent on these microbial communities for nutrient cycling, plant nutri- ent uptake and crop yields (Liao et al., 2018). In organic farming systems, nutrient availability to plants is pri- marily achieved through the mineralization of plant and microbial organic compounds in soils (Araújo and Melo, 2010) and the conservation of an active soil microbiota through mulching, erosion control and organic matter inputs (Lori et al., 2017). The mineralization of soil organic matter is carried out by soil heterotrophs, which accounts for the over- whelming majority of microbial species and also the majority of the soil microbial biomass. Although this is true for both organic and conventional farming systems, in organic farming, a significantly large soil microbial biomass represents an essential pool of rapidly cycling and readily available nutrients (Araújo and Melo, 2010; Hartmann et al., 2015; Liao et al., 2018). Soils on organic farms may contain nearly twice as much microbial bio- mass carbon, microbial biomass nitrogen, total phos- pholipid fatty-acids than conventional systems (Lori et al., 2017). This microbial biomass acts not only as a plant nutrient reservoir, but also as the source the bio- logical processes or functions related to plant nutrient uptake, such as the production of extracellular enzymes involved in the soil organic matter decomposition e.g. carbohydrases, dehydrogenases, ureases and proteases (Lori et al., 2017). Soil microbes regulate not only the carbon cycle pro- cesses, but nitrogen fixation, nitrification and denitrifi- cation, and nearly all the soil sulfur oxidation-reduction reactions (Yadav et al., 2018). Symbiotic diazotrophs (Rhizobium), rhizosphere bacteria and mycorrhizal fun- gi are considered to play a crucial role in mediating plant access to essential nutrients in organic farming (Zarb et al., 2005). Organic farming systems show a more com- plex fungal network with higher abundance of keystone mycorrhizae from the orders Glomerales, Paraglomera- les, and Diversisporales than other farming systems (Ba- nerjee et al., 2019). Many recent studies have documented the effect of cropping systems on the response of microbial commu- nities. On a global scale, soil microbial biomass, diver- sity and activity are higher in organic farming systems than under conventional farm management (Liao et al., 2018; Lori et al., 2017; Yadav et al., 2018). When organic farming systems are involved, these effects are often sep- arated into the following components: a) organic fertility inputs and the cycling of soil organic matter, including crop residues and the use of legume-based cover-crops and green manures; b) the effect of controlling plant pathogens with ‘alternative’ and ‘environmentally-friend- ly’ pesticides; c) the effect of management practices such as tillage (depth, frequency) and crop rotation); and d) bioaugmentation of the soil with beneficial microorgan- isms (Lupatini et al., 2017; Zarb et al., 2005). In com- parison to organic amendments, crop protection inputs (application of pesticides) has a significantly lower im- pact on soil microbes (Hartmann et al., 2015; Zarb et al., 2005). Other factors such as climate, soil management, type and life cycle of the crop also impact the microbi- al responses to organic amendments (Lori et al., 2017). There are numerous studies that document the sig- nificant influence of both quantity and type of organic farming fertility inputs on soil microbial biomass size, activity, function and taxonomic diversity (Araújo and Melo, 2010; Hartmann et al., 2015; Liao et al., 2018). In general, the amount of the microbial biomass tends to increase with higher inputs of organic fertility amend- ments. Soil organic matter is also widely postulated as one of the key drivers of soil microbial diversity (Tsiknia et al., 2014), since chemo-organo-heterotrophs are pre- dominant in all soils. In summary, organic fertility amendments increase soil microbial species richness, diversity, heterogeneity, dominance, and results in a different microbial commu- nity structures in comparison to the use of mineral fer- tilization (Hartmann et al., 2015; Lupatini et al., 2017). Organic inputs, and organic farming management as a whole, increase the amount and diversity of keystone soil microbial species that are directly beneficial to plants (Banerjee et al., 2019; Liao et al., 2018; Yadav et al., 2018). Organic inputs create a less selective environ- ment, allowing for a higher taxonomic diversity, whereas systems that do not receive organic inputs are dominated by oligotrophic organisms (Hartmann et al., 2015; Lupa- tini et al., 2017). This higher microbial biomass and tax- onomic diversity are generally assumed to lead to a high- er microbial functional redundancy, ecosystem stability, productivity and resilience (Torsvik and Øvreås, 2002). Background soil properties such as soil texture, mois- ture regime, alkalinity and salinity condition the extent in which organic amendments will be effective in shap- ing a healthy soil microbial community. Among these, soil salinity is especially relevant for Omani farms. On the one hand, organic farming management may help to decrease salinization-related land degradation, and on the other, it will reduce the capacity of organic farming practices to sustain crop yields and a diverse and func- tional soil microbiome (Daffonchio et al., 2015; Lozu- pone and Knight, 2007; Rietz and Haynes, 2003). Soil bacterial communities are more prone to be affected by salinity than any other soil environmental conditions (Lozupone and Knight, 2007), tending to be enriched by halophilic Actinobacteria (O’Brien et al., 2019), Proteo- bacteria and Firmicutes (Das et al., 2019). Soil salinity decreases microbial biomass, respiration, enzymatic ac- 49Research Paper Janke, Blackburn, Khan, Al Busaidi, Al Busaidi, Al Hinai, Ahmed tivities, diversity and increases the microbial metabolic quotient (Wasserstrom et al., 2017; Zhang et al., 2019). The metabolic quotient (an indicator of stresses) is usu- ally unaffected by organic farming systems (Lori et al., 2017). The abundance of heterotrophic diazotrophic bacteria, essential for nutrient management in organ- ic farming systems, are expected to be significantly decreased by soil salinity (Moradi et al., 2011), even though some research shows that high additions of or- ganic matter can reduce the effect of salinity on plants. Soil Microbiology Studies Specific to Oman In terms of research specific to Oman, one of the ear- liest studies of soil microbiology documented the economically important fungal diseases (Waller and Bridge, 1978). This has been updated, as a more gen- eral list of fungi found in Oman (Maharachchikum- bura et al., 2016). Surveys of the biological soil crusts have previously been one of the least explored areas for fungal diversity. However, in Oman 226 fungal isolates were recovered from both free-living and li- chen-forming fungal communities in non-cultivated areas (Abed et al., 2012). These fungi were classified using DNA pyrosequencing, and the authors conclude that the desert crusts of Oman harbor a large diversi- ty of fungal communities, and that the presence of li- chens in these crusts improves soil characteristics. Studies of date palm plantations and natural veg- etation have found a large number of unique arbuscu- lar mycorrhizal fungal communities (Al-Yahya’ei et al., 2011). Spore abundance and species richness was high- er under date palms than native vegetation. Overall, 25 morphospecies were detected across both habitats, some of which seemed to be site specific. Two of the most globally abundant species (Glomus intraradices and Glomus moseae) were not detected. In addition, the study found four previously undescribed morphos- pecies and a considerable degree of molecular diversity seems to make these sites unique. In a follow-up study in north-central Oman using both morphological and molecular identification techniques, researchers recov- ered four species of arbuscular mycorrhizal fungi that had previously been described for the Arabian Penin- sula, but are reported for the first time in Oman (Sy- manczik et al., 2014a). In a second study of the same region of north-central Oman, three new species of arbuscular mycorrhizal fungi were discovered and re- ported for the first time; two from non-cropped areas and one from a date palm plantation (Symanczik et al., 2014b). The authors conclude that further work is need- ed to understand the functional significance of these new taxa for conservation efforts and for agriculture. The effect of a newly discovered endophytic fungus, Talaromyces omanensis, was studied on tomatoes sub- jected to drought-stress under greenhouse conditions (Halo et al., 2020). The presence of the fungus result- ed in improved tomato characteristics under drought, such as shoot dry weight, root length, the number of flowers and fruit weight, as compared to the non-in- oculated control. Even without the effect of drought, the inoculated plants performed better than the con- trol. The inoculation lead to some interesting ana- tomical and chemical changes in the plants, such as increased phloem and cortex width, improved chlo- rophyll fluorescence, and higher concentration of gib- berellic acid, which may enhance drought tolerance. Both endophytic fungi and bacteria may help plants with other biotic stress, such as salinity, and the plants may also affect the rhizosphere. Date palm seedlings were inoculated with soil from existing date palms grow- ing at the Sultan Qaboos University campus, and half were subjected to saline growing conditions (Yaish et al., 2016). DNA extraction and barcoding revealed that 30% of bacterial and 8% of fungal species had statistically different abundance when subjected to the salinity treat- ment. Some of the endophytes identified in this study were previously isolated from saline and marine envi- ronments. The authors suggested that there may have been a buffering effect by the host plant on the colonized root environment. The microbes in the environment may also affect each other, and respond differently to moisture or oth- er soil conditions. When four arbuscular mycorrhizal fungal (AMF) species from the Arabian desert were added to the crop, Sorghum bicolor, as the host plant in a microcosm experiment, they responded differently when an invasive species (Rhizophagus irregularis) com- monly found in commercial inoculant was introduced (Symanczik et al., 2015). Three watering regimes were used. In the well-watered plants, hyphal length density was slightly higher, but not significantly different than when the commercial inoculant was added. However, the drought stressed treatment had the opposite result, with more hyphal length density in the native only (no commercial) inoculant treatment. Sorghum root and shoot dry weight were similar in both inoculant treat- ments in the watered treatment, but lower in the dry- ing cycle and drought stressed treatment with the com- mercial inoculant. Other researchers have found that commercial introduced species sometimes do not sur- vive when in competition native fungi (Hamel, 1996) or simply do not have any effect under field conditions, and when not in a sterile lab situation. A similar response can be seen with bacteria that were used as a manure inoculation. A native phytase-producing bacterium had significant reduction in abundance after only 6 days of incubation after inoculation (Menezes-Blackburn et al., 2016). In another experiment, E. coli survival time was reduced when introduced to soil with high microbial di- versity, as compared to low diversity (Xing et al., 2019). Many crops commonly grown in Oman benefit from mycorrhizal associations. Citrus, for example, are strong- ly mycorrhizal dependent (Ortas, 2012). Bananas with AMF benefit from enhanced plant nutrition and reduced 50 SQU Journal of Agricultural and Marine Sciences, 2022, Volume 27, Issue 1 A 25 Year History of the use of Organic Soil Amendments in Oman: A review nematode lesions (Jaizme-Vega and Pinochet, 1997), some protection from Fusarium wilt disease (Castillo et al., 2019), but there can be significant differences between colonization and benefits from the AMF that are differ- ent among different banana cultivars (Jefwa et al., 2012). Compost can be a rich reservoir of beneficial fungi. An organic compost product originating from Oman made from green waste, wood, cow and buffalo manure, processed for 60 days, was sampled using direct plating methods on potato dextrose agar, and also through DNA extraction and identification (Al-Mazroui and Al-Sadi, 2016). Pyrosequencing detected 94 species, belonging to 6 phyla and 12 classes. Sixty-seven of the identified fungi are reported in Oman for the first time. Only 5 species were identified using direct plating. Though composts in the past had been considered potential sources of pathogenic fungi, this experiment found that the organic compost sample was dominated by sapro- phytic fungi and fungi with biocontrol characteristics. The three dominant fungi revealed by direct plating were Aspergillus, Trichoderma and Penicillium. Patho- genic species such as Fusarium and Phoma occurred at very low levels (less than 1% of the total species). In a similar experiment, 72 compost, organic fertilizer and potting media samples were compared from 14 coun- tries. Fungal diversity analysis found high fungal diver- sity, especially in products from the Netherlands. Again, Trichoderma, Aspergillus and Penicillium predominated in most samples. Saprophytic fungi and fungi with bio- control characteristics dominated in general, but Fusar- ium spp. were recovered at high frequencies in some samples (Al-Mazroui and Al-Sadi, 2015). When both local and imported potting soils were compared to cultivated (farmed) and uncultivated soils of Oman, a surprising number and diversity of Trich- oderma species were found in the uncultivated soils (Al-Sadi et al., 2015). Thirteen isolates, representing four species, were collected from 65 uncultivated soil samples with no known history of prior cropping. How- ever, some samples were collected from dams, where soil that may have washed from farms upstream had accumulated. Fifteen isolates representing five species were obtained from 45 samples collected from farms growing crops such as date palm, cucumber, tomato, bean, alfalfa, pepper and potato. Twenty four isolates were obtained from 84 potting media samples produced in Oman, The Netherlands, Estonia, Germany, Finland, Latvia or the UK. Nine species were identified from the potting media isolates. On a percentage basis, the culti- vated soils resulted in a higher frequency of both isolates and species per sample collected (33 and 11% respec- tively), but the authors were surprised by the number of species found in uncultivated soils, including T. har- zianum, which is often used as a fungal inoculant. The percent genetic variation among the populations of T. harizianum obtained from the potting media, cultivated and uncultivated soil was very low, indicating high gene flow among these three sources. The authors also note that the high level of fungal diversity found on farms in Oman is a sign of soil health and makes it possible to find and select isolates with high antagonistic proper- ties. Three species identified in this study had not been before reported in Oman. Fungal diversity can vary between organic farms and conventional farms. A non-organic semi-oasis farm in Oman growing date palm, acid limes and cucumber was compared to an organic farm growing cucumber and to- mato (Kazeeroni and Al-Sadi, 2016). The three crops on the non-organic farm had been fertilized using animal manures, but the cucumber also received conventional fertilizer and soil fungicide for the past 5 years. The or- ganic farm crops only received organic fertilizer and had never received soil fungicide. Species richness estimates showed that the two organic farm samples were signifi- cantly higher than the three samples from the non-or- ganic farm. The Shannon diversity values indicated that soils from date palm and lime have higher fungal diversi- ty than cucumber on the non-organic farm, but that the organic cucumber and tomato soils had higher diversity than the non-organic cucumber. The diversity values were likely high for two crops on the non-organic farm, because manures are applied to the dates and limes, and there was no known use of fungicide. However, the cu- cumber on the non-organic farm had received fertiliz- ers, and also soil fungicide, and both may have played a role in the lower fungal diversity. In this study, the soil organic carbon was highest under the date palm, proba- bly due to repeated applications of manure, but not sig- nificantly different among the other crops, though it was slightly lower for the non-organic cucumbers. Indigenous rhizobacteria can also have very specific plant protective properties. Thirty-eight native bacterial isolates from the rhizosphere soil of muskmelon and cu- cumber were screened for antagonism against Monospo- rascus root rot and vine decline disease in muskmelon. Five isolates showed antagonism grown on media in the lab, and one of the isolates, Pseudomonas resinovorans, reduced the incidence of vine declined by 93% relative to an infected control when used as a seed treatment or soil application (Al-Dagjaro et al., 2020). Native bacte- ria isolated from compost can suppress Pythium and Fu- sarium-induced damping off in cucumber (et al., 2020). Seven bacterial isolates from compost were screened, and two showed a significant level of antagonism. Both isolates were identified as Pseudomonas aeruginosa Soil application of this bacterial were effective in controlling damping-off of cucumber. Plants can also benefit from endophytic bacteria iso- lated from wild plants. Endophytic bacteria live inside the plant, and can improve plant growth by improving plant nutrient uptake, modulating growth and stress re- lated phytohormones, and improve plant health by tar- geting pests and pathogens with antibiotics, hydrolytic enzymes and by priming plant defenses (Afzal 2019). In 51Research Paper Janke, Blackburn, Khan, Al Busaidi, Al Busaidi, Al Hinai, Ahmed Oman, an experiment was conducted by first isolating endophytic bacteria from leaves of the mangrove tree, Avicennia marina (Ali et al., 2017a). From 28 bacterial isolates obtained, nine different genera were identified using DNA sequencing, and from these, two salt tol- erant species (Bacillus pumilus and Exiguobacterium sp.) were chosen for further study on tomato. Tomatoes grown in a non-soil substrate were subjected to salinity stress and treated with these two strains of bacteria as a liquid inoculant. The inoculated tomatoes had signifi- cantly higher biomass, photosynthetic rate and pigment accumulation as compared to controls. The authors con- clude that endophytic bacterial strains play a regulatory role in salinity stress, and possible mechanisms such as reducing oxidative stress by regulating antioxidants and related enzymes are discussed. The type of crop and the growing conditions can also lead to different results when looking at the effect of microbial association benefits. For example, on to- matoes inoculated with Trichoderma harzianum and subjected to salt stress, the inoculated tomatoes without salt benefited from the inoculation in both fertilized and unfertilized experiments. However, once salt stress was introduced, only the non-fertilized tomatoes showed en- hanced growth from the effect of the inoculation (Rubio et al., 2017). The type of crop can also make a difference, as well as the diversity of fungi in the inoculum. Wheat, barley, corn, and sorghum were compared in their re- sponse to either a single or a mix of four AM fungi. Wheat was unresponsive to any fungal treatment, and barley responded positively to the mix of four species, but only under low P conditions. The corn and sorghum responded positively to fungal inoculation, but only the sorghum responded to the increased fungal diversity and had higher colonization rates (Frew, 2019). Most of the papers reviewed in this section focus on the species of microorganisms found in the soil, or the effect they have on the plant, whether it is to stimulate growth, protect from abiotic or biotic stress. Some of the species occur naturally in the soil or compost, and some have been introduced as inoculants, sometimes af- ter being isolated from natural sources. In some organic farming literature, these broad categories of organisms and their effects are lumped together, along with others known for nitrogen fixation in a category called “biofer- tilizer” (Venkateswarlu, 2008). This is not a very precise term, and can mask the actual mode of action or mech- anism responsible for the various effects. However, this term did not come into use by accident, but rather as a response to efforts to commercialize, patent, and sell microbial inoculants (Harman, 2010). As noted in the diverse studies covered in this sec- tion, some microoransims have more than one effect on a plant, including both protection and growth promo- tion. If a company in the US or other country wants to bring a product to market as a “biopesticide,” it can take as long as 6 years and up to $8 million for toxicology and efficacy testing, patenting, production and formula- tion in addition to identifying an effective organism. To register a product as a “biofertilizer, inoculant or plant strengthening agent” requires some of those same steps but can usually be accomplished in only 1 to 2 years and $1.8 million (Harman et al., 2010). Other models pro- posed that are even less expensive include government sponsored production, or community-based local pro- duction. The local model has been successfully used for strawberries in Costa Rica and in other countries, but quality control mechanisms must be in place to be sure that the final product is efficacious and not contami- nated, but can be a cost-effective solution, with local production in close proximity in time and space to the end-users so that shelf-life of the product is not an issue. On organic farms, including in Oman, some farmers purchase biological inoculants, especially Trichoderma, and add it to the potting soil mix for both plant stimula- tion and protection from root disease. However, for the Trichoderma or any other inoculant to work in the real world, the soil system must be conducive to growth. This effect is sometimes called “biostimulation,” and is usually achieved by adding organic matter compounds such as manures or composts. This brings us back to the begin- ning of this review, where it was noted that many farmers in Oman are already adding both unprocessed and pro- cessed manures and composts. Future research in Oman should look at both the organisms, and the soil environ- ment into which they are found or introduced, to better understand these processes as a whole system. Applica- tion of organic matter to the soil, especially composted crop, animal manures and mulches is also a recommen- dation in the “Strategic plant for combating water and soil salinity in the Sultanate of Oman” (Hussain, 2005). Cropping Systems Research and Organic Farm- ing Over the years, long term cropping systems studies have made a significant contribution to our understanding of organic agriculture; especially in terms of nutrient and carbon cycling (Drinkwater et al., 1998), and also yield and pest management during the conversion to or- ganic (Liebhardt et al., 1989). Most current textbooks on organic farming emphasize the need for a systems approach (e.g. Lockeretz, 2007; Francis, 2009; Barker, 2010). In a systems approach, organic treatments are not simply removing an input such as a synthetic fertil- izer, but designing a diverse crop rotation with organic fertility amendments, non-chemical pest control, and different methods of planting, tillage, and mulching as compared to conventional monoculture systems. Un- derstanding soil processes and differences between con- ventional and organic systems is key, especially when these are understood and studied as agro-ecosystems (Drinkwater, 2009). Much relevant research has been conducted in Oman, in short term focused studies. For example, carbon and 52 SQU Journal of Agricultural and Marine Sciences, 2022, Volume 27, Issue 1 A 25 Year History of the use of Organic Soil Amendments in Oman: A review macro-nutrient balances were studied in organic vegeta- ble systems within a crop rotation over a 2-year period in northern Oman (Siegfried et al., 2011). Tools such as genetic/molecular identification of beneficial soil micro- organisms such as Trichoderma are also increasing our understanding of presence of these microorganisms in both cultivated and uncultivated soils in Oman (Al Sadi et al., 2015a), and fungal diversity in potting media and fertilizers used in organic systems (Al Sadi et al., 2015b). This level of detail is needed to explain the sometimes apparently contradictory results in the literature. For example, that Trichoderma can induce systemic resis- tance in host plants for a variety of plant diseases (Hoit- ink et al., 2006), but that the effect depends on whether the potting mix contains compost. These complex details can be best studied in side-by- side replicated long term research plots, where complete control over soil conditions is established and monitored every year. Trials such as these have been conducted in the US since the early 1980’s. A review of lessons learned from six of these experiments demonstrate accumu- lation of soil carbon, nitrogen, and other measures of improved soil health over the long term under organic management (Delate et al., 2015). Competitive econom- ic returns are also realized from these cropping systems, but sometimes only after organic price premiums are considered. Diverse crop rotations along with organic amendments were key to good crop production, but also the level of experience of the farm manager made a dif- ference in terms of the success of organic weed control. A review of 132 long-term studies of long-term exper- iments (greater than 10 years) world-wide also found that the application of manure resulted in gains in soil organic matter and Olsen P, but not always equivalent yields to treatments with inorganic fertilizer or manure plus fertilizer (Chen et al., 2018). The authors concluded that adding fertilizer to plots with organic amendments resulted in more resilience, especially if the crops were grown under tropical conditions or the inherent fertility of the soil was low. In contrast, a long-term experiment from the Neth- erlands, summarized after 13 years found less variability (lower coefficient of variation) in yields or the organic as compared to conventional systems with fertilizer, af- ter initially finding lower yields in the organic plots in the first few years (Schama et al., 2018). Soil properties showed significantly higher soil organic matter, soil ag- gregates, nutrient availability, and soil biota in the or- ganic systems. The organic system also resulted in lower groundwater nitrate concentrations and fewer plant par- asitic nematodes. An analysis of 14 long term field trials in the US and the UK, ranging from 20 to 120 years, was used to compare the effect of fertilizers and manures (Edmeades, 2003). Manured soils had higher contents of organic matter and soil microfauna, and were also higher in P, K, Ca and Mg in the topsoil, and nitrate N in the subsoil. They also had lower bulk density, higher porosity, hydraulic conductivity and aggregate stability as compared to fer- tilized soils. This experiment cautioned that over-appli- cation of manures over a long period of time can lead to excess P and the potential leaching of nitrate if not ap- plied in moderation, and it recommended to match the rates of application to the nutrient needs of the crops. Long-term in-ground greenhouse production sys- tems (also called high tunnels), can also result in changes in soil properties over time as compared to field plots (Knewtson et al., 2012). After eight years of organic production the total carbon and particulate organic matter (readily available carbon) were both increased in the organic high tunnel and open field plots, but not in conventionally managed/fertilized high tunnels or field plots, with a vegetable crop ro- tation. Soil salinity increased slightly in the high tun- nels as compared to the field in both systems but was not high enough to be detrimental to crop growth. Organic Farming in Oman - Benefits and Poten- tial In addition to soil improvement, other benefits of or- ganic farming have been well documented in terms of environmental quality, and also human health. In Oman, over-fertilization has been found to contribute to high levels of nitrate in groundwater supplies (Ahmed et al., 2001). Mis-use of both legal and illegal pesticides in Oman has also been documented in a farmer survey (Al Zadjali et al., 2014), and pesticide testing for shipments of fruits and vegetables going into and out of Oman has become mandatory, due to un-safe levels of pesticide residue detection on some shipments. Several reviews have revealed links between human pesticide exposure and certain forms of cancer (Bassil et al., 2007), and also links between pesticides and oth- er negative non-cancer health impacts (Sanborn et al., 2007). Organic food has been demonstrated to have lower levels of pesticide residues than non-organic foods (Smith-Spangler et al., 2012), though similar levels of nutrient content (Dangour et al., 2009). However, other research found higher levels of anti-oxidants and other beneficial phytochemicals in organic foods (Zhao et al. 2006). Children who eat diets of organic food consis- tently for 2 weeks have less pesticide metabolites in their urine than the control group (Curl et al., 2003; Lu et al., 2006), so less pesticide in the diet can translate into less pesticide in the body. But does eating organic food can also lead to a lower risk of cancer? Two extensive, long term co-hort stud- ies have been conducted in France (Baudry et al., 2018) and in the UK (Bradbury et al., 2014). Participants were asked to report their consumption of organic foods, and data were controlled for other risk factors such as smoking, age, and gender then co-horts were followed over a period of years. The study in France over 7 years included 68,946 participants and found links to several 53Research Paper Janke, Blackburn, Khan, Al Busaidi, Al Busaidi, Al Hinai, Ahmed types of cancer in the non-organic consumption group. The UK study included 623,080 individuals over 9 years and found a statistically significant reduction in the inci- dence of non-Hodgins forms of leukemia for the group that consumed organic foods. Globally, organic food and drink sales have been growing at a rate of about 20% per year for at least 2 de- cades, and has reached $97 billion USD. The land clas- sified as “certified organic” has increased from 11.0 to 69.8 million ha from 1999 to 2017 (Willer and Lernoud, 2019). In addition, there is probably much land that is being managed organically but is not certified. The cer- tification process is expensive, and requires registration with a certification organization, annual fees and inspec- tions, in addition to documentation and records show- ing that only the allowed inputs are being used on the farm. Certification is usually only pursued by farmers with markets that will pay a premium price for organic products, or allow the farm to achieve product differen- tiation or distinction in the market. Attitudes towards organic farming in Oman seem to be generally positive. A recent survey conducted at a farming conference in Oman found that 50% of the farmers preferred to use organic compost, 30 % pre- ferred organic non-composted manures, and only 6% preferred to use chemical/synthetic fertilizers (Al Toubi, 2015). She also found that when asked about the possi- bility to adopt organic farming systems in Oman, 68% believed it is possible, while 19% thought it isn’t possible due to pest and/or soil fertility issues. Neighboring countries such as the UAE and Saudi Arabia have recently developed Organic Agricultural policies and certification schemes, demonstration farms, and also dedicated organic research plots (Hartmann et al., 2012). Al Toubi (2015) concludes her thesis by sug- gesting that long-term research is needed to address the future research questions for organic farming in Oman, while a general case was made by Drinkwater (2009) for long term ecological studies to move the science of or- ganic farming forward. Understanding the soil food web, especially the microbial component, can help us design cropping systems for the future that are more resilient to drought (De Vries et al., 2012). Benefits of Improved Soil in Oman A note about audience for this research; there are 154,010 land-holding farms in Oman (MAF, 2015) on 315,011 Feddan (a Feddan is 0.42 ha). Of these, 90% of them are 2 Feddan in size or smaller (Figure 3). The 10% of farms that are larger than 2 Feddan are potentially sell- ing product in the marketplace. Similar percentages are seen in the marketing statistics (MAF, 2015), as the Cen- sus of Agriculture shows that 88% of farms are growing primarily for family consumption, 9.5% to sell at markets in Oman, and 0.5% is sold into the export market. The remainder is unknown or sold for processing. Research on soil quality, and the results from the research needs to be disseminated to both market and non-market ori- ented farms, of various size categories, in addition to farms on different soil types, crops, and using differ- ent irrigation systems described in the earlier sections. Urban home gardeners are also an important recipi- ent of this information, as many are growing fruits and vegetables for home consumption (Al Mayahi, 2019). Many small farms/gardens may already be using primar- ily organic practices, and may not care about certifica- tion, but could be encouraged to continue to use their soil and water resources wisely. A recent survey of 125 households in a suburban area in Muscat Oman, found that 57% of them had home gardens. Of those that had a garden, 75% were growing fruit crops, and 48% vegetable crops, in addition to or- namental plants and grass (Al-Mayahi et al., 2019). In terms of soil fertility inputs, only 17% were using fertil- izer alone, 54% were using only organic sources (com- post, green manure and livestock manure) and 25% used a combination of fertilizer and manure. About 4 % didn’t use any soil amendment or fertilizer, but the majority of the gardeners brought in new soil every year from an outside source (66%). Most were applying their fertilizer only once a year. In most cases, family members, or fam- ily members along with a gardener or a housemaid were caring for the garden, but in 27% of the gardens, the care was left up to either the housemaid or gardener. Sources of information for the gardeners in the survey area were the internet, family, friends, social media, neighbors and the plant nursery, in that order (Al-Mayahi et al., 2019). In the farm survey (Al Salmi, 2020), the farm plant- ing decisions were primarily made by the owner (85%), but with workers (8%), a farm manager (5%), or family (2%) also making the decisions on some farms. Sources of information for the farms include [note, survey re- spondents could select more than one answer] parents (82%), neighbors (65%), and extension agent (45%). Less mentioned sources were the internet/newspaper (32%), vegetable traders/processors (31%), consumers (25%) and input suppliers (14%). Conclusion This review has summarized the research on a wide vari- ety of organic soil amendments used in agriculture, with a focus on the research that has been done in Oman. There are many benefits to using these amendments, ranging from increased organic carbon, improved water holding capacity, better habitat for soil microbial diver- sity and function, resulting in better nutrient mineral- ization, plant protection, and crop yield. Amendments vary from organic mulches, compost, manures, and municipal biosolids and wastewater. Municipal biosol- ids and wastewater are not allowed in certified organic farming, but have beneficial effects on the soil and its properties, and are good for the environment as long as the anti-quality factors that may be present are either 54 SQU Journal of Agricultural and Marine Sciences, 2022, Volume 27, Issue 1 A 25 Year History of the use of Organic Soil Amendments in Oman: A review controlled at the source, or tested and only applied to crops not for human consumption. The audience for this research varies from home gardeners, to fami- ly farms with small plots for home use, to larger scale farms that grow for both domestic and international ex- port markets. Crops range from forages, grains, fruits and vegetables, and irrigation systems include both drip from well water and flood irrigation from springs/aflaj. The soils suitable for agriculture in Oman are limited to only 6.5% of the total land area, and are further limited by soil and water salinity and the availability of water. However, the papers summarized in this review show that organic matter added to the soil can help soils con- serve water and reduce the effects of salinity. The most recent Oman Agriculture Census data shows that 88% of agricultural land received non-processed manures from some source, and 45 % received processed manure, or compost, which is positive. In addition, many of the Ministry recommendations for soil fertility for a range of crops include organic sources. Organic farming is a trend that continues to grow world-wide at a rate of about 20% per year, and farm- ers who choose to get the organic certification usually enjoy a price premium for their products. Long term experiments on organic cropping systems show that it is possible to get the same yield using organic methods as compared to conventional NPK fertilizer and pesti- cides, but it requires excellent management, long-term application of organic fertility sources, and in some cas- es, cultural and biological control of some insect and disease pests. As the research in these areas progress, organic farming will become more feasible in the future, and is prepared to feed the anticipated 9 billion people. Continuous farming with only NPK fertilizer degrades land, contributes to the salt index, does not build organ- ic matter, and has no long-term benefits. In addition, fertilizers and pesticides can leach into groundwater and are associated with higher rates of cancer. On the other hand, organic farming methods lead to increased organic matter in the soil, which can be achieved by us- ing both traditional and non-traditional carbon sourc- es. This increased soil organic matter is also associated with higher microbial biomass, diversity, and can result in enhanced crop growth and successful biological con- trol of plant pathogens. This review has covered a wide range of topics, which are all inter-linked, and we hope this will provide a road map for future research in these areas including long-term systems studies on both farms and experiment stations. Figure 3. Number of farms and area by size class in Oman (MAF, 2015). 55Research Paper Janke, Blackburn, Khan, Al Busaidi, Al Busaidi, Al Hinai, Ahmed Acknowledgement The authors would like to thank Sultan Qaboos Univer- sity and the Sultanate of Oman for funding numerous research projects and graduate students over a sever- al-year period, including, but not limited to the Dean’s Research Fund, Internal Grants, Research Council Grants and His Majesty Trust Fund Research Grants. References Abed RM, Al-Sadi AM, Al-Shehi M, Al-Hinai S, Rob- inson MD. (2012). 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