325 J. Hortl. Sci. Vol. 17(2) : 325-332, 2022 This is an open access article d istributed under the terms of Creative Commons Attribution-NonCommer cial-ShareAl ike 4.0 International License, which permits unrestricted non-commercial use, d istribution, and reproduction in any med ium, provide d the original author and source are credited. Original Research Paper INTRODUCTION A properly designed and operated drip irrigation system has to supply the water amount required by the crop and should also wet enough soil volume. The wetting patterns which develop from dripping water onto the soil depend on discharge and soil type. Two of the key factors in the design of micro-irrigation systems to obtain the maximum benefits are the amount of water used and the volume of soil to be wetted. The partial soil wetting pattern by micro irrigation requires assessment of the percentage of soil volume that is wetted (Sne, 2006). Distance between emitter on lateral pipe and distance of lateral pipes from each other should be determined based on the degree of wetted soil diameter by emitters. Duration of irrigation also depends on the fact that at what time after commencement of irrigation, the wetting front reaches depth of plant’s root or a multiple of it. Distance of outlets, discharge rate and time of irrigation in drip irrigation have to be determined so that volume of wetted soil is close to volume of plant’s root as much as possible. This is because volume of wetted soil surface and moisture depends on soil texture and layering, soil homogeneity, dripper flow rate, primary moisture of soil, consumption water and land slope. A truncated ellipsoid is assumed to best represent the geometry of the wetted soil volume under an emitter. The restricted volume of the wetted soil under drip irrigation and depth-width dimensions of this volume are of considerable practical importance. The volume of the wetted soil represents the amount of soil water stored in the root zone, its depth dimension should coincide with the depth of the root system while its width dimension should be related to the spacing between the emitters and lines. The parameters which influence the wetted soil volume are the available water holding capacity of the soil and the peak daily crop water use representing specific field conditions. The irrigation interval and the management-allowed deficit are additional parameters which affect the wetted volume and could be changed depending on crop sensitivity as well as water and irrigation equipment accessibility (Li et al., 2004). Irrigation water applied should be adequate for crop water use in irrigation interval. The applied water should not be beyond crop root zone to avoid deep Standardisation of soil volume wetting for drip irrigation in mango (Mangifera indica L.,) Manjunath B.L.*, Nair A.K., Laxman R.H. and Abhilasha C.N. ICAR-Indian Institute of Horticultural Research Hesaraghatta, Bengaluru - 560 089 *Corresponding author Email : blmanjunathagri@gmail.com ABSTRACT Field experiments were conducted in mango for four years during 2017-2020 at ICAR-Indian Institute of Horticultural Research to standardise optimum soil volume wetting for drip irrigation. Wetting soil volume upto 70% recorded higher mean fruit yield of 34.8 kg/plant (9.68 t/ha)and with further increase in the level of soil volume wetting irrigation (upto 80%), there was a decline in the mango yield (7.40 t/ha). Similarly, significantly increased response was observed in fruit weight upto 70% soil volume irrigation (226 g) although there were no significant differences in the TSS of the fruit. Significantly higher water use efficiency was observed for 30% soil volume wetting irrigation (274.1 kg/m3) and further no significant differences were observed in water use efficiency between 50% and 70% soil volume wetting irrigations indicating that in areas of water scarcity, it is enough to scheduling the irrigation only upto 50% soil volume wetting in mango for economising the water (232.1 kg/m3). Keywords: Mango yield, scheduling irrigation, soil volume wetting, water use efficiency 326 Manjunath et al J. Hortl. Sci. Vol. 17(2) : 325-332, 2022 percolation. Although the wetted soil is based on soil type, flow rate, and crop water use, the horizontal and vertical water movements are related to both emitter flow rate and soil intake rates. As such there is a need to optimise the wetted volume taking into account soils, crop, crop stage and seasons. Ma ngo is the ma in fr uit cr op of India a nd is extensively cultivated under rain fed conditions (68%) with wider spacing without much inputs. At present mango is cultivated in an area of 22, 93,000 ha with a production of 2, 07, 98,000 MT, the productivity being 9.66 t/ha (Anon. 2019). Most of the fruit development of on-season mango fruits takes place during the dry season and farmers have to irrigate mango trees to ensure high yields and good quality. Mango responds well to irrigation especially during fruit set to fruit development. Mango fruit production and quality at fruit growth stage were significantly affected under different irrigation water amounts. Variation in soil water content not only had effects on fruit size, but also on fruit yield (Wei et al., 2017). Deficit irrigation strategies are needed to increase water use efficiency and solve the problem of fruit weight reduction during development (Srikasetsarakul et al., 2011). Further, the amount of water to be irrigated and the per cent soil volume to be wetted need to be standardized to a given crop situation for enhanced water use efficiency especially under scarce situations. Keeping these points in view, efforts were made to standardise the optimum soil volume wetting irrigation for mango. MATERIALS AND METHODS Field experiments were conducted for four years during 2017 to 2020 at ICAR- Indian Institute of Horticultural Research, Hessaraghatta, Bengaluru located at a latitude of 13°8’12"N and a longitude of 77°29’45"E, to standardize the optimum wetted soil volume for irrigation in 18 years old mango (variety Ra spur i) spa ced a t 6m x 6m. T he ma ximum temperature during the experimental period ranged from 240C to 360C and the minimum temperature ranged between 100C to 220C. The period between March to May are the warm months with higher temperatures and evaporation while the period between November to January were the cooler months with low temperature and evaporation. The average relative humidity was higher during September and October months. The average rainfall of the location is around 850 mm with two peak periods of rainfall during June- July and September-October months. Pre-experimental soil had a pH of 4.73 with moderate salts (1.00 dSm-1). The organic carbon content of the soil was good (2.91 %). The available nitrogen (471.4 kg/ha), available phosphorus (23.8 kg/ha) and the available potassium content of the soil was on higher side (350 kg/ha). The experiment involved comparison of three levels of soil volume wetting irrigation (30%, 50% and 70%) with normal drip irrigation (80% soil volume wetting) as control in RBD design with six replications. The mango crop was maintained with recommended package of practices except for irrigation. The evaporation data was collected from USWB Class A open pa n eva por imeter of meteor ologica l observatory situated in the experimental farm of ICAR-IIHR. Irrigation scheduling was done based on pan evaporation data as per the treatments coinciding with the period from fruit set to fruit development stages. The volume of the active root zone (soil volume) was arrived by excavating the moist soil carefully (without da ma ging the r oots) a r ound the ba se befor e experimentation in the representative plants. Plastic barriers were introduced to the required length and depth of the root zone to demarcate the required per cent wet and dry zones. Three different percentages of the surface soil areas were wetted by the use of single (for 30% and 50% soil volume wetting) and double drip laterals (for 70% soil volume wetting). The amount of rain was taken into account and irrigation paused or reduced accordingly. Total water applied was mea sured and wa ter a pplied per tree wa s calculated based on application time and nominal flow ra te. The ca lculated amount of water for ea ch irrigation was either partially wetted or fully wetted in the root zone depending on the treatment. An irrigation level of 80% ER was fixed based on the results of earlier experiments in mango and water use was calculated to wet the required per cent soil volume in the active root zone based on the wetted area basis as per the treatments.Soil moisture variations were monitored both in dry and wet zones periodically through gravimetric method. Mango was applied with recommended FYM with a fertilizer dose of 730g N, 180g P2O5 and 680g K2O per plant per year and the crop was managed with r ecommended pa cka ge of pra ctices except for irrigation. Plant hopper s were controlled using 327 Imidacloprid 0.3% and powdery mildew with wettable Sulphur. All the growth and yield parameters were recorded in mango each year. The canopy volume in mango was computed as per standard procedure (Mark et al., 2002). At harvest, yield was determined separately for each tree in alltreatments by the use of a mechanical balance. Water use efficiency (kg fruits m-3of water applied) was worked out based on the total water applied through drip irrigation according to FAO recommendations (Doorenbos and Kassam, 1979). Dropped fruits under all trees in the experiment were collected, counted and weighed. Fruit drop was recorded periodically in number and weight. After harvest the number of all dropped fruits per tree and all harvested fruits were added up to estimate the total fruit retention. The retention rate was calculated as the percentage of fruits attached to the tree at harvest as compared to the calculated initial fruit set. The mean data was analysed as per standard statistical procedures (Panse and Sukhatme, 1985). RESULTS AND DISCUSSION Soil moisture The moisture studies in the root zone during different periods indicated that there exist significant variations in the soil moisture across the treatments. In the wet zone, the soil moisture increased with increase in the per cent volume of soil irrigated. The highest soil moisture in the wet zone (14.5 %) was recorded with 80% soil volume wetting with a record of 179.9% increased moisture over the dry zone. It was noticed that even with 50% soil volume wetting, the per cent soil moisture difference in the wet zone was over 158.3 % as compared to dry zone.The higher moisture with increased level of irrigation meeting higher volumes of soil may be attributed to the fact that increasing the water application rate allowed more water to distribute in horizontal direction, while decreasing the rate allows more water to distribute in vertical direction for a given volume applied (Li et al., 2004). Moreshet (1983) attributed this to the differences in the water depletion a s well to the root density distribution pattern between the partially irrigated and that of the fully irrigated one. Plant growth in mango The growth parameters in general increased upto 50% soil volume wetting and declined thereafter. Further at 80% soil wetted volume irrigation,significantly lower canopy spread of the plant was observed compared to lower levels of soil volume wetting suggesting that the growth in mango is not favoured much with irrigation above 70% soil volume wetting. Vellame (2015) attributes this to the plant acclimation which is caused by an increase in root concentration in the irrigated area. After a period of acclimation, if the entire root system is wetted, soil water extraction becomes proportional to the percentage of wetted area after a short period of time. Fruit retention in mango The fruit r etention in mango was significantly influenced by different wetted volumes of irrigating the Standardisation of soil volume wetting for drip irrigation in mango Table 1 : Mean soil moisture variation in dry and wet root zones in mango basin Irrigation treatment Soil moisture in Soil moisture in % increase in soil wet zone dry zone moisture in wet zone (%) (%) over dry zone 30% soil wetted volume 7.71 3.06 152.0 irrigation 50% soil wetted volume 11.96 4.63 158.3 irrigation 70% soil wetted volume 12.77 5.01 154.9 irrigation 80% soil wetted volume 14.50 5.18 179.9 irrigation S.Em± 1.05 0.52 C.D (P=0.05) 3.35 NS J. Hortl. Sci. Vol. 17(2) : 325-332, 2022 328 soil with increase in the retention as the percent soil wetting increased with irrigation although the trend was not continuous. Significantly higher fruit retention (49.3 %) was observed at 70% soil volume wetting irrigation which although was found at par with 50% soil volume wetting irrigation (47.2%), differed significantly from the other two levels. The increase in fruit retention with increased moisture levels may be attributed to the reduction in fruitlet drop as a consequence of favourable moisture conditions. These differences may also be attributed to the accumulation of abscisic a cid in buds at flor a l initiation in optimizing leaf water potential and sap flow besides optimizing carbohydrate availability and cytokinin in sustaining differentiation activity in growing buds (Makhmale Sandip et al., 2015). Similar observations of maximum fruit retention at harvest stage and delayed maturity in the mango trees with irrigation was also observed by Malshe et al., (2020). Yield attributing characters and the fruit yield The number of mango fruits per plant increased significantly with increase in soil volume wetting irrigation upto 70% (191.9 fruits/plant) decreasing thereafter suggesting that mango responds only upto this level of soil moisture. Higher fruit number with 70% soil volume wetting irrigation may be attributed to higher fruit retention with reduced fruit drop owing to favourable soil moisture conditions at the critical phase. Morshet et al., (1983) also observed that there was a considerable difference in flower abscission between irrigation levels especially at the beginning Manjunath et al Table 2 : Percent wetted soil volume irrigation in influencing the plant growth characters in mango Plant height Canopy volume Girth Primary Secondary Treatment (m) (m3) (cm) branches/plant branches/plant 2019 2020 2019 2020 2019 2020 2019 2020 2019 2020 30% soil wetted 3.20 3.34 35.64 31.98 64.38 70.00 3.00 3.00 3.38 3.38 volume irrigation 50% soil wetted 3.70 3.98 53.06 45.94 64.13 67.75 4.00 4.00 2.95 2.94 volume irrigation 70% soil wetted 3.52 3.70 46.10 37.74 67.75 70.25 2.50 2.50 3.30 3.30 volume irrigation 80% soil wetted 3.44 3.58 38.24 34.22 63.50 67.25 2.75 2.75 3.05 3.04 volume irrigation S.Em± 0.19 0.19 4.10 4.50 3.38 3.90 0.42 0.42 0.27 0.28 C.D (P=0.05) NS NS 12.79 NS NS NS NS NS NS NS Treatment Fruit retention in plant (%) Fruit no. /plant 2017 2018 2019 Mean 2017 2018 2019 2020 Mean 30% soil wetted 38.9 39.7 47.9 42.2 120.6 205.7 136.6 65.9 132.2 volume irrigation 50% soil wetted 41.2 36.1 38.8 38.7 124.2 198.7 155.3 131.5 152.4 volume irrigation 70% soil wetted 44.2 38.2 59.1 47.2 154.5 214.0 230.4 168.8 191.9 volume irrigation 80% soil wetted 45.3 37.8 64.7 49.3 66.6 177.0 169.8 146.7 140.0 volume irrigation S.Em± 2.8 3.7 2.8 1.6 18.4 22.7 18.5 25.5 11.91 C.D (P=0.05) NS NS 8.7 4.73 56.1 NS 56.2 NS 36.23 Table 3 : Mean fruit retention and fruit number per plant in mango as influenced by different wetted soil volume irrigation during different years J. Hortl. Sci. Vol. 17(2) : 325-332, 2022 329 of the flowering season. The flower abscission rate in the partially irrigated trees was higher than in the fully irrigated trees while the abscission of fruitlets was lesser in the partially irrigated treatment. The mean fruit yield per plant increased significantly with increase in soil volume wetting irrigation upto 70% decreasing there after indicating the graded response to moisture levels in mango. Significantly higher mean fruit yield of 34.80 kg/plant (9.68 t/ha) was recorded with 70% soil volume wetting irrigation. This suggests that it is worth giving irrigation to meet 70% level of evaporation demand in areas where water is not scarce. The results also suggests that with further increase in the level of soil volume wetting irrigation (upto 80%), there was a decline in the mango yield (7.40 t/ha) indicating that beyond 70% of soil volume wetting, it is a luxury consumption for the plant. The increase in mean fruit yield with 70% soil volume wetting irrigation over the control (80% soil volume wetting) was 26.1 per cent indicating the deleterious effects of excess irrigation in mango that too with loss of precious irrigation water (24.5%). Earlier studies in mango also revealed that meeting 70% evaporative demand is the best for higher fruit yield and quality (Srinivas et al., 2016). Fruit weight and TSS The number of fruits rather than the fruit size influences the total yield. Higher fruit yield and favorable fruit size distribution are counteracting and the exact control of both parameters by means of irrigation seems to be difficult. While there is a negative correlation between the number of fruits onthe tree and the average fruit size, the influence of irrigation on fruit size remains important. Significantly increased response for irrigation in mango fruit size was observed upto 70% soil volume irrigation (226 g) and decreased there after suggesting that beyond this level, the rate of increase in fruit size is only marginal. Lesser fruit weight at lower levels of irrigation may be attributed to the water stress for the full growth of the fruit. Noitsakis et al., (2016) also inferred that higher level of water stress was observed when 50% irrigation water of fully watered pomegra nate pla nts was applied resulting in a significant decrease in mean fruit weight and diameter. The TSS in mango fruit was although not significantly influenced by different irrigation wetted volumes of soil, relatively higher T.S.S. of 18.840 B was observed at 50% as compared with 80% (17.760 B) although found at par with rest of the treatments. Further, this may be attributed to the ability of mango to survive short periods of water deficits as a result of drought tolerance that reduces vegetative growth allowing better penetration of light into the canopy. Water use efficiency A perusal of the amount of water used / ha during different years for each of the treatment showed that ther e was a consider able differ ence acr oss the treatments. The amount of water used / ha under 80% of soil wetted volume was substantially higher (78.7 m3/ha) as compared to 30%, the latter depicting a saving of 67.5 % water. Similarly, 50% wetted soil volume irrigation showed a saving of 46.2% water Standardisation of soil volume wetting for drip irrigation in mango Table 4 : Mean fruit yield as influenced by percent soil volume wetting irrigation in mango Fruit yield (kg /plant) Fruit yield (t/ha) Treatment 2017 2018 2019 2020 Pooled 2017 2018 2019 2020 Pooled Mean Mean 30% soil wetted 22.1 36.6 21.8 12.0 21.4 6.13 10.17 6.05 3.32 5.93 volume irrigation 50% soil wetted 22.4 34.9 28.3 25.0 25.8 6.21 9.68 7.87 6.95 7.15 volume irrigation 70% soil wetted 33.7 40.2 45.9 30.4 34.8 9.35 11.18 12.76 8.43 9.68 volume irrigation 80% soil wetted 12.2 31.7 31.2 31.5 26.6 3.38 8.79 8.66 8.73 7.40 volume irrigation S.Em± 3.5 3.8 4.0 4.7 2.0 0.97 1.06 1.11 1.30 0.56 C.D (P=0.05) 10.7 NS 12.2 14.2 6.1 2.96 NS 3.38 3.96 1.70 J. Hortl. Sci. Vol. 17(2) : 325-332, 2022 330 compared to nor mal (80% soil wetted volume) irrigation indicating that by following the 50% soil volume wetting, nearly double the area of the crop can be irrigated. Significant variations were observed in the mean water use efficiency across the treatments. Higher water use efficiency was observed for 30% soil volume wetting irrigation (274.1 kg/m3) differing significantly with other levels suggesting that more yield could be obtained per unit amount of water used with the treatment. Further, as the per cent soil volume wetting irrigation increased, the water use efficiency decreased Manjunath et al Table 5 : Mean fruit weight and total soluble solids as influenced by percent soil volume wetting irrigation in mango Mean fruit weight (g) T.S.S. (0B) Treatment 2017 2018 2019 2020 Mean 2017 2018 2019 2020 Mean 30% soil wetted 181.4 177.9 156.7 172.9 172.2 19.64 16.66 18.16 18.3 18.2 volume irrigation 50% soil wetted 183.5 178 181.7 185.4 182.2 19.40 17.96 20.52 17.44 18.84 volume irrigation 70% soil wetted 218 192.8 197.4 183.5 197.9 17.98 17.76 19.32 19.28 18.58 volume irrigation 80% soil wetted 214.3 179.7 183.2 230.2 201.8 18.54 15.84 17.48 19.12 17.76 volume irrigation S.Em± 13.71 6.873 4.69 18.01 7.5 0.79 0.48 0.72 0.52 0.28 C.D (P=0.05) NS NS 14.27 NS 22.7 NS 1.49 NS NS NS Table 6 : Water use efficiency in mango (over four years) as influenced by different levels of per cent soil volume wetting irrigation Mean Savings water in Water used (m3/ha) used water WUE (kg/m3) Treatment (litres/ (%) plant) 2017 2018 2019 2020 Mean 2017 2018 2019 2020 Mean 30% soil 25.28 33.18 34.73 8.9 25.52 91.8 67.5 242.3 306.6 174.3 373.3 274.1 wetted volume irrigation 50% soil 41.90 54.84 57.67 14.9 42.33 152.3 46.2 148.2 176.5 136.4 467.3 232.1 wetted volume irrigation 70% soil 58.98 76.71 81.03 20.8 59.38 213.6 24.5 158.6 145.7 157.5 405.8 216.9 wetted volume irrigation 80% soil 77.03 95.15 112.75 29.7 78.70 283.1 - 43.9 92.4 76.8 293.7 126.7 wetted volume irrigation S.Em± - - - - - - 23.3 16.8 20.1 93.3 27.0 C.D (P=0.05) - - - - - - 70.8 51.0 61.1 - 82.2 J. Hortl. Sci. Vol. 17(2) : 325-332, 2022 331 drastically. This may be attributed to the fact that evaporation is minimised by restriction in wetted soil area and such reduction is influenced by the number of days after the beginning of partial irrigation, a tmospher ic eva por a tive dema nd a nd pla nt phonological stage (Vellame et al., 2015). It was noted that there was non-significant differences in the WUE between 50% (232.1 kg/m3) and 70% soil volume wetting irrigation (216.9 kg/m3) indicating that in areas of water scarcity it is worth irrigating only upto 50% of soil volume wetting so that we can also save another 21.7% water. Spreer et al. (2009) also inferr ed tha t wa ter use efficiency was always significantly higher in the deficit irrigation treatments as compared to the control. Further, Wei et al. (2017) also concluded that when the soil moisture content was controlled at about 65±70% of the field water moisture capacity, water demand in the growth and development of mango could be ensured and maximum production efficiency of irrigation and the best quality of fruit could be achieved. irrigation only upto 50% soil volume wetting in mango for economising the water (232.1 kg/m3). ACKNOWLEDGEMENT The authors are thankful to the Indian Council of Agricultural Research for the financial assistance in the conduct of the field experiment through the net work project on Consortia Research Platform on Water coordinated by ICAR-Indian Institute of Water Management, Bhubaneshwar. The work is C ont r ib u t i on N o. 2 3 / 2 0 2 1 of I C AR - I I H R , Hessaraghatta, Bengaluru REFERENCES Anonymous, 2019. Area and production of horticulture crops for 2018-19 (3rd Advance Estimates), Na tiona l Hor ticultur a l Boa r d, India . www.nhb.gov.in. Diamantopoulos, E. and Elmaloglou, S. 2012. 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