V01.6(1997): 313-322. Effect of fertilization on soil phosphorus in a long-term field experiment in southern Finland Antti Jaakkola, Helinä Hartikainen and Riitta Lemola Department ofApplied Chemistry and Microbiology. PO Box 27, FIN-00014 University ofHelsinki, Finland, e-mail: antti.jaakkola @helsinki.fi A field experiment was established in 1978 on a loam soil (pH in CaCl2 7.1) to monitor gradual changes in the soil P status as response to different P fertilization regimes. For 18 years, cereals or grass were cultivated without P fertilization (P 0 ) or with annual P application of 35 kg ha* 1 (P ( ) or 70- 79 kg Pha 1 and 71-83 kg Kha ' (P 2 K). The effects of the treatments on the crop yield varied yearly. The Chang and Jackson fractionation analysis revealed that fertilizer P not taken up by the plant crops was mostly in the NH 4F extract and to a lesser extent in the NaOH extract. The NH 4F-extract- able Pproved also to be the main P source for plants. However, the changes in the reserves of inorgan- ic and organic P did not agree very well with the calculated P balance in soil (applied P minus plant P uptake). This disproportion was partly explained by the soil movement from plots to the neighbour- ing ones during the experiment. Phosphorus extractable in acid ammonium acetate or water decreased gradually when no P was applied and increased with increasing P accumulation. The changes in the inorganic P reserves due to different P fertilization history were reflected a little more sensitively in the water extraction test than in the acid acetate test. Key words: acid ammonium acetate extraction, inorganic P fractions, P accumulation, P balance, P depletion, water extraction ntroduction Effective sorption of phosphorus by hydrated oxides of iron and aluminium is typical ofFinn- ish mineral soils (Kaila 1963, Hartikainen 1979). This efficient retention restricts losses with seep- age water, but also decreases the availability of phosphorus to plants. This is why more phos- phorus has been applied to soils over the years than has been removed with the plant crops. This has resulted in accumulation of phosphorus in the cultivated soils, the excess has been calcu- lated to average 800 kg ha’ 1 since 1920(I. Saare- la, personal communication). The most marked accumulation has taken place during the last three decades. The enrichment of phosphorus in the surface soils may increase the load of phosphorus dis- solved in the surface runoff water and bound in © Agricultural and Food Science in Finland Manuscript received March 1997 313 AGRICULTURAL AND FOOD SCIENCE IN FINLAND https://www.c-info.fi/en/info/?token=pHW41OMii-Lr2Ijc.fIoTZgM5gcEK32S49IpvfA.FVx3zLps-gAT2OcCt-1CKSjYJsn0DINDjo7tsVPnKBdQZO0Umfp4SvOaJxG9gLdoV7n2zB7zp4jBCvLbt3DjHPf3mSLD0I5PMONNb-FW2Oz68SysBCgMsZTYUEz7-G_WEOjA5Owcgp0FSsoPuPzjP-Ti3gOrAT4CAG0AFB7nNlbPfLvGorbqQPcPJB-FKdidqNZ1c7WgyOl3ScoFNfjREZQBmxmsHAbmD_NeVlzFRznX8LEBHcsCZyxQMOMuKCBgFfYivhaHX8rynVkhmg_Hk8H317ZzMlAw5mCn68PEwQySOc5YysBas7DOhL-Z i Jaakkola, A. et al. Effect offertilization on soil phosphorus eroded soil material and, thus, contribute to eu- trofication (Yli-Halla et al. 1995). On the other hand, accumulation gradually leads to saturation of the sorption sites and decreases their reten- tion strength (Barrow 1974, Hartikainen 1991), which improves the efficiency of fertilizer phos- phorus added to crops. Long-term residual con- tributions ofphosphorus fertilizer to crop yields and phosphorus uptake has been shown in nu- merous studies (e.g. Campbell 1965, Halvors- son and Black 1985). In terms of crop produc- tion, accumulation is therefore beneficial until an economical optimum is reached. In this study changes of various soil phos- phorus fractions as a function of different ferti- lization history and withdrawal by plant crops were investigated in a long-term field experi- ment. The sensitivity of two phosphorus tests to describe these changes was compared to evalu- ate their use as a basis for fertilization recom- mendations. Material and methods The field experiment was established on a farm in southern Finland (60° 47’ N, 24° 55’ E, I 10 m asl) in 1978 and it is still continuing. The field slopes to the south east c. 6%. The particle size distribution determined according to Elonen (1971) was; fraction >2OO pm 8%, 60-200 pm 23%, 20-60 pm 22%, 2-20 pm 35% and <2 pm 12% (a loam soil). The concentration of organic carbon determined by a modified wet digestion method (Graham 1948)was 2.8%, and pH in 0.01 M CaCl, was 7.1. The dry bulk density of the soil samples ground to pass a 2-mm sieve was 1.14 kg I '. Amorphous Fe and A 1 oxides extract- ed by an acid ammonium oxalate solution(Tamm 1922, modified by Niskanen 1989) were meas- ured by atomic absorption spectrophotometry (AAS). Their concentrations were; 16 mmol kg' 1 Fe and 76 mmol kg IAI . The soil had been OX ° ox cultivated conventionally for decades before the experiment started. The design of the experiment consists of two factors with two levels each: without and with PK fertilizer as well as without and with an ex- tra superphosphate application. The treatments PO, P,, P,K and P,K are replicated 6,9, 4 and 6 times, respectively. They were arranged in plots of 20 by 15 m (Po and P ( K) or sbyls m (Pj and PjK). Nitrogen has been applied at 100 kg ha’ 1 yearly. The fertilizers were placed at depths of 6-8 cm as calcium ammonium nitrate (NH 4:N03 = 1:1) or NPK compound fertilizer (NH 4:NO, = 2:1). The treatments were as follows: Treatment Nkg ha’ 1 Pkg ha’ 1 Kkg ha’ 1 P 0 100 0 0 P, 100 35 0 P,K 100 44 (35*) 83 (71*) P,K 100 79 (70*) 83 (71*) * in 1992-1995 Treatments P O, P, and P 2K are discussed in this paper. Four replicates situated in separate blocks were chosen for each treatment. The ce- real crops were harvested with a combine har- vester. The straw was ploughed into the soil in 1978 and 1979. Thereafter it was removed year- ly with a pick-up baler. Both grain and straw yields of cereals were weighed from a net plot of 4by 11 m except for the years 1978 and 1979 when the straw was not weighed. Grass yields (1988, 1992) were harvested once in each sea- son and weighed from a net plot of 1.5 by 10m. Plant samples representing the net plots were taken for the determination of dry matter yield and analyzed for phosphorus from ashed sam- ples with the ammonium vanadate-molybdate method. Soil samples representing the plough layer (23 cm deep) of the net plots were taken yearly after the harvest and air dried. The samples from the years 1978, 1984, 1990 and 1995 were stud- ied more closely for this paper. The inorganic phosphorus reserves bound by various soil com- ponents were investigated by the fractionation method of Chang and Jackson (as modified by Hartikainen 1979). Organic phosphorus was de- 314 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Vol. 6 (1997): 313-322. termined as a difference between H,SO, extract--2 4 able P (O.IM, extraction ratio of 1:100) in the ignited (1 h at 550°C) and untreated sample. The plant available reserves were estimated by an acid ammonium acetate test used routinely in Finland (Vuorinen and Mäkitie 1955) and by the van derPaauw and Sissingh water extraction test modified by Hartikainen (1979). Results Yields and P balance The yearly variation of grain yields was remark- able (Fig. 1). Without P fertilization the yields were lower, but therelative decrease varied dur- ing the experiment. As compared to the P 0 treat- ment, the increase in the average grain yield in the F, treatment remained statistically insignifi- cant (Table 1). The P 2K treatment on the contra- ry, increased the yield significantly. This posi- tive response was at least partly due to K fertili- zation, because the treatment P t K did not differ from P 2K. The straw and grass yields also varied con- siderably annually (Fig. 2). Significant differences due to the treatments were found in five years but the differencesappeared to be similar in most years. However, the average straw or grass yield was not increased statistically significantly by P and K applications (Table 1). The concentration of phosphorus in grain varied annually between 2.8 and 5.3 g kg' 1 (Fig. 3). However, all treatments had the same average concentration of 4.0 g kg '. The phos- phorus concentration in straw variedbetween 0.5 and 4.0 g kg' 1 (Fig. 4), being 2.2 g kg' 1 on aver- age. It was not affected by the fertilization. When calculating the P uptake (Fig. 5) the unweighed straw yields in 1978 and 1979 were expected to be equal to the grain yields. During the eighteen years of experiment the average yearly uptake of phosphorus was 12.0, 14.2 and 16.9 kg ha' 1 in the treatments P O, P, and P 2K, re- spectively. The variation in the treatments and individual years was between 3.1 and 23.6 kg ha 1 . The P balance in soil was calculated as the difference between the phosphorus application and the removal with the yields. Cumulative P balance from the very beginning of the experi- ment is given in Fig. 6. In treatment P 0 it was negative while in the other treatments it was posi- tive increasing rather linearly in the course of the experiment. During eighteen years the re- moval ofP from the plot cultivated without phos- phorus fertilization amounted to 206 kg ha' 1 while the P ( and P 2K treatments resulted in a phosphorus accumulation of384 and 1037kg ha 1, respectively. Soil P fractions The soil pH was not significantly altered during the experiment (Table 2). To study the response of soil P to differentfertilization regimes in more detail, the changes in the various inorganic P fractions were investigated. The P fertilization Table I.Average yield 1978-1995 (kg ha 1 DM) in the various fertilization treatments. 1 Treatment P, (PjK) P 2K Grain yield 2198 a 2563ab 2839b 3057 b Straw yield* 2028" 2466" 2836“ 3115* Means in each row followed by a common letter do not differ significantly (Tukey, P = 0.05). * Grass yield 1988 and 1992. 315 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Jaakkola, A. et al. Effect of fertilization on soilphosphorus Fig. 1. Grain yield. Letter after year denotes the followingcrops: B=barley, W=spring wheat, o=oats, G= rye grass. *signif- icant difference between treatments (P=0.05). Fig. 2. Straw or grass yield. Letter after year denotes the followingcrops: B=barley, W=spring wheat, o=oats, G= rye grass, *significant difference between treatments (P=0.05). 316 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Vol. 6 (1997): 313-322. Fig. 3. P concentration in grain (DM). Letter after year denotes the following crops: B=barley, W=spring wheat, o=oats, G= rye grass. *significant difference between treatments (P=0.05). Fig. 4. P concentration in straw or grass. Letter after year denotes the following crops: B=barley, W=spring wheat, o=oats, G= rye grass. *significant difference between treatments (P=0.05). 317 AGRICULTURAL AND FOOD SCIENCE IN FINLAND 2 Jaakkola, A. et al. Effect of fertilization on soilphosphorus Fig. 5. P uptake by the crop. Letter after year denotes the following crops: B=barley, W=spring wheat, o=oats, G= rye grass. *significant difference between treatments (P=0.05). Fig. 6. Cumulative P balance. Letter after year denotes the following crops: B=barley, W=spring wheat, o=oats, G= rye grass. *significant difference between treatments (P=0.05). 318 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Vol. 6 (1997): 313-322. Table 2. pH and inorganic P extracted sequentially by the fractionation procedure as well as organic P. Year Treatment pH NH4CI NH 4F NaOH H2 S04 Organic P I mg kg ' mg kg ' mg kg' mg kg' mg kg' mg kg 1 1978 P 0 7.1“ 9“ 246“ 155“ 195“ 244“ 849“ P, 7.2“ 10“ 245“ 161* 198“ 231“ 845“ P,K 7.2“ 12“ 265“ 161“ 191“ 284“ 9l3b 1984 P 0 7.0“ 8" 231“ 148“ 195“ 203“ 785“ P, 7.1“ 8“ 259 “b 164“b 208“ 209“ 848“ P2 K 7.0“ 10“ 301b 176b 189“ 245“ 921“ 1990 P (| 7.1“ 3“ 210“ 145" 201“ 198“ 757“ P, 7.1“ 6“b 260 “b 169“b 212“ 188“ 835“ P2 K 7.1“ 10b 343 b 195b 206“ 227“ 98l b 1995 P„ 7.1“ 3* 203“ 158“ 193“ 193“ 750“ P, 6.9“ 5“b 247“ 182“ 200“ 196“ 830“ P, K 7.0“ 9b 356 b 215 b 198“ 207“ 985 b Means of the same year and the same column followed by a common letter do not differ significantly (Tukey, P =0.05). increased the NH 4CI extractable P significantly in 1990 and 1995, but some tendency could be seen earlier. In the P 0 treatment this fraction de- creased with time. However, a similar tendency was obvious even in P,K. All the samples were dominated by the NH4F-P in which significant differences between the treatments were detect- ed since 1984. As compared to the beginning of the experiment, NH 4F-P showed a slight decrease when no P was given, an increase at the P ferti- lization level 70-79 kg ha 'a 1, and no clear change at 35 kg Pha 1 a l . Similar differences between the treatments were observed in NaOH-P. However, in contrast to NH.F-P, this fraction was not depleted by the continuous cultivation without P fertilization. The H,SO, soluble frac--2 4 tion did not respond to the treatments. The or- ganic phosphorus tended to decrease during the experimental period but there were no signifi- cant differencesbetween fertilization treatments. The sum of the fractions responded to the treat- ments rather logically. However, the difference between P 0 and ?! was not statistically signifi- cant. Phosphorus extractable with acid ammonium acetate (P MC ) decreased with time in the soil not amended with P (Table 3). No changes were found at the lower P fertilization level although the P balance was positive. In the P2 K treatment, P aac showed an increasing trend. The differences between treatments became significant in 1990. At the beginning of the experiment, the con- centration of the water extractable P (Pw ) was about half of the acid ammonium acetate extract- Table 3. Phosphorus extractable with acid ammonium acetate, (mg kg ') and waterPw (mg kg '). Treatment 1978 1984 1990 1995 PP PP PP PPaac w aac w aac w aac w P 0 31“ 14k 25“ 12k 19“ 9k 20* 6k P, 33“ 14k 32“ I6k 31” 17' 34“ h 17' P,K 35“ 14k 37“ 17k 43“ 22' 50 h 26'" Each extract is tested separately. Means in the same column followed by a common letter do not differ significantly (Tukey, P = 0.05). 319 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Jaakkola, A. et al. Effect offertilization on soil phosphorus able P (Table 3). In the plots cultivated without P fertilization it decreased very clearly with time being at the end of the experiment only one third of P .In treatment P, K, on the contrary, P al- aac 2 ’ J w most doubled until 1995. A small increase might have taken place also in treatment P r The dif- ference between treatments became significant from 1990 on but was obvious already in 1984. Discussion No significant trend of increasing yield differ- ences between treatments over time could be observed although the unfertilized plots were exhausted and the fertilized ones were enriched with P. At the higher P fertilization level, the yield increase is, no doubt, attributable to K ap- plied with the fertilizer. The P application af- fected the P concentration in the crops very lit- tle as compared with the variationbetween years due to plant species, harvesting timeand weath- er during the growing season. In general con- centrations were at the range reported by Jaak- kola et al. (1982). The conclusion can be made that the fertilization-induced increase in the P uptake was mainly caused by an increase in crop yield. According to Saarela et al. (1995) a yield increase ofcereals or hay seldom takes place due to P application in a soil with over 30 mg 1'P aac. Not even the 20 mg 11 to which the P concen-~ aac tration dropped in the non-fertilized soil would usually be low enough to facilitate a yield re- sponse. Therefore, the positive yield response to the P application probably obtained in the present study is surprising. One reason for the unexpect- ed behavior might be that in the present study the soil pH was much higher (6.7-7.3) than nor- mally found in Finnish soils. The extraction pow- er ofacid acetate is highly dependent on soil pH (Griffin 1971, Hartikainen 1989a). In fact, the acid-acetate test is shown to overestimateplant- available P reserves in limed soils (Hartikainen 1989a). On the other hand, the experimental soil was initially very rich in Pw as compared to the val- ues reported in some previous studies (Hartikai- nen 1982,Sippola and Saarela 1986). Pw amount- ed only to 30 to 55 per cent of Paac but responded during the experimental period relatively more sensitively to the P fertilization regimes. Also the NH 4CI-P obtained by the Chang and Jackson fractionation was initially rather high. Its range in Finnish soils is usually 1-4 mg/kg, and values above 10 mg/kg can be found only occasionally (see Kaila 1965b, Hartikainen 1979). Actually, this fraction comprises a part of P w and reflects the P saturation degree on the surfaces of Al- and Fe-oxides (Hartikainen 1982). In Finnish soils, the fertilizer P has been shown to accumulate on these compounds (Kai- la 1965a, Hartikainen 1989b, Yli-Halla 1989). The dominance of the NH 4F-P assumed to rep- resent Al-bound P over the NaOH-P supposed to be bound by Fe agreed with the relative high- er abundance of the Al oxides. Accordingly, the bulk of the accumulated fertilizer P was recov- ered in the Al-bound fraction. The high soil pH favouring the P retention by Al oxides at the ex- pense of Fe oxides (Kaila 1965c, Hartikainen 1981, 1983) might contribute this reaction pat- tern. As in previous studies (Kaila 1965a, Har- tikainen 1989b) the H,S04-P representing Ca- bound fraction was found to be inactive in P re- tention. The depletion in the NH4F-P in the plots cul- tivated without P fertilization being higher than that in the NaOH-P can be taken to indicate that the Al-bound P was more labile than Fe-bound P and, thus, more important as a P source for the crop. The H,S04 -P (Ca-P), in turn, showed to be of low availability. This is in agreement with the observation made in an earlier 7-year field experiment with 16 acid soils: this fraction was not affected in 10 soils and reduced in 6 soils (Hartikainen 1989b). According to regression analysis the mean decrease of the sum of fractions in the P 0 treat- ment was 5.8 mg kg ' per year. Assuming a ploughing depth of 23 cm and dry bulk density of 1.14 kg 1 1 the annual decrease of P reserves 320 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Vol. 6 (1997): 313-322. was 15 kg ha It was greater than the mean an- nual P balance of-11 kg ha In the P, and P 2K treatments the mean annual changes of the sum of fractions were -1.0 and 4.9 mg kg ' corre- sponding to -3 and 13 kg ha', respectively. These figures do not agree with the mean annual bal- ances which were 21 and 58 kg ha 1. In long-term field experiments the movement of soil and fertilizers from a plot to a neighbour- ing plot and vice versa is a well-known problem discussed in detail by Sibbesen (1986). The plots with treatments ?! and P 2K were only 5 m wide while the P 0 and P,K treated plots being neigh- bours to P, and P 2K, respectively, were 20 m wide. The length of all the plots was 15 m. A net plot of 4 by 11 m was usually harvested length- wise in the middle of each plot from south west to north east. Calculations according to the model of Sib- besen (1986) revealed that during this 18-year experiment the exchange of soil between plots may have increased the P content in the treat- ment P 0 at a rate of 30 kg ha’ 1 (for transport co- efficient D=o.4 m 2 a 1). So, the corrected annual P balance in the P 0 treatment should have been about -9 kg ha 1 . Considering the large random variation of the analytical results as well as the inaccuracies in determining the ploughing depth and bulk density the agreement with the change in soil Preserves (-15 kg ha’ 1 a 1) is satisfactory. in the P amended net plots the decrease due to soil movement would be about 14 kg ha 'a 1 . The decrease is similar for P, and P 2K because the difference in P application rate between neigh- bouring plots is equal. The corrected annual P balances shouldbe 7 and 44 kg ha’ 1 as compared with changes in soil P reserves of -3 and 13 kg ha’ 1 a’ 1 for P ( and P2K, respectively. So, the agree- ment is not very good. It is obvious that the soil movement has been more vigorous than the val- ue of the transport coefficient (D=o.4 m 2 a 1) adopted from Sibbesen (1986) presupposes. Some P may also have moved to the deeper lay- ers. In addition, on this sloping field 1-2 kg ha’ 1 of phosphorus might have been lost by leaching and surface runoff (Rekolainen 1989). References Barrow, N.J. 1974. Effect of previous additions of phos- phate on phosphateadsorption by soils. Soil Science 118: 82-89. Campbell, R.E. 1965. Phosphorus fertilizer residual ef- fects on irrigated crops in rotation. Soil Science So- ciety of America Proceedings 29: 67-70. Elonen, P. 1971. Particle-size analysis of soil. ActaAgralia Fennica 122: 1-122. Graham, E.R. 1948. Determination of soil organic matter by means of a photoelectric colorimeter. Soil Science 65: 181-183. Griffin, G.F. 1971. Effect of liming on the soil test level of phosphorus as determined by three methods. Soil Science Society ofAmerica Proceedings 35: 540-42. Halvorsson, A.D. & Black, A.L. 1985. Fertilizer phosphorus recovery after seventeen years of dryland cropping. Soil Science Society of America Journal49: 933-937. Hartikainen, H. 1979. Phosphorus and its reactions in terrestrial soils and lake sediments. Journal of the Scientific Agricultural Societyof Finland 51: 537-624. - 1981. Effect of decreasing acidity on the extractabil- ity of inorganic soil phosphorus. Journal of the Sci- entific Agricultural Society of Finland 53: 16-26. - 1982. Water soluble phosphorus in Finnish mineral soils and its dependenceon soil properties. Journal of the Scientific Agricultural Society of Finland 54; 89-98. - 1983. Effect of liming on phosphorus in two soils of different organic matter content. 1. Changes of na- tive and applied phosphorus in incubation experi- ment. Journal of the Scientific Agricultural Society of Finland 55: 345-354. -1989a. Evaluation of water and ammonium acetate tests as indices for available P in limed soil. Journal ofAgricultural Science in Finland 61: 1-6. -1989b. Effect of cumulative fertilizer dressings on the phosphorus status of mineral soils. 1. Changes on inorganic phosphorus fractions. Journal of Agricul- tural Science in Finland 61: 55-59. - 1991. Potential mobility of accumulated phosphorus in soil as estimated by the indices of Q/l plots and by extractant. Soil Science 152: 204-209. Jaakkola, A., Syvälahti, J. & Saari, E. 1982.Contents of mineral elements in Finnish cereal straw. Journal of the Scientific Agricultural Society ofFinland 54: 385- 394. Kaila, A. 1963. Dependence of soil sorption capacity on the aluminium and iron in Finnish soils. Journal of the Scientific Agricultural Society of Finland 35:165- 177. 321 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Jaakkola, A. et al. Effect offertilization on soil phosphorus -1965a. The fate of water-soluble phosphate added to some mineral soils. Journalof the Scientific Agri- cultural Society of Finland 37: 104-115. -1965b. Some phosphorus test values and fractions of inorganic phosphorus in soils. Journal of the Sci- entific Agricultural Society of Finland 37: 175-185. -1965c. Effect of liming on the mobilization of soil phosphorus. Journal of the Scientific Agricultural Society of Finland 37: 243-254. Niskanen, R. 1989. Extractable aluminium, iron and man- ganese in mineral soils. 2. Extractability by oxalate and pyrophosphate. Journal of Agricultural Science in Finlandet: 79-87. Rekolainen, S. 1989. Phosphorus and nitrogen load from forest and agricultural areas in Finland. Aqua Fenni- ca 19: 92-107. Saarela, 1., Järvi, A,, Hakkola, H. & Rinne, K. 1995. Fosfo- rilannoituksen porraskokeet 1977-1994. Vuosittain an- netun fosforimäärän vaikutus maan viljavuuteen ja pel- tokasvien satoon monivuotisessa kenttäkokeessa. Maatalouden tutkimuskeskus, Tiedote 16/95. 94 p. Sibbesen, E. 1986. Soil movement in long term field ex- periments. Plant and Soil 91: 73-85. Sippola, J. & Saarela, I. 1986. Some extraction methods as indicators of need for phosphorus fertilization. Annales Agriculturae Fennica 25: 265-271. Tamm, 0.1922. Eine Methode zur Bestimmung deranor- ganischen Komponente des Gelkomplexes im Boden. Statens Skogsförsöksanstalt, Meddelande 19: 387- 404. Vuorinen, J. & Mäkitie, O. 1955.The method of soil test- ing in use in Finland. AgrogeotogicatPublication 63: 1-44. Yli-Halla, M. 1989. Effect of different rates of P fertiliza- tion on the yield and P status of the soil in two long- term field experiments. Journal of Agricultural Sci- ence in Finlandet: 361-370. -, Hartikainen, H., Ekholm, P., Turtola, E. & Kallio, K. 1995. Assessment of soluble phosphorus load in sur- face runoff by soil analyses. Agriculture, Ecosystem S Environment 56: 53-62. SELOSTUS Maan fosforitilan muutos pitkäaikaisessa kenttäkokeessa hietamaalla Antti Jaakkola, Helinä Hartikainen ja RiittaLemola Helsingin yliopisto Vuonna 1978 Etelä-Suomeen hietamaalle perustetun kenttäkokeen tarkoitus oli seurata erilaisten vuosit- taisten fosforilannoituskäsittelyjen vaikutusta maan fosforitilaan. Koekentällä kasvoi viljakasveja tai rai- heinää 18 vuoden ajan. Koejakson aikana yhtä koe- jäsentä ei lannoitettu lainkaan fosforilla ja muille koe- jäsenille annettiin 35 kg ha 1 tai 70-79 kg ha 1 vuo- tuinen fosforilannoitus. Fosforilla lannoitetuissa koejäsenissä jyväsato oli keskimäärin suurempi kuin ilman fosforilannoitusta viljellyssä koejäsenessä. Alemmalla fosforilannoitustasolla ero ei kuitenkaan ollut tilastollisesti merkitsevä. Ylemmällä tasolla yht’aikaa annettu kaliumlannoitus aiheutti sen, ettei tilastollisesti merkitsevää eroa voitu kiistatta lukea fosforilannoituksen ansioksi. Ilman fosforia viljelty koejäsen köyhtyi fosforin suhteen ja fosforilla lannoi- tettuihin koejäseniin kertyi fosforia. Maan epäorgaanisia fosforireservejä luokittelevan Changin ja Jacksonin fraktiointianalyysin perusteel- la maahan kertynyt fosfori näytti pidättyvän pää- asiassa alumiinioksideihin ja vähäisemmässä määrin rautaoksideihin. Alumiinin sitoma fosfori osoittautui myös kasvien pääasialliseksi fosforilähteeksi ilman fosforilannoitusta viljellyssä koejäsenessä. Lannoitus- käsittelyt eivät vaikuttaneet kalsiumin sitomaan fos- foriin. Maan kyntökerroksen epäorgaanisissa ja or- gaanisissa fosforivaroissa tapahtunut muutos ei täy- sin vastannut fosforitasetta, joka laskettiin maahan lisätyn fosforin ja kasvin fosforinoton erotuksena, vaikka maa-aineksen todennäköinen kulkeutuminen vierekkäisten koeruutujen välillä otettiin laskennal- lisesti huomioon. Kokeen aikana annetun lannoituksen vaikutusta fosforireservien käyttökelpoisuuteen arvioitiin vesi- uuton ja Suomessa viljavuusanalyysissä käytettävän happaman ammoniumasetaattiuuton avulla. Kummal- lakin menetelmällä mitattu kasveille käyttökelpoisen fosforin määrä väheni ilman fosforia viljellyssä koejäsenessä asteittain kokeen edetessä ja kasvoi fos- forilannoituksen saaneissa koejäsenissä maahan ker- tyneen fosforin määrän lisääntyessä. Vesi uutti BO- SS % happamaan ammoniumasetaattiin uuttuvan fos- forin määrästä. Vesiuuttoinen fosfori kuvasi erilaisen lannoituksen aiheuttamaa muutosta maan fosforiva- roissa vähän paremmin kuin happamaan ammonium- asetaattiin uuttuva fosfori. 322 AGRICULTURAL AND FOOD SCIENCE IN FINLAND