JOURNAL OF THE SCIENTIFIC AGRICULTURAL SOCIETY OF FINLAND 16 Maataloustieteellinen Aikakauskirja Voi. S3: 16-26, 1981 Effect of decreasing acidity on the extractability of inorganic soil phosphorus HELINÄ HARTIKAINEN Department of Agricultural Chemistry, University of Helsinki, 00710 Helsinki 71 Abstract. The extractability of P by the water and anion exchange resin methods and reactions of soil inor- ganic P were investigated with seven acid mineral soil samples incubated with KOH solutions of various concen- trations. The results were compared with the analytical data obtained from three soil samples incubated in a pro- longed liming experiment. The resin extraction method proved more effective than the water extraction method. The amounts of P desorbed by both methods seemed to increase exponentially as the pH in the soil suspensions rose. The factors involved were discussed. On the basis of fractionation analyses P reacting to changes in the pH and participating in desorption proc- esses was supposed to originate from secondary NH 4F and NaOH soluble reserves. In general, as the acidity de- creased NH 4F-P increased at the expense of NaOH-P. In heavily limed gyttja soil also H 2 S0 4 -P increased. This was possibly induced by the precipitation of mobilized P as a Ca compound. The significance of pH in the extractability of soil P seemed somewhat to lessen as the amount of secondary P increased. The results were in accordance with the conception that liming improves the availability of inorganic P to plants and reduces the need for P fertilization. However, increasing of the soil pH involves the risk that P is more easily desorbed to the recipient water by the eroded soil material carried into the watercourse. Therefore, intensive liming is not recommendable close to the shoreline. Further, it should be taken into account that liming of lakes may also result in eutrophication as desorption of sedimentary inorganic P is enhanced. Introduction The effective retention of phosphorus by acid Finnish soils decreases the avail- ability of phosphorus to plants and results in a continuous accumulation of fertilizer phosphorus in the surface soils. The improvement of the utilization of phosphorus is of importance not only in plant production but also in protection of waters. Al- though the leaching of phosphorus normally is insignificant, considerable amounts of phosphorus bound by eroded soil material may be carried into waters. The possible eutrophication risk due to this phosphorus is dependent on the properties of soil and the conditions in the recipient water (HARTIKAINEN 1979). However, the eroded soil material being carried into watercourses always causes potential loading. This means that environmental conditions in the water may alter, resulting in mobiliza- tion of certain phosphorus reserves from the sedimented soil material to the over- lying water. https://www.c-info.fi/en/info/?token=9cKDcXFD4a6ZW0FX.4_lgcX7zfWGYuz5ffKRn7Q.n59cwjbG2F1GdyRIX8Z73chyBGcxadgJ2zJIo3eduKcKB1w7DfNpchKTqEkZk9yKemZQ-Gf3lERR3HPsMdNcnuz602XgieKCf9UhF3pU47uldZ2vuFPZ79Hgt6p_UkxhmTQRYqwMzAO91iUG8wPx5L8iafddTOsmIN4ota-2VSNzPRIq3Q 17 The purpose of this study was to investigate the effect of acidity on the extracta- bility of soil phosphorus. The results were considered to elucidate not only the influ- ence of liming on the availability of phosphorus in cultivated soils but also the de- pendence between pH and the liability of eroded soil material and lake sediments to desorb phosphorus. Materials and methods The experiment was carried out with seven acid mineral soil samples represent- ing the plough layer: two silt (samples 1 and 2), two silty clay (3 and 4) and three heavy clay soils (5—7). In addition, comparative studies were made with sand (8), heavy clay (9) and gyttja clay soil samples (10), one of each, incubated in an earlier experiment at 20 °C for more than half a year with and without liming. The sand and heavy clay soil were limed with 0.375 % of CaCOj and the gyttja clay soil with 0.375—3.00 % of CaCOj. Some characteristics of the soil samples are listed in Table 1. Soil pH was measured by a Beckman pH-metcr in a 0.01 M CaCl 2 suspension in the ratio of 1 to 2.5. The content of organic carbon in the samples was deter- mined by a modified WALKLEY and BLACK wet combustion method (GRAHAM 1948). When calculating the analytical results, it was assumed that 80 % of C in the soils was covered. The amorphous aluminium and iron were extracted by 0.3 M acid ammonium oxalate (TAMM 1922) in the ratio of soil to solution of 1 to 20 (w/v). After the organic matter in the extract was destructed by ignition, A 1 was de- termined by a modified Aluminon method (McLEAN 1965) and Fe by the sulfosalicylic acid procedure (KOUTLER-ANDERSSON 1953). Exchangeable aluminium was extracted by four portions of 1M KCI in the ratio of soil to solution of 1 to 5 (w/v). The particle size composition of the mineral material in the soils was determined by a pipette method (ELONEN 1971). The effect of increasing pH on the desorption ofphosphorus was studied by wa- ter extraction and by anion exchange resin extraction. Reactions of soil phosphorus were investigated by a modified CHANG and JACKSON (1957) fractionation Table 1. Characteristics of experimental soils. P (ppm) extracted sequentially by Exch. Oxalate extr. AI AI FSoil Clay Org. C % A 1 Fe No % pH of D.M, NH„CI NH 4 F NaOH H 2SO„ ppm ppm ppm 1 15 4.63.4 2.3 159 2 15 4.74.1 1.3 153 3 34 4.57.7 2.9 73 4 44 4.33.2 2.1 134 5 61 4.86.5 1.0 89 6 65 4.66.6 1.3 147 7 84 4.86.7 1.2 66 8 14 5.92.9 10.3 250 9 78 5.61.0 0.8 19 10 59 3.53.1 3.3 33 145 91 134 6175 5050 138 110 122 5720 5528 108 155 116 3290 4663 530 188 181 6750 16250 427 230 67 6550 14583 384 155 183 6900 12368 171 71 72 5285 7500 250 260 n.d. n.d. n.d. 78 292 n.d. n.d. n.d. 388 145 n.d. n.d. n.d. n.d. = not determined 18 procedure. The various extracts were analysed for phosphorus by a molybdenum blue method modified by KAILA (1955). Addition of CaC0 3 to the soil does not immediately decrease the soil acidity, only after incubation. Therefore, it is possible that microbiological processes and or- ganic matter affect the extractability of soil nutrients. In order to avoid this, the pH was quickly raised by KOH solutions in this experiment. Further, the slow dissolu- tion of CaCO ; was assumed possibly to cause some experimental errors, because free CaC0 3 is found to disturb the fractionation analysis of phosphorus (WILLIAMS et al. 1971). On the other hand, the rate-limiting step in exchange reactions often is the ion diffusion to or from the colloid surface. Because the ion movement was not speeded up in this experiment by shaking the samples, it is not certain that the soil acidity was completely attacked in the KOH treatments. Therefore it is more accurate to deal with pH in the soil suspension instead of the soil pH. One gram of soil weighed into a centrifuge tube was moistened with one ml of KOH solution (0.01—0.4 M) and incubated over night. The control sample was in- cubated with one ml of distilled water. In water extraction the incubated samples were shaken for one hour with 50 ml of distilled water and centrifuged. The super- natant solutions were filtered through a 0.2 u m membrane filter. Resin extraction was performed by the method described by AURA (1978 a), using one gram of soil, 100 ml of distilled water and 2 g of anion-exchange resin (Dowex 21-K, 16—20 mesh, Cl-form). The extraction time was one hour. The extracts were analysed for phosphorus by the ascorbic acid method (ANON. 1969). In order to investigate changes in the pH caused by the base treatment, 10 g of soil was moistened with 10 ml of KOH solution (0.01—0.4 M) or with 10 ml of distilled water. To the samples incubated over night 25 ml of 0.01 M CaCl2 solu- tion was added and the pH in the suspension was measured after they were allowed to stand for four hours. The tests were carried out with four replicates, excluding the pH measurements performed with two replicates. Results The treatment of the various soils with KOH solutions affected the pH in the soil suspensions to various degrees (see Figures 1 and 2). After incubation with 0.4 M KOH solution the pH rose in some suspensions much over 9, while in one sam- ple it was only 7.7. Because Finnish soils do not involve so hight pH values, discus- sions were concentrated mainly on the results from samples with pH below 7. Figures 1 and 2 show that the amounts of phosphorus desorbed by water ex- traction as well as by resin extraction increased with decreasing acidity. In the above figures the points over pH 7 are joined together by dotted lines. On the other hand, the results obtained in the fractionation analyses (not presented) demontrated that the treatments with KOH solutions of various concentrations caused no statistically significant changes in the total quantities of fractionated inorganic phosphorus in a given soil. But with increasing pH the base soluble phosphorus reserves tended to decrease at the same time as the fluoride extractable fraction became greater. The 19 Fig. I. Soil phosphorus desorbed by water extraction at different pH levels. 20 acid soluble reserves seemed unattacked. The NH 4CI-P markedly rose only at very high pH values. In some cases, in water or resin extraction, the desorption of phosphorus from a given soil at different pH levels seemed to be connected with the amount of the fluo- ride or/and base soluble phosphorus: desorption was intensified with increasing Fig. 2. Soil phosphorus desorbed by anion exchange resin extraction at different pH levels 21 NH4F-P fraction and decreasing NaOH-P fraction. However, when calculating partial correlation coefficients between the phosphorus desorbed and these fractions by eliminating the influence of the pH, a statistically significant positive dependence between the water soluble and NH 4F extractable phosphorus was found only in samples 5 and 7. The corresponding dependence in resin extraction was found in samples 3 and 5. Thus it can be concluded that the pH alone is of greater impor- tance on the extractability of soil phosphorus than the distribution of secondary phosphorus in various chemical fractions. This supposition is supported by the fact that neither the decrease in NaOH-P nor the increase in NH4 F-P, respectively, quantitatively corresponded to the intensifield desorption observed in water and resin extraction. Desorption followed the exponential equation y = ab*, where y is desorbed P mg/kg soil, x is pH, a and b constants. However, the equations presented in Figures 1 and 2 give evidence that the factors a and b are dependent on each other. When b increases, it seems as if a would decrease logarithmically. When comparing the curves in Figures 1 and 2, it can be seen that at low pH values desorption from various soils differed relatively more in resin extraction than in water extraction. Further, the equations calculated show that the water soluble phosphorus is more strongly dependent on pH than the resin extractable phospho- rus. This means that, although at low pH level the extraction of phosphorus by the water treatment is not as effective as that by the resin treatment, at higher pH values the differences between these methods tend to become equalized. For the sake of comparison, the extractability of phosphorus was investigated also with three soil samples incubated in a prolonged experiment with and without CaC0 5 . The results are presented in Table 2. In the sand soil sample (8) liming doubled the already exceptionally high NH 4CI soluble fraction. The NaOH-P was markedly decreased, but the other fractions seemed to remain unchanged. In the heavy clay (9) and gyttja clay soil samples (10) the NH4F-P significantly rose at the expense of the NaOH-P as the pH increased. Further, it was interesting to observe that in the gyttja clay soil also the H 2S04-P distinedy tended to increase with intensified liming. The NH 4CI-P likewise was somewhat augmented when the pH rose to 6.9. Table 2. pH, water and resin extractable P (ppm) in three soils incubated with and without liming. Lime Soil 8 Soil 9 Soil 10 applied P cxtr. by P extr. by P extr. by % pH water resin pH water resin pH water resin