Einfluß von Bodenreaktion,Redoxbedingungen und organischer Substanz auf die Phosphatgehalte der Bodenlösung |
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| Authors: | G Welp U Herms G Brümmer |
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| Abstract: | Influence of soil reaction, redox conditions and organic matter on the phosphate content of soil solutions Samples of seven agriculturally used soils of different composition (Ap-horizons), one marine underwater soil and two garbage composts were adjusted in suspensions (soil-water-ratio 1:3) to different pH values between 3 and 8 by additions of NaOH or HCl. By a different degree of aeration the redox potential was kept constant at selected values between +600 and ?300 mV. After an incubation period of 22–24 days under controlled Eh-pH-conditions the content of total phosphate and orthophosphate was measured in the equilibrium solutions. At oxidizing conditions all soil and compost samples show the lowest phosphate content in solution at pH S6. At higher and lower pH values the phosphate content increases. The results indicate that the phosphate concentration is determined by adsorption/desorption processes – mainly connected with iron oxides – and not by dissolution or precipitation of definite phosphorus compounds. Under reducing conditions the phosphate content increases in the equilibrium solutions of all samples. Especially in samples with high content of sulphides a considerable increase of the phosphate concentration could be measured at Eh values below +300 mV at pH 5, +200 mV at pH 6, and ± 0 mV at pH7 and 8. Below these values phosphate containing iron(II1) oxides were reduced and – with further decreasing redox potentials – transformed to iron sulphides. In samples without sulphide formation the phosphate mobilization is much lower. With increasing amount of soluble organic matter the phosphate content of the solutions also increases because of phosphate desorption by organic anions or complexation of aluminium and iron from phosphate adsorbing compounds. But also the content of soluble organically bound phosphate increases and may amount to 70 % of the total phosphate content in solution. |
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