ORTHOPHOSPHATE AND ORGANIC PHOSPHATE IN THE SOIL SOLUTION OF 4 SANDY SOILS IN RELATION TO PH-EVIDENCE FOR HUMIC-FE-(AL-) PHOSPHATE COMPLEXES

Soil from the Ap-horizon of four acid sandy soils differing mainly in Corg content was adjusted to pH values between 3 and 7.5 with NaOH and HCl, respectively and incubated for two weeks. Afterwards, displaced soil solution was obtained and analyzed. The concentrations of Fe, Al, and P showed a broad minimum in the pH range from 4 to 6. The concentration of these elements strongly increased with the increase of pH to 7.5. Acidification below pH values of 4 led to a slight increase. Separation of dissolved organic carbon by ultrafiltration before the photometric orthophosphate determination decreased measured concentrations in comparison to direct determination in two of the four soils. This decrease was more pronounced for soil solutions with higher concentrations of organic carbon. The effect of acid hydrolysis of organic phosphorus during orthophosphate determination can be explained by existence of humic- Fe- (Al-) phosphate complexes in the soil solution. These complexes can account for more than 50% of the total organic P in solution.