Five loamy and four sandy soils were incubated with either CaHPO4 or two sewage sludges with the P:Fe ratios 1:1.5 (sludge Sh) and 1:5.4 (sludge Gö) for 1 and 6 months. P sorption and inorganicP(Pi) concentration of the soil solution were measured. In a pot experiment P uptake of maize was examined. The application of sludge Sh did not modify P sorption compared to soils without sludge. However, after application of sludge Gö the P sorption increased to 116 % in loamy and 112 % in sandy soils. After application of sludge Sh the mean Pi concentration increased in loamy soils to 124 % and in sandy soils to 113 %. On the other hand the Pi concentration decreased after applying sludge Gö to 82 % (loamy soils) and 62 % (sandy soils) as compared to the controls of the respective soils. One month after the application of increasing amounts of sludge Gö (5, 10, 15 t DM ha−1) both the concentration of oxalate-soluble Fe in the soil and the P sorption were increased. The relationships between these two parameters were highly significant (r2=0.87−0.97). Plant uptake of P was lower after application of sludge Gö than after application of sludge Sh and much less than P uptake from CaHPO4. Sewage sludges with a P : Fe ratio of 1 : 5 should not be recommended for agricultural use, as the P availability is significantly reduced. Iron salts should not be used for conditioning of sludges. |
