Influence of iron content in sewage sludges on parameters of phosphate availability in arable soils

The use of iron salts for the P elimination in sewage plants is widely used . But it is not clear whether the P availability in arable soils is negativ ely influenced by iron compounds or not. The aim of the investigations was, therefore, to study the influence of two sewage sludges with a high and a low Fe content respectively on P sorption and phosphate concentration (P-i) in the soil solution after application of CaHPO4 or sewage sludge to 5 loa my and 4 sandy soils (pot experiments and 1 silty loam (field experiment)). Soils were analyzed 1, 6, and 13 months after P application. Sludge Ga con tained 12 kg P and 65 kg Fe (t DM)(-1) (P : Fe = 1 : 5.4) and sludge Sh 25 kg P and 39 kg Fe (t DM)(-1) (P : Fe = 1 : 1.5). The basic P application wi ts 60 kg P ha(-1) (= 30 mg P (kg soil)(-1) in the pot experiment, as sludge or as CaHPO4). P uptake by maize was determined in a separate pot experime nt with a loamy soil and the same P application rate. The P sorption capacity remained similar in all soils after application of sludge Sh (P : Fe = 1:1.5) compared with soils without sludge, however, aft er application of sludge Ga the P sorption increased by 16% (0-59%). After application of sludge Sh the mean P-i concentration increased in loamy soil s by 34% and in sandy soils by 15% On the other hand the P-i concentration decreased after applying sludge Ga by 13% and 36% as compared to the contro ls of the respective soils. In the field experiment the Pi concentration of plots with a high P level (50 mg lactate soluble P (kg soil)(-1)) was also significantly decreased after application of 10 t sludge Ga (126 kg P ha(- 1)) in comparison with triple phosphate. One month after the application of increasing amounts of sludge Go (5, 10, 15 t DM ha(-1)) both the concentra tion of oxalate-soluble Fe in the soil and the P sorption were increased. T he elevated relationship between these two parameters was highly significan t (r(2) = 0.6 - 0.97). Plant uptake of P was less after application of slud ge Ga 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 net be recommende d for agricultural use, as the P availability is significantly reduced. Iro n salts should not be used for conditioning of sludges.