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Phosphorus fertilization effect of sewage sludges treated with iron compounds
No full textThe result of waste water treatment with iron salts are sewage sludges with higher P but also Fe contents. The effect of such sludges on P availability in soils is not clear and was, therefore, compared with CaHPO4 as a P mineral fertilizer. In experiments (Mitscherlich pots, 6 kg soil) two sandy soils (pH 4.3 and 4.9), two luvisols ( pH 6.3 and 7.2), a loess (13 % CaCO3), and quartz sand all differing in their P status were treated with two sewage sludges which differed in their molar Fe:P ratios (sludge GO: 1:0.3, sludge Sh: 1:1.2). For sludge GO the P elimination had been carried out with FeSO4 and the sludge was stabilized with FeCl3 + Ca(OH)(2) (filter press). For sludge Sh P was eliminated by FeCl3 in the aerobic basin. The first crop was maize (total shoot dry matter), the second red clover. In case of sludge Sh both crops took up in comparison to CaHPO4 the same amount of P from all substrates with the exception of quartz sand. The effect of sludge GO was quite different: from the neutral luvisol soils maize was able to take up only 64-82 % P and red clover 77-82 % P only as compared to CaHPO4. On the sandy soils maize and red clover grew after sludge GO as good or better than after CaHPO4 application and P uptake was quite similar (83-106 %). This result was concomitant to an increase of pH values (from 4.3 to 4.9, 4.9 to 5.6, respectively). The negative efficiency of P uptake in heavy soils after application of sludge GO resulted from an increase of P sorption and decrease of orthophosphate concentration in soil solution. This sludge contains a high proportion of iron hydroxides/oxides with free sorption sites for P. In future P elimination from waste water should be done without iron salts. At least during the process of conditioning no Fe salts should be used
Influence of iron content in sewage sludges on parameters of phosphate availability in arable soils
No full textThe 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 negatively 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 loamy 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 contained 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 wits 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 experiment 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, after 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 soils 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 controls 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 concentration of oxalate-soluble Fe in the soil and the P sorption were increased. The elevated relationship between these two parameters was highly significant (r(2) = 0.6 - 0.97). Plant uptake of P was less after application of sludge 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 recommended for agricultural use, as the P availability is significantly reduced. Iron salts should not be used for conditioning of sludges
Einfluss eisenhaltiger Klärschlämme auf Kenngrößen der P-Verfügbarkeit von Lehm- und Sandböden
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