Interaction of Ca(H2PO4)2applied to an oxisol and previous sludge amendment: Soil and crop response

Treble superphosphate (TSP) was applied at 0 and 120 kg P/ha to a highly weathered Oxisol which had received various levels of a sewage sludge 5 years earlier. Lettuce (Lactuca sativa L.) was grown as a test crop. Plant and soil chemical composition indicated that the efficiency of TSP was significantly increased in the presence of sludge: 120 kg P/ha added to soils amended with 45, 90, and 180 Mg/ha sludge were correspondingly as effective as 306, 314, and 196 kg P/ha as TSP required by the unamended soils. This is probably because organic anions and molecules produced by the sludge reacted with P fixing sites on the soil surface and/or chelated‐precipitated soluble Al. Furthermore, increased Ca levels in soil solution by sludge or TSP addition improved plant growth and reduced Al uptake. For lettuce, the adequate Ca range was 400–500 mg/L in soil solution and 1.9–2.0% in leaf. Because of an unexpected additional input, adequate range of soil solution P could not be clearly determined for lettuce, but the plant's internal P requirement appeared to be 0.45–0.50%.