INCUBATING SUPERPHOSPHATE IN DRY SOIL CAN REDUCE ITS EFFECTIVENESS

Single superphosphate was incubated for six months at 25 degrees C in soil which had been subject to one of three moisture treatments. These were: dried in a glasshouse, dried at a constant temperature of 25 de grees C, or moist soil. Phosphorus (P) effectiveness was then compared with effectiveness of P from freshly-applied superphosphate using yie lds of wheat (Triticum aestivum) and triticale (x Triticosecale) tops in pot experiments. Incubation in soil which had been dried at 25 degr ees C did not decrease the effectiveness of the P. Incubation in moist soil decreased it to about 20% of the effectiveness of freshly-applie d P in one case and to about 50% in the other case. Incubation in soil which had been dried in a glasshouse also decreased its effectiveness . The decrease varied with conditions, but in two cases the P was 70% as effective as freshly-applied P, and in one case only 45% as effecti ve. Presumably sufficient moisture was present in the soil dried in th e glasshouse to enable water-soluble P present in the fertilizer to re act with the soil.