Changes in inorganic and organic Phosphorus in soil under maize (Zea mays) cultivation

The changes in applied and residual phosphorus (P) were examined in Avalon and Clovelly soils under cultivation by using the P fractionation method of Hedley, Stewart and Chauhan (1982). Soil P was separated into six major fractions of which some fractions were subdivided into organic (Po) and inorganic (Pi) P forms: P extracted by an anion exchange resin (Resin-P); 0.5 M NaHCO3-P (NaHCO3-Pi and Po); 0.1 M NaOH (NaOH-l-Pi and Po); ultrasonication in 0.1 M NaOH at 75 watt (NaOH-ll-Pi and Po); 1.0 M HCI(HCI-Pi); and H2SO4 digestion and H2O2 oxidation, (H2SO4P). General trends in soil P were evident over 15 years of cultivation. Resin-P, NaHCO3-Pi, NaOH-l-Pi, NaOH-ll-Pi and HCl-Pi were reduced in the Avalon soil, while only the NaHCO3-Pi and NaOH-ll-Pi decreased in the Clovelly soil. It is hypothesised that NaOH-l-Pi, which showed the largest decrease, was mainly responsible for the replenishment of the labile P pool. In the Po fraction, NaHCO3-Po and NaOH-ll-Po decreased with time in the Avalon soil, in con...