LONG-TERM PHOSPHORUS TRENDS IN VERTISOLS UNDER CONTINUOUS CEREAL CROPPING

Vertisols originally carrying brigalow vegetation (Acacia harpophylla F. Muell. ex Benth.) and cultivated for cereal cropping for up to 45 y ears were examined for trends in available phosphorus (P) [sodium bica rbonate extractable P (bicarb. P) and dilute acid extractable P (acid P)], organic P, inorganic P, and total P. The soils (0-0.1 m depth) in their virgin state contained 60 mg/kg of bicarb. P, 168 mg/kg of acid P, 239 mg/kg of organic P, and 330 mg/kg of inorganic P. All fraction s of soil P declined following first-order decay with the period of ce real cropping; the rates of bicarb. P and acid P (available P) decline were 0.047 and 0.08/year. The organic P, inorganic P, and total P dec lined more slowly than available P; the respective rates were 0.026, 0 .019, and 0.021/year. The rates of loss of total P and inorganic P wer e much higher from the clay-size fraction than the silt-size or sand-s ize fraction, with the t(1/2) value of inorganic P in the sand-size fr action being almost 20 times greater than any other fraction. On the o ther hand, organic P loss from the clay-size fraction was much less; p resumably, clay provides physical protection to soil organic matter an d hence to organic P from decomposition. On average, bicarb. P and aci d P declined at the rate of 1.3+/-0.3 and 4.6+/-1.4 mg P/kg soil . yea r. The declines in organic P and inorganic P were 3.0+/-0.4 and 4.1+/- 1.2 mg P/kg soil . year, with a loss in total P of 7.2+/-1.3 mg P/kg s oil . year. There was no significant shift in the ratio organic P:tota l P (38+/-7%) with the period of cultivation and cereal cropping. Orga nic P was closely correlated with organic C and total N in these soils . The mean amounts of P contained in the grain and the dry matter of e ach cereal crop from 1981 and 1984 were 7.6+/-1.8 and 8.3+/-2.4 kg P/h a, respectively. Thus, most of the soil total P loss could be accounte d for by crop removal, of which organic P contributed about 40%. Howev er, the continuous decline in available P, especially below 15 mg P/kg soil, warrants remedial measures to arrest the decline in the yields of crops grown on these Vertisols.