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.