
TY  - JOUR
A1  - Dalal, R. C. 
T1  - Long-term phosphorus trends in Vertisols under continuous cereal cropping
JF  - Soil Research
JO  - Soil Res.
Y1  - 1997
VL  - 35
IS  - 2
SP  - 327
EP  - 340
KW  - 
UR  - https://doi.org/10.1071/S96052 
N2  - Vertisols originally carrying brigalow vegetation (Acacia harpophylla F. Muell. ex Benth.) and cultivated for cereal cropping for up to 45 years were examined for trends in available phosphorus (P) &#91;sodium bicarbonate extractable P (bicarb. P) and dilute acid extractable P (acid P)&#93;, organic P, inorganic P, and total P. The soils (0-0 &#183; 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 fractions of soil P declined following first-order decay with the period of cereal cropping; the rates of bicarb. P and acid P (available P) decline were 0 &#183; 047 and 0 &#183; 08/year. The organic P, inorganic P, and total P declined more slowly than available P; the respective rates were 0 &#183; 026, 0 &#183; 019, and 0 &#183; 021/year. The rates of loss of total P and inorganic P were much higher from the clay-size fraction than the silt-size or sand-size fraction, with the t&frac12; value of inorganic P in the sand-size fraction being almost 20 times greater than any other fraction. On the other hand, organic P loss from the clay-size fraction was much less; presumably, clay provides physical protection to soil organic matter and hence to organic P from decomposition. On average, bicarb. P and acid P declined at the rate of 1 &#183; 3&#177;0 &#183; 3 and 4 &#183; 6&#177;1 &#183; 4 mg P/kg soil &#183; year. The declines in organic P and inorganic P were 3 &#183; 0&#177;0.4 and 4 &#183; 1&#177;1 &#183; 2 mg P/kg soil &#183; year, with a loss in total P of 7 &#183; 2&#177;1 &#183; 3 mg P/kg soil &#183; year. There was no significant shift in the ratio organic P: total P (38&#177;7%) with the period of cultivation and cereal cropping. Organic 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 each cereal crop from 1981 and 1984 were 7 &#183; 6&#177;1 &#183; 8 and 8 &#183; 3&#177;2 &#183; 4 kg P/ha, respectively. Thus, most of the soil total P loss could be accounted for by crop removal, of which organic P contributed about 40%. However, 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. 
ER  -

