Fertility changes in soils cultivated for wheat in southern New South Wales.

Abstract

A study has been made of red-brown earthÆs cultivated for wheat on a narrow rotation of fallow-wheat-volunteer pasture in the Ariah Park district of Xew South Wales. Surface soil samples from 55 cropped paddocks currently under fallow and from matched adjoining areas with no history of cultivation were examined in laboratory, pot culture, and field studies. Many of the soils had carried at least 20 crops since being brought under cultivation and had lost an average of 30% of their organic matter during this period. However, only 17% of the organic phosphorus had been mineralized. The mean ratio of carbon : nitrogen : sulphur : organic phosphorus in the uncultivated soils was 178.3 : 10 : 1.37 : 0.63 while that for the cultivated soils was 170.9 : 10 : 1.41 : 0.74. As a result of the decrease in organic matter content the cation exchange capacity decreased under cultivation. Losses of the exchangeable cations also occurred, but the relative proportions of the metal ions in the cultivated soils were the same as those in the uncultivated. Removal of cations in crops could not account for all of the losses observed. There had been a net loss of sulphur from the surface soils over and above that likely by crop removal. The nitrate content of the surface samples was insufficient to provide the nitrogen uptake by the crop either in pots or in the field. Evidence is presented suggesting that other sources, including leached nitrate, were utilized in the field. Mineralization of organic phosphorus and fertilizer addition had increased the amounts of inorganic, plant-available forms, but the total phosphorus was unchanged in the cultivated soils. However, the phosphorus status of' the cultivated soils was still extremely low and responses to phosphorus were obtained at all sites. There seems little prospect of reducing current superphosphate rates on these soils.