The influence of alternative tillage systems on the distribution of nutrients and organic-carbon in some common Western Australian wheatbelt soils

Abstract

The effect of reduced cultivation on the chemical fertility of three agricultural soils important in Western Australia was investigated. The experiment compared the effect of different tillage systems for continuously cropping wheat on the distribution of extractable P, extractable K, total N, organic C and pH for soils ranging from a sand to a sandy clay loam. Three tillage systems were applied (conventional cultivation, direct drilled with a combine, direct drilled with a triple disc drill) and the distribution of nutrients was measured to a depth of 25 cm. Developments were monitored for a period of 9 years. Clear differences between treatments were evident within the first 3 years of the experiment. Relative differences between treatments remained constant after this time. The concentrations of extractable P, extractable K, total N and organic C in the soil were all higher with direct drilling than with conventional cultivation, regardless of soil type or environment. The concentration of these elements declined in all treatments throughout the duration of the experiment. Major differences between treatments occurred in the surface 5-10 cm of the soil, although trends differed slightly depending on the element measured. Ammonium nitrate application substantially reduced the pH of the sandy soil, regardless of tillage treatment, but had no effect on the other soil types. Conventionally cultivated sandy and sandy clay loam soils also had a slightly higher pH than the direct drilled soils. The yield of plants was not directly related to the concentration in the soil of any elements measured. Accumulation of nutrients near the surface indicated that, if the surface soil is dry, then nutrients may be less available to plants established with direct drilling than with conventional cultivation. Further research is needed to establish whether present 'soil testing-P recommendation' relationships, based on conventional cultivation systems, are appropriate under direct drilling systems in Western Australia. Changes in pH with different tillage systems may have a bearing on fertilizer application strategies. Finally, the decline in organic C with conventional cropping indicates that the structure of soils in this experiment remains fragile even with direct drilling.