Using soil and plant analyses in decisions about fertilisers: a farmer’s perspective

Abstract

Summary. This paper describes the part played by soil and plant analysis in decisions about fertilisers used on a mixed pastoral and cropping farm at Scone, in the Northern Midlands of Tasmania. Measurements of phosphorus and potassium concentrations in soils provide an essential framework for choosing fertilisers for crops and pastures. Use of lime with certain crops is determined by pH measurements. Neither soil nor plant analysis are used in choosing nitrogenous fertilisers. Fifteen paddocks were tested 7–8 years after initial soil tests (Colwell extracts). Phosphorus concentrations (mean ± s.e.) had risen by 13 ± 5 µg/g (P<0.05) and potassium by 89 ± 19 µg/g (P<0.001). In addition, the relationship between changes in these concentrations and the total phosphorus or potassium applied to individual paddocks was examined. Phosphorus concentration rose (P<0.001) by 5.9 µg/g for each 10 kg/ha additional phosphorus applied over the 7–8 years. Application of phosphorus at 7 kg/ha annually was enough to maintain soil phosphorus levels. No such relationship was detected for potassium. Application of lime at 4 t/ha raised the pH by about 1 unit. These results provide some reassurance about the behaviour of soils on Scone in response to the practices used in commercial mixed farming. The importance for the farmer of analyses being reliable is stressed, as is the need for well-based interpretation of the analytical results in order that the yield responses to fertilisers can be foreseen. Evidence about the financial returns produced by the increased yields is often lacking, especially for grazing enterprises.