The effect of previous phosphorus additions to soils on phosphorus sorption and related parameters

Detrimental environmental effects resulting from the overuse of phosphorus (P) fertilisers in agriculture is a global issue which can cause the eutrophication of water systems. Phosphorus fertilisers are commonly used in Australia (and worldwide) and there remains no single simple model available which takes into account the many factors which affect P sorption and solubility; thus the potential for adverse environmental effects resulting from inefficient and excessive use is substantial. In this study six Australian soils were analysed in order to ascertain the effect previous P fertiliser additions have on the behaviour of subsequent additions of P. In addition other physical and chemical factors affecting P sorption in soils were investigated in order to achieve a more ʻcompleteʼ view of sorption across a range of soil types and under varying conditions. Each of the six soils studied had five different rates of fertiliser (as triple superphosphate) applied two years prior to the commencement of this study. Phosphorus sorption experiments were conducted on the soils and the subsequent sorption data obtained was fitted to two commonly used sorption models, the Langmuir and Freundlich. Additional tests including pH, organic carbon content, mineralogy as well as other commonly used P extraction procedures were carried out. All soils displayed effects on sorption of the new P additions as a results of the previous fertiliser treatments which had caused a residual ʻpoolʼ of P to remain in the soils, thus the higher the previous P additions, the less new P can be sorbed. The Langmuir model fit the data best, although acceptable R2 values were also observed in the Freundlich curves. Chemical parameters also affected by the previous P additions were Colwell P, labile P and oxalate extractable P. Further research is still required in this field in order for a model which accurately describes P sorption in soils to be created.