Relationships between agronomic and environmental phosphorus analyses of selected soils

Abstract

Phosphorus (P) is an essential nutrient in crop production and is therefore typically applied as phosphatic fertilisers.  This can induce soil P concentrations that may contribute to freshwater eutrophication.  Soil P tests developed from a water pollution protection perspective are unlike those developed for agronomic purposes and are also not easily adapted to routine analyses.  The aim with this study was therefore to establish whether the values of P extracted from a range of soils by various agronomic and environmental P determination methods are related or not.  Topsoil samples were collected from virgin soils in central South Africa and treated with KH2PO4 to induce different P concentrations and then incubated for 3 months, while subjected to various wetting and drying cycles.  The samples were then analysed for P using the extractants of Bray 1, citric acid, ISFEI, Olsen, and Truog, commonly employed to establish the agronomic P status of soils.  Environmental P status was determined with calcium chloride (CaCl2) and ammonium oxalate (DPSox) extractants.  Generally good relationships were found between CaCl2 and ISFEI (R2 = 0.72), between DPSox and Truog (R2 = 0.79), and between DPSox and citric acid (R2 = 0.82).  Agronomic P soil tests therefore have the potential to be used in regulating application of phosphatic fertilisers to ensure optimum crop yields, while simultaneously limiting freshwater pollution.  Further field studies are, however, recommended to determine threshold values from an environmental point of view.