Risk assessment of phosphorus (P) loss and guidelines for P use in lower Herbert soils Final report on SRDC Project No CLW010

Abstract

In project CSS3S (Bramley et aI., 1998), a field and laboratory-based survey of the behaviour of phosphorus (P) was carried out on the soils of the lower Herbert River catchment, and sediments derived from them. The aim was to explore the factors governing P sorption or desorption in Herbert soils, and in suspended sediments in associated riverine and estuarine waters, so that the extent of any problem associated with sugarcane and soil-derived inputs to strearnwaters could be defined and advice on the development of best management practices for P fertilizer could be provided. Accordingly, an assessment of the risk of P loss from selected lower Herbert soils was made based on their P sorption characteristics and an assessment of the susceptibility of the lower Herbert soils to runoff following rainfall events. One of the recommendations made at the conclusion of CSS3S was that "spatial analysis of the assessment of P desorption risk based on digital maps of the CSR soil survey would enable more precise guidelines for better P management to be derived.". Following the recent availability of the CSR 1:5,000 soil survey in geo-referenced digital form, this report details the results of the suggested spatial analysis. Nine hundred and thirty four soils for which detailed soil property data are available in the database accompanying the 1:5,000 CSR survey of lower Herbert sugarcane soils were classified according to a range of indices of P sorption and the results mapped using either a geostatistical interpolation routine (kriging) or the mean values for each soil type identified in field survey. The results were coupled with an analysis of the susceptibility of these soils to runoff to produce maps of the potential for P loss.