Mike Wong Ting Fook

Soils across the grains cropping regions of Western Australia were inherently low in P and other nutrients. Development of agriculture would not have been possible without the use of P and other fertilisers. In these soils, profitable rates of P resulted in increased yields and a gradual build-up of soil available P (measured as Colwell P). Once P deficiency is corrected and the Colwell P values for near maximum crop production (critical values) are reached, the recommended practice is to maintain the soil at these critical values. This prevents reversion to deficiency and risk of yield and profit loss. The P maintenance practice uses less P than the build-up phase. The amounts of P applied during maintenance are designed to balance (1) removal in harvested grains and other products such as hay and sheep sold off farm (2) amounts of fertiliser P that becomes tied-up by strong adsorption with soil and (3) unavoidable losses due to leaching, runoff and erosion. The WA cropping industry has a long history of P use that started with the development of the industry. It is now time to assess if we have succeeded in correcting the P deficiency and if we are ready to move to the maintenance phase. This is also an opportunity to assess what other soil constraints are limiting production and profits so that money freed-up by transitioning from build-up to maintenance could be re-invested in managing these constraints.

Phosphorus (P) transfer from agricultural land via runoff, throughflow and leaching is of increasing concern for land managers worldwide. Previous studies suggested that organically bound P comprises a large proportion of soil P reserves in south west Western Australia (WA), but the implications of these findings for the processes and rates of P transport in soils are not known. Two contrasting soil profiles (sand and clay) from cropping land of the upper Fitzgerald River catchment in the south coast region of WA were studied in packed boxes to examine the P forms and fluxes in runoff, throughflow, leachate and soil solution after three rates of P application (equivalent to 0, 20 and 40 kg P/ha). Soil solution was collected at 5, 10 and 15 cm depths with inert soil solution samplers, and leachate was collected at the bottom of the 30 cm of packed boxes. Solutions were analysed for particulate P (PP), dissolved reactive P (DRP) and total dissolved P (TDP) while the dissolved uneactive P (DURP) was calculated by difference (TDP-DRP). Phosphorus transport increased with P rate. In the sand, DRP comprised < 35 % of TP in runoff whereas DURP and PP are about 90% of TP in runoff and leachate. In clay soil, 90 % of P losses in DURP and PP form via thoughflow and leaching while DRP constituted < 33 % of total P lost. The result suggested that major portion of mobilized P appeared to be associated with DURP and PP in runoff and leachate in association with dispersed inorganic colloidal compounds < 0.2 mm.

Phosphorus (P) export by erosion, surface runoff, throughflow and leaching are considered the main sources of P loss from agricultural land. The present study was conducted on the upper Fitzgerald River Catchment in the South coast region of Western Australia (WA) to examine the process of P mobilization at different P rates (0, 20 and 40 kg P/ha). Intact column leaching, packed box and field runoff plot studies were conducted on contrasting soils from the catchment. Soil solution was collected at 5, 10 and 15 cm by installing Rhizon soil solution samplers, and leachate collected at 30 cm. Runoff and soil solutions were analysed for particulate P (PP), dissolved reactive P (DRP), and total dissolved P (TDP) and dissolved organic P (DOP) was calculated by difference (TDP-DRP). Overall, DRP comprised <35 % of TP in runoff while about 90% or more of relative P losses via runoff, throughflow and leachate were in DOP and PP forms. The DOP and soluble organic carbon (SOC) in soil solution were well correlated in sand (R2 = 0.78, P <0.05) and clay soils (R2 = 0.56, P <0.05) at 0-5 cm suggesting that amounts of organic matter dissolved in soil solution influences P sorption and mobility. The higher PP concentration for the clay soil at the interface of clay and sandy layers indicates subsurface lateral flow is exacerbated by dispersive clay which might be an additional concern regarding P mobility in clay and duplex soils of the catchment. Ponding of water at the surface or lateral movement of water at the interface of sand and clay layers in the profile would increase the risk of P losses in the form of DP or PP in dispersion-prone sodic soils.