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) 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.

Boron (B) is removed from soils when crop products are harvested. However, B has never been used as a fertiliser in broadacre agriculture in WA. A GRDC-funded project was started in 1998 to consider the sustainability of this mining of soil B. The project aims are to (1) map the spatial distribution of B deficiency in canola and lupin; (2) investigate the role of B in soil and plants for grain yield of canola and lupin, and; (3) develop management options for B to take account of soil properties and crop requirements. The present paper reports preliminary results that begin to define the extent of low B soils in the W A wheatbelt and its likely implications for crop production. Young leaves of canola and lupin crops were sampled at 136 sites in the wheatbelt during vegetative growth in 1998. Potentially B deficient crops representing 10- 20 % of sites were widely distributed throughout the wheatbelt. In 73 Reference Soils of south-west Australia, extractable soil B was positively correlated with soil pH and clay content. A preliminary study of the response of farmers' canola crops to a B spray was conducted at 7 sites with sandy to sandy loam texture in the Great Southern Region of WA in 1998. For all sites, a mean increase of 14% in seed yield was found from the B spray application at flowering. Preliminary pot trial results suggest that responses to B in canola and lupins will occur on a number of low B soils. Our results tend to confirm that the risk of B deficiency warrants further consideration. The key questions now are: where is it most likely to occur, under what crops is it most likely, what seasonal factors bring on B deficiency and what are the management options for dealing with the problem?