Ground Cover Supplement : GC Supplement - Nutrient management
17 NUTRIENT MANAGEMENT GROUND COVER NUTRIENT DECISIONS P-rated films outshine their rivals A MORE ACCURATE AND RAPID SOIL TEST FOR PHOSPHORUS AND MICRONUTRIENTS HAS BEEN DEVELOPED FOR AUSTRALIAN SOILS AND IS UNDERGOING EXTENSIVE FIELD TESTING IN 2007 BY SEAN MASON A neW MeTHoD of analysing available phosphorus and micronutrients is going to be extensively field- tested in 2007. Diffusive gradients in Thin Films (DgT) is a system developed at lancaster University in the UK. Placed on the surface of moist soil for 24 hours, DgT is considered to mimic plant-root behaviour by locally lowering nutrient concentrations in the soil solution. For the past four years as a gRDC-funded PhD student at the University of Adelaide, together with colleagues at CSiRo land and Water i further developed and tested the DgT technique in the glasshouse, using Australian soils and different crop varieties DgT technology from the UK was designed to measure selected negatively or positively charged elements in water systems or soils, but at concentrations characteristic of highly contaminated systems. in the first part of this work, we developed a mixed binding gel capable of measuring phosphorus, zinc, copper, manganese, molybdenum and cadmium at concentrations more representative of uncontaminated agricultural soils, sediments and water. The performance of the DgT was compared with three other commonly used techniques for testing soil phosphorus – Colwell P, olsen P and resin, a method widely used in South America. The test was designed to establish if the technique could predict the responsiveness of wheat, canola and barley to a one-off application of phosphorus fertiliser. Frame wheat was grown in 21 Australian agricultural soils and a plant response, not always a significant one, was recorded with the addition of phosphorus (30 milligrams per kilogram). The DgT accurately predicted the plant response in all 21 soils. in contrast, olsen P predicted plant response in 76 per cent of the soils, the resin in 71 per cent and Colwell P in only 53 per cent. Similar high levels of accuracy were found for DgT when tested with barley and canola. This suggests that DgT can predict available phosphorus on the selected soils more accurately than other traditional testing methods. Rate-response experiments were also conducted and DgT was found to predict plant responses to phosphorus more accurately than the comparison resin method. zinc and manganese deficiency thresholds in wheat and canola were tested. The DgT, using the mixed binding gel, successfully determined the threshold for both nutrients. Due to the low levels of these nutrients, detecting deficiency thresholds has proved elusive with other methods. Before DgT can become established as an alternative soil-testing method, validation is required for field-grown crops. The project aims to assess crop response to applied phosphorus and correlate these responses to DgT measurements at about 20 field sites throughout the western, southern and northern cropping regions. The performance of the DgT method will be compared with resin, Colwell and olsen tests for phosphorus. glasshouse trials and on-farm field trials will be conducted to test the effect of field conditions on plant response and grain yields when phosphorus fertiliser recommendations are based on DgT results. in the glasshouse the aim is to explore further the capabilities of the DgT method to assess accurately micronutrient (zinc and manganese) deficiency in Australian agricultural soils. GRDC Research Code UA00095 More information: Dr Sean Mason, 08 8303 8107, firstname.lastname@example.org Sean Mason places the clear diffuse gel layer over the orange mixed binding layer to make up the DGT, which has the potential to predict plant responses to phosphorus more accurately than other methods. A filter paper is placed on top of the gels and the DGT is placed face-down on moist soil, where it acts like a plant root, drawing in the targeted nutrients from the soil.
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