Ground Cover Supplement : GC Supplement - More profit from crop nutrition II
16 Clay amelioration and nutrients TESTS URGED BEFORE CLAYING WATER-REPELLENT SOILS An estimated 200,000 hectares of Australia’s water-repellent soils have been ameliorated with clay, through spreading or delving, but how has this changed plant-available nutrients such as nitrogen, phosphorus, potassium and sulfur? content of the ameliorated paddocks is a concern because a survey of clay pits on WA's south coast showed wide variation in the clay and nutrient content of the subsoils collected (Table 1). David Hall, senior research officer from the Department of Agriculture and Food, WA, with GRDC support, collected subsoils from 82 clay pits that growers had opened during the past 15 years to use the clay to ameliorate non-wetting soils. His analysis of the subsoils showed the average clay level was 40 per cent, which is considered adequate for claying. However, some subsoils only comprised 10 per cent clay, which Professor Bell says would have little impact in helping to ameliorate water-repellent soil. On the other hand, Professor Bell says, subsoils comprising 75 per cent clay may present difficulties for handling and incorporation, and may possibly lead to surface crusting and crop emergence issues if not incorporated adequately. Professor Bell says the survey shows phosphorus, potassium and sulfur levels can vary greatly depending on the subsoil source and that pre and post-claying soil tests are essential to ensure that a limitation or toxicity in one or several nutrients would not hamper crop growth. "Some trials within the GRDC's More Profit from Crop Nutrition II initiative have shown subsoil clay can increase phosphorus, potassium and sulfur. However, these nutrients can be diluted if levels are low in the added subsoil," he says. "For example, in a five-year trial at Bolgart, north-east of Perth, we didn't observe any yield increase in response to added clay because what was applied did not contain any freely available potassium." As a consequence, Professor Bell encourages all growers considering claying to test subsoil clays via a comprehensive nutrient analysis, including phosphorus buffering index and clay content, and to be aware of any potential for toxicities from boron and salinity. "Knowing how subsoil clay contents and nutrients vary from one paddock to the next or from patch to patch can help put a value on the subsoil and assists in designing the subsequent crop nutrition program," he says. "On bigger farms a comprehensive nutrient analysis may enable certain paddocks to be prioritised so dollars can be invested on areas likely to generate a quicker return." □ G RDC Research Codes UMU00041, DAW00204 More information: Professor Richard Bell, 0405 131 429, firstname.lastname@example.org; David Hall, 0447 914 013, email@example.com TA BLE 1 Clay and nutrient content of 82 clay pits on the WA south coast. Clay (%) Electrical conductivity (dS/m) pH (CaCl2) Colwell phosphorus (mg/kg) Colwell potassium (mg/kg) KCl40 sulfur (mg/kg) Cation exchange capacity (cmol/kg) Soluble boron (mg/kg) Phosphorus buffering index Average 40 0.4 6.8 3 320 51 11 4 94 Maximum 75 2.3 8.7 13 1090 337 28 20 502 Minimum 10 0.0 4.7 1 19 3 3 0 7 Legend: Nutrient beneft Potential issue Nutrient limitation SOURCE: PROFESSOR RICHARD BELL, MURDOCH UNIVERSITY, AND DAVID HALL, DAFWA. PHOTO: DAVID HALL By Nicole Baxter A LEADING SOIL SCIENTIST is reminding those planning to add clay to water-repellent soils to test the clay source and the ameliorated paddocks to ensure clay and nutrient levels are sufficient to produce a crop yield response. Professor Richard Bell, from Murdoch University, says a GRDC-supported survey of 30 growers in Western Australia showed 87 per cent had seen strong yield responses from claying. "In a long-term trial near Dalyup, west of Esperance in WA, yield increases of 40 to 50 per cent over 15 years were observed where the clay content was raised from one per cent to seven per cent within the topsoil," Professor Bell says. However, only 27 per cent of growers surveyed undertook a laboratory test of the subsoil clay source before they used it and only 13 per cent changed their fertiliser practice after claying. Professor Bell says failure to test the clay source and the resulting nutrient www.grdc.com.au/gctv19 GROWER TIP The deep-phosphorus calculator will be able to indicate the possible optimum deep-phosphorus rate and the economic outcomes for a multi-season crop rotation.
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