SOIL scientists continue to see stronger responses in northern grain crops to combined nutrients placed deep in the soil profile than individual nutrients alone.
Dr Mike Bell, Queensland Alliance for Agriculture and Food Innovation (QAAFI), University of Queensland and David Lester, Department of Employment, Economic Development and Innovation (DEEDI) say this is especially the case where sufficient nitrogen is available.
They are working with funding from the Grains Research and Development Corporation (GRDC).
"Combinations of nutrients including phosphorus, sulfur and potassium are proving effective, especially when enough nitrogen is available to allow higher yield targets to be reached," Dr Bell said.
"Improving the ability of root systems to explore large areas of soil can greatly boost yield responses.
"In the recent wet summer and winter crop seasons, the reliance on deep placement of phosphorus or potassium has generally been reduced in favour of overall profile (0-25 centimetre) enrichment, but residual effects of deep phosphorus applications are still being recorded six crop seasons after application."
Dr Bell told advisers at the GRDC Update at Goondiwindi to base strategies for phosphorus and potassium fertiliser around periodic deep (15-20cm) applications in bands 50cm apart (or closer) for best results.
This proposal is in addition to the current use of starter phosphorus applications in the seeding row when Colwell P tests in the 0-10cm layer indicate a deficiency.
"Application rates should be high enough to at least meet likely crop removal in grain until the next fertiliser application, while redistribution of deep applied nutrient in crop residue will enrich the topsoil layers," he said.
"Applications of the more mobile nitrogen and sulfur can be made to target individual crops and yield targets, as is currently practiced."
Researchers continue to see evidence of declining soil organic matter and inorganic and organic nutrient reserves in northern cropping systems, although the rates of decline and the amount of reserves vary greatly between regions, soil types within regions and even between paddocks.
"Nutrient budgeting has been clouded by uncertainty about the size of plant available nutrient reserves in different soils, the rate of release to meet crop demands and climate-related uncertainty about crop yield targets," Dr Bell said.
"In recent years we have seen that a number of our longer term cropping soils are showing signs of severe depletion of these plant available reserves, so significant yield and water use efficiency constraints have increased the urgency to adopt new methods of soil fertility monitoring and nutrient application.
"This has been accentuated to some extent by a string of potentially wetter seasons with higher yield potentials.
Such seasons also result in greater risks of off-site losses of mobile nutrients like nitrogen and sulfur due to leaching (both nutrients) or denitrification (nitrogen)."