Applying phosphorus to best improve pasture was the topic of CSIRO Agriculture and Food research scientist Dr Richard Simpson’s talk.
Dr Simpson said there were differing views on what improvements, if any, were made to pasture once phosphorus was applied, with some found to be incorrect through recent research.
‘‘Typically we think in a very linear fashion about what happens when phosphorus fertiliser is applied to improve or maintain high pasture production,’’ he said.
‘‘Some of us assume that each application of superphosphate feeds the pasture directly to boost pasture growth.
‘‘Others have heard about ‘P-fixation’ in soils and have a pessimistic view of how fertiliser works.
‘‘Recent research has demonstrated that neither view is quite right and that by using soil testing to inform P-fertiliser use, we can objectively enhance the cycling of phosphorus in a grazing system to support high pasture growth rates and animal production.’’
Dr Simpson said there were three important phases of phosphorus cycle management.
‘‘The first is the sub-optimal soil phosphorus fertility phase, which is characterised by unrealised potential for pasture growth and stock carrying capacity.
‘‘Optimum soil phosphorus fertility coincides with the attainment of the critical STP (soil test phosphorus levels) concentration in soil and should be accompanied by a recognisable improvement in the clover content of the pasture.
‘‘And over-fertilised pastures occur if critical STP concentration of the soil is grossly exceeded,’’ Dr Simpson said.
To identify the ideal STP level for your soil, Dr Simpson said if using the Olsen test it would usually be 15mg P (phosphorus)/kg, while if using the Colwell P test, he suggested using the Phosphorus Buffering Index soil test to determine the critical Colwell P value that is appropriate for your soil.