A large body of Australian research has been used to work out the soil test levels to which pastures can be fertilised with phosphorus, sulphur and potassium appropriately for high production.
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These soil test benchmarks can be used to check if major elements, like phosphorus, are deficient and to choose the right fertiliser or fertiliser blend to correct deficiencies.
Suitable soil tests for phosphorus covered by the work include Colwell P, Olsen P and, when using Colwell, also the Phosphorus Buffering Index (PBI).
Details of this research are published in the latest edition of the Grassland Society of NSW newsletter.
The paper titled 'Phosphorus soil test benchmarks for productive legume-based pastures' is authored by Richard Simpson of CSIRO Agriculture and Food, Canberra, and Cameron Gourley of Agriculture Research Victoria.
Dr Gourley led industry-supported research known as the Better Fertiliser Decisions for Pastures (BFDP) project, to develop "critical" soil test benchmarks for pastures by collating yield responses from about 650 Australian fertiliser experiments over a 50-year period.
Critical soil test values (for example plant-available phosphorus measured in milligrams per kilogram soil) are the soil fertility level for top 10 centimetre depth samples, at which the pasture is expected to achieve near-maximum yield.
As such, they provide targets for fertiliser use and soil fertility improvement.
Adding fertiliser in amounts that cause soil nutrient levels to exceed these benchmarks is unlikely to achieve a lot more pasture growth.
A breakthrough in soil test interpretation for phosphorus occurred with the finding that critical Colwell P levels can be predicted when the soil Phosphorus Buffering Index (PBI) measurement is known.
Soils with low PBI readings will reach maximum pasture production at lower Colwell P readings than soils with high PBI values.
It is now easier to interpret the widely used Colwell P test results to decide on fertiliser programs, Dr Simpson notes.
Also noteworthy was the critical Olsen P levels (also a well-used soil test) were independent of soil type or PBI reading.
Numerous alternative pasture legumes have been developed in Australia, mainly to provide more options where sub clover is less reliable, and little has been known about their phosphorus requirements.
In recent times, Dr Simpson and his colleagues have assessed many of these legumes for phosphorus responsiveness over three years at four sites near Yass and Wagga Wagga.
This research is part of a national project looking at phosphorus efficient plants. It is supported by the Department of Agriculture as part of its Rural R&D for Profit program. Other funding includes MLA, Dairy Australia and AWI.
Numerous alternative pasture legumes have been developed in Australia, mainly to provide more options where sub clover is less reliable, and little has been known about their phosphorus requirements.
- Bob Freebairn
The research found that the critical soil test P levels for many alternative legumes did not differ consistently from that of sub clover, indicating phosphorus fertiliser use for most pasture legumes can be guided by the current guidelines for temperate Australian pastures.
Especially interesting results were that a small number of pasture legumes species grew at full production, with much lower soil phosphorus levels than most pasture legumes, including sub clover.
In soils with a PBI between 40-80, where sub clover and many of the alternative legumes need to be fertilised to 30-35mg Colwell P a kilogram (or 15mg Olsen P a kilogram), these phosphorus-efficient alternatives (crimson, purple and arrowleaf clovers, and yellow and French or pink serradella) could be fertilised to 20-25mg Colwell P a kilogram (or 7.5-10mg Olsen P a kilogram) to achieve near-maximum production.
Critical phosphorus requirement of lucerne was higher than that of sub clover.
Phosphorus requirements of traditional legumes like sub clover and white clover, are higher than the grasses with which they are commonly grown.
This research is vital, as legumes drive the overall pasture system via their nitrogen fixation.
Next week: Research points the way to better soils. Facts not fiction are important.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact (0428) 752 149.
