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Prioritising a fertiliser program for grazing pastures

By Sue Briggs, Senior Land Services Officer - Sustainable Agriculture

There are a number of factors that influence stocking rate, with soil fertility a key component for driving profitability.  Fertiliser is one of the main expenses for grazing properties, but many producers do not approach this expenditure strategically.

Soil testing program
Relying on one or two soil test results every three to five years for a fertiliser program has the potential to result in a wasted resource.  This approach is unable to identify the variation that exists between paddocks and hence does not provide a basis for prioritising expenditure between paddocks. To develop a comprehensive fertiliser program, soil testing every paddock (or production zone) is the ideal approach. This may be a considerable investment in the first year, but it has the potential to reduce fertiliser expenditure. Strategic soil tests in subsequent years can be used to monitor the response to your fertiliser application.

Target phosphorus levels (Critical Value) 
Colwell P soil test interpretation should be based on the soil’s Phosphorus Buffering Index (PBI), as critical values increase with increasing PBI.  PBI is a measure of the P-sorbing capacity of the soil and varies among soil types.  The table (from Making Better Fertiliser Decisions for Grazed Pastures in Australia) shows the PBI category and critical Colwell value for a sub clover mix pasture.  For example, a PBI of 60 indicates a critical Colwell P soil test value between 28 - 31mgP/kg to produce 95 per cent of maximum pasture yield, whereas a soil with a PBI value of 180 will have a critical range of 37 – 44mgP/kg.

The soil testing results from the Holbrook Landcare Network sub clover nodulation saw 58% per cent of the phosphorus results over target P level, 17 per cent close to target P, and 25 per cent under target.

Table 1: Target phosphorus levels (critical value)
PBI CategoryCritical value1 for midpoint of PBI category
Colwell value (Range)
< 1523 (20 - 15)
15 - 3526 (25 - 28)
36 - 7030 (28 - 31)
71 - 14034 (31 - 37)
141 - 28041 (37 - 44)
281 - 84056 (45 - 65)
> 840n/a

Soil test values above target P
One option for managing high soil test values is to not apply phosphorus for a season or two.  There have been numerous research projects in many differing rainfall zones that demonstrate no loss of production when no phosphorus is applied to high P soils (especially if all other limiting factors are addressed).  Addressing other limiting factors can become a priority in these paddocks.  It is important to monitor phosphorus levels over time to make sure the phosphorus level does not drop below the target P.

Maintenance P
A maintenance rate (replacing what is taken off) can be applied when soil test results are close to target P levels.  The old rule of thumb of 1 kgP/DSE/ha has been refined through research, and now factors such as soil type, landscape, grazing management and rainfall are used, resulting in a variation in maintenance rate from 0.4 to 1.4 kgP/DSE/ha.

Capital P
If the soil test result is below target value, a capital rate is required to raise phosphorus levels.  It may not be possible or desirable to fix the deficiency in one year.  Capital rate can be spread over two or more years.  This will be sufficient time to achieve potential stocking rate or improve feed utilisation.
Phosphorus is only one of the essential nutrients required by legume-based pastures. If the soil has additional nutrient limitations, such as potassium, sulphur and molybdenum, or if the soil pH is very low, then the response to applied P can be held back by other soil deficiencies.

Most importantly, it is essential to treat fertiliser as you would any other long-term investment.  This means continuing to monitor soil fertility outcomes using soil tests and conducting an annual stocking rate review.

Cows grazing pasture

Annual stocking rate reviews and soil fertility monitoring are essential in managing your fertiliser.