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Measuring critical P

As highlighted in Figure 1, where critical P levels are not met (under-fertilising), pasture growth rates are inhibited.

Exceeding critical P (over-fertilising):

  • does not support additional growth
  • results in additional soil phosphorus ‘fixation’
  • can be environmentally hazardous.

Carrying out regular soil testing is the first step in developing a cost-effective and efficient phosphorus
fertiliser program.

Soils always contain more phosphorus than is available to plants during the current growing season. Most of the phosphorus is in compounds plants cannot use directly, is tightly bound to soil particles or is in compounds that are only sparingly soluble.

Soil tests aim to extract a phosphorus fraction that indicates how much phosphorus is available for plant growth.

Strategic phosphorus application

Determining how much phosphorus to apply will depend on whether the aim is to ‘increase’ soil phosphorus fertility or maintain the current level of fertility by holding the paddock at about the same soil phosphorus test value.

For more information on how to implement a cost-effective soil testing and fertiliser program, see the Five Easy Steps phosphorus tool.

Predicting how many head of stock you can carry as soil fertility increases is often the most difficult task.

Use MLA’s stocking rate calculator and Rainfall to pasture growth outlook tool, in conjunction with the Five Easy Steps phosphorus tool, to ensure additional pasture growth – as a result of improved soil phosphorus fertility – delivers associated enterprise profits.