In 2003 the Better Fertiliser Decisions project began with the aim of providing a series of tools to help producers and their advisers improve fertiliser practices.

The project team collated all the research on fertiliser use in pastures across Australia and created an extensive database.

The key outputs of the Better Fertiliser Decisions project are:

The establishment of critical soil test values for phosphorus, potassium and sulphur which include different soil test methods

An interactive database for advisers containing all the pasture response data for applied Nitrogen, Potassium, Phosphorus and Sulphur fertiliser

The Farm Nutrient Loss Index, a decision support tool to assess the risk of nutrient loss from a paddock.

The database allowed the project team to determine the critical soil test values for key nutrients in pastures.

The ‘critical soil test value’ is the soil test value where 95 per cent of maximum pasture production occurs.

Knowing this value allows producers to target the correct amount of fertiliser.

An important inclusion is the relationship between the critical soil test value and the Phosphorus Buffering Index (PBI) for the Colwell Phosphorus test.

The PBI is a measure of the soil’s capacity to hold on to, or release, phosphorus and is closely related to soil texture.

Light textured soils tend to have a low PBI, whereas heavy soils have higher PBI values.

So rather than one critical soil test value for all soil types, the critical value will change, with PBI allowing producers to modify fertiliser application according to soil type.

In a practical sense, a high PBI soil will have a higher critical soil test value (median value 55, range 44 - 64), compared to a low PBI soil (median value 34, range 31 - 36) to achieve 95pc of maximum production.

The range of critical soil test values for a given PBI category reflects the influence of other factors driving pasture production, such as environmental conditions.

For most producers in NSW the values will be appropriate but they will still need to be interpreted in your own farm context.

Producers with an interest in soil tests can talk to their local agronomist who can assist with the interpretation of soil test results and help fine tune fertiliser strategy.

The critical soil test values, advisers database and Farm Nutrient Loss Index can be accessed from the CSIRO Australian Soil Resource Information System (ASRIS) internet site: http://www.asris.csiro.au/themes/nutrient.html