Phosphorous (P) is an essential nutrient for plant growth and production.  An understanding of the amount and availability of P in your soil is essential to efficient fertiliser management. 

The “Colwell P” test has been the standard soil P test used in South Australia since 1976 and has been widely calibrated across a range of different soil types.  Colwell P provides an estimate of the “quantity” of P in the soil potentially available to plants.

However, it is important to note that Colwell P does not always provide a good indication of P availability to the plant.  Recently, a new test has become available that improves our understanding.
 
When applied as granular fertiliser P dissolves rapidly and is then subject to a range of processes including combining with other soil elements (such as iron, calcium and aluminium), adsorption to clay and organic carbon particles and leaching.  PBI supports Colwell P analysis by providing a measure of “tie up” and leaching potential.

High PBI levels (greater than 280) indicate a strong potential for “tie up” and a significant proportion of P applied will be unavailable to plants.  A very low PBI (below 70) indicates that there may be potential for phosphorus to leach (these soils generally have low Colwell P levels as well).

Colwell P, PBI and the forecast needs of the crop for P all need to be taken into account when deciding on fertiliser application. It is strongly recommended that the soil test results should be discussed with your agronomic consultant or fertiliser supplier when making decisions based on test results.

Kits are available from any PIRSA office to test Colwell P, PBI and free lime and cost $90.00 for four tests.

AUTHOR: Rachel Trengove, Soils & Land Management Consultant
CONTACT:  Rural Solutions SA 1300 364 322.

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