Test starting phosphorus levels during “P” month.

With the rising cost of fertiliser an understanding of the amount and availability of Phosphorous (P) in your soil is essential to maximising the efficiency of your fertiliser applications. 

A simple laboratory analysis of a soil sample from your paddock can tell you the level of phosphorous that you are starting with prior to the cropping season and give you some indication as to its availability. 

During April Rural Solutions SA, in conjunction with the Advisory Board of Agriculture (ABA) and PIRSA via the ACML/ SASPAS laboratory, are offering growers the opportunity to have 4 soil samples analysed for P at a cost of only $90.

The results of this analysis include extractable phosphorous (“Colwell P”), free lime and PBI (Phosphorous Buffering index).

Colwell P is an estimate of the “quantity” of P in the soil potentially available to plants.  The table below provides recommended Colwell P levels for dryland agriculture in South Australia. 

Recommended adequate soil P levels

Colwell P however does not always provide a good indication of “plant-available” P. This is due to the potential for soil processes to combine P with other elements, clays or organic particles in plant unavailable forms (P “tie-up) or leach P from the system. 

PBI can be used to give a measure of potential “tie-up” and leaching and help estimate P availability.

Soils with a high PBI (>280) indicate a strong potential for “tie up” (P unavailable to plants) and would need higher Colwell P levels.  A very low PBI (<70) indicate high potential for P to leach.  The recommended minimum P levels are appropriate for soils with a PBI between these levels (70-280).

Where PBI is 70-280 and Colwell P levels exceed the recommended minimums it may be possible for growers, in consultation with their agronomic consultant or fertiliser supplier, to cut back on P applications. 

“P” month phosphorous testing kits are available from your local Rural Solutions SA/ PIRSA office.  Or call 1 300 364 322

AUTHOR: Brett Masters- Soil and Land Management Consultant, Rural Solutions SA
CONTACT: Rural Solutions SA, Telephone: 1300 364 322 or ACML/SASPAS