There are many soil tests that measure extractable phosphorus. All of them aim to measure the amount of phosphorus available for plant growth (extractable phosphorus). As such, their objective is to be useful as a predictor of likely response to fertiliser phosphorus applications.
Regardless of the test used, it is important to understand the phosphorus buffering index (PBI) of your soil. PBI is a measure of a soil’s ability to readily bind phosphate from soil solution.
The PBI is a one-step test adopted as the national standard method for measuring the phosphorus sorbing capacity of soil. Phosphorus sorption determines the partitioning of phosphorus between the solid and solution phases of the soil (i.e. whether or not the phosphorus is available to be taken up by plant roots). This characteristic is useful for interpreting some tests of plant-available phosphorus in soil. In particular, it allows us to predict the Colwell extractable phosphorus value of a soil that corresponds with maximum pasture growth (critical Colwell P).
