What is the phosphorus buffering index?

The Phosphorus Buffering Index (PBI) approximates a soil’s ability to ‘fix’ phosphorus (P). Soils with a high PBI quickly and tightly bind fertiliser P making it unavailable to plants. Low PBI soils can’t lock-up much P, leaving most applied P for plant uptake.

Use PBI to improve interpretation of critical Colwell P values. As PBI increases so does the amount of fertiliser P required, as more of the applied P is bound to the soil. Phosphorus is more likely to move deeper into soils a low PBI, more so in high rainfall areas.

Soil PBI is quite stable and only needs testing every few years.

What is low or high PBI?

PBI Category Rating
<15 Extremely low
15 – 35 Very very low
36 – 70 Very low
71 – 140 Low
141 – 280 Moderate
281 – 840 High
> 840 Very high

Source: Moody, 2007

The above PBI ratings will vary depending on cropping region.

What affects soil PBI?

PBI tends to increase as soil gets more clayey. Sands often have a low PBI. PBI can also be higher when soils are:

  • high in reactive iron and/or aluminium
  • calcareous and alkaline.  

Using PBI with the DGT Phosphorus test

In calcareous soils, Colwell P overestimates P so the DGT phosphorus test is recommended.

Research into incorporating PBI and DGT-P, to provide recommended P rates, is underway.



Explanation and use of the phosphorus buffering index

What is the DGT Phosphorus test and when should you use it?

How critical is the critical level?

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