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?
|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.