Phosphorus buffering index (PBI) is a single point measure of a soil's ability to sorb phosphorus (P). When used in conjunction with Colwell P it can assist in the estimation of P application rates for pasture production yield targets/relative yields (RY) through the application of P response curves.

It has been identified that soils with low PBI (sand) pose a high risk of P loss via leaching. Defining threshold soil PBI levels where P leaching increases aids in management of nutrients in agriculture.

A trial was conducted with ryegrass and clover using soils with ten PBI levels ranging from 2.5 (extremely low, sand) to 800 (very high, clay loam) and ten P application rates for RY of 0-+99%. Biomass, leachate and P concentrations were measured.

Phosphorus application rates based on desired RY and the soil PBI influenced the amount of P leached. Soils with a PBI >=50 displayed little difference in P leached between PBI groups up to 95% RY, leaching <0.5mgP/kg. Soils with a PBI <50 increased P leached with decreasing PBI and increasing P application rates, leaching losses were >6 mg P/kg soil RY of 95%.

Soils with a PBI >50 leached P (2-4 mg P/kg) at extremely high application rates (RY+99), indicating that high PBI soils can still leach P if not maintained at agronomic targets.

The trial identified that leaching of P becomes a key loss pathway for soils with a PBI <50. To reduce leaching losses, RY would need to be well below current agronomic targets for extremely low PBI soils (likely an RY <=70). Therefore, even in situations where nutrients are at evidence-based critical values, soils with a PBI of <50 are likely to lose P to the environment via leaching.