Soil phosphorus: its measurement, and its uptake by plants

Abstract

Phosphorus (P) is the most important nutrient element (after nitrogen) limiting agricultural production in most regions of the world. It is extremely chemically reactive, and more than 170 phosphate minerals have been identified. In all its natural forms, including organic forms, P is very stable or insoluble, and only a very small proportion exists in the soil solution at any one time. Plant-available P may be considered in either its quantitative or intensive dimension. The quantity of available P is time-specific and crop-specific, because it is the amount of P that will come into the soil solution and be taken up by the crop during its life cycle. The intensity of available P (availability) is most easily identified with its concentration in the soil solution.

The soil property controlling the relationship between the solid phase P and its concentration in solution is known as the buffering capacity. The solid phase P involved in this relationship is only a small proportion of the total P, and is known as labile P. It is usually measured by isotopic exchange, but this exchangeable P component does not include the sparingly soluble compounds that also replenish the soil solution as its concentration is depleted by plant uptake. The buffering capacity is the ability of the soil solution to resist a change in its P concentration as P is removed by plant uptake or added in fertilisers or organic materials.

Buffering capacity is synonymous with sorptivity, which is a preferable term in the context of the reactivity of P fertiliser with soil. It is usually measured from an adsorption isotherm. By fitting a suitable equation, such as the Langmuir, the total sorption capacity as well as the sorption strength can be determined. Both parameters are important in understanding P availability in soils.

Buffering capacity has a major effect on the uptake of labile P because it is inversely related to the ease of desorption of solid phase P and its diffusion. Available P therefore is a direct function of the quantity of labile P and an inverse function of buffering capacity. This has been demonstrated in plant uptake studies. Similarly, the most effective methods of measuring available P (soil tests) are those which remove a proportion of labile P that is inversely related to buffer capacity. Soil tests which measure the concentration of P in solution actually measure availability rather than available P, and their efficacy on a range of soils will depend on the uniformity of the soils" buffer capacities.

The most effective soil test usually consists of an anionic extractant. Acidic lactate or fluoride have been found most effective in New South Wales, on a wide range of soils, except calcareous soils which neutralise the acidic component (usually hydrochloric or acetic acid) of the extractant. Sodium bicarbonate (pH 8 · 5) has been found effective on calcareous soils and is widely used throughout the world. It has proved unreliable on NSW soils, and may need more thorough evaluation on non-calcareous soils in other parts of Australia.