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.
Phosphorus availability in a low pH highly weathered soil as affected by added salts
