eChapter Name: Estimation of Available Phosphorus in Soil [Bray’s Method (Bray and Kurtz, 1945) – Acid Soils]
9789390591954
eBook Name: A PRACTICAL MANUAL ON GEOLOGY AND SOILS
by A. S. Mailappa
[Bray’s Method (Bray and Kurtz, 1945) – Acid Soils]
Introduction
The term available phosphorus (P) refers to the inorganic form, occurring in the soil solution, which is almost exclusively ‘orthophosphate’. This orthophosphate occurs in several forms and combinations, and only a small fraction of the total amount present may be available to plants, which is of direct relevance in assessing the P fertility level.
The phosphate concentration in solution is governed by heterogeneous equilibria in which it takes part. This situation can be represented as follows:
P adsorbed in solid phase ↔ P in the soil solution ↔ P precipitated
The phosphorus absorbed by plants from soil solution comes from the soil solution in which it exists as inorganic orthophosphate ions viz., H2PO4 –, HPO4 2– and PO4 3–. The most accessible ion is H2PO4 –, with the greatest activity coefficient, followed by HPO4 2–. The quantity of P accessible to the plants is influenced by a series of soil properties. The relative abundance of these ions is, however, relatively dependent on the soil pH. For soils having a pH between 4.5 and 7.5, ions of H2PO4 – as well as HPO4 2– exist in soil solution. At a pH of 7.2, H2PO4 –and HPO4 2– ions have an equal activity, and, when the pH is strongly alkaline (>8.3), ions of HPO4 2– predominate in solution. Above pH of 9.0, the trivalent ion (PO4 3–) becomes more important than H2PO4 –, but even at a pH of 12, the HPO4 2– concentration is still greater than that of PO4 3–. Soluble phosphorus may be adsorptively retained at the surface of colloidal particles. This retention is more marked when the higher amounts of clay and sesquioxides are present. Relatively, the available inorganic P tends to accumulate in its most stable state under prevailing conditions, thus, in calcareous soils, the available inorganic P would be acid-soluble, whereas, in acid soils the adsorbed P would be more available. In flooded soils, with low oxidation potential, certain forms of P, normally considered as unavailable, for example, iron and phosphate, can be regarded as available. In most soils, the main source of orthophosphate is organic matter unless, of course, direct fertilization, with soluble phosphate, has been made.