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Forms of Occurrence of Inorganic Phosphorus and Its Chemical Availability in the Limestone Soils of Barbados

ABSTRACT

In the coral island of Barbados the soils at higher elevations where there is greater rainfall are like Terra Rossas and those in lower and drier parts are like Grumusols. There are also soils formed from lagoonal deposits on coral at low elevations which have properties similar to Grumusols. The soils do not respond to phosphorus fertilization. They have about the same level of total phosphorus on the surface, although the level for the Grumusols is slightly higher. In the Terra Rossas total P decreases sharply with depth to less than half the surface values; in comparison there is an increase with depth in the Grumusols.

All the soils are alkaline but the Terra Rossas have many times more free Fe₂O₃ than the grumuso's and this difference seems to determine the chemical forms in which phosphorus occurs. The Terra Rossas have most of the inorganic phosphorus associated with iron, the reductant-soluble fraction being greater than the alkali-extractable fraction; in this respect they resemble highly weathered tropical soils. The Grumusols have most of their phosphorus bonded by calcium and aluminum.

Four chemical methods were used to estimate available phosphorus. Troug's and Bray's reagents were found to be unsuitable since they dissolved free CaCO₃ releasing associated phosphorus, thus giving a false indication of availability. For the Grumusols and soils high in CaCO₃ but low in iron oxides, Morgan's reagent is recommended; for the Terra Rossas where iron phosphates are important, Olsen's method appears to be more suitable. As a result of this study no immediate phosphate fertilization is suggested for the Grumusols but for the Terra Rossas fertilization should be practiced if only to the extent of replacing losses by the crop. Strict antierosion measures are also recommended since most of the soil phosphorus is concentrated on the surface.