The distribution of phosphorus in forest and savannah soils of the Gold Coast and its agricultural significance

Abstract

The amount of phosphorus in composite surface samples and profile samples from a range of agriculturally important soils in the forest and savannah regions of the Gold Coast has been determined. The total phosphorus in each sample has been divided into an acid-soluble, an inorganic alkali-soluble, an organic and a residual fraction. On the surface samples these fractions, together with the soil pH, organic carbon and Bray ‘quick-test’ available phosphorus, have been related to superphosphate responses in the savannah region. The following conclusions have been reached: Surface samples The total phosphorus in the soils is low by comparison with other parts of the world, but its distribution among the soil fractions is normal for slightly acid soils. Soils developed over basic rocks have more total phosphorus, and a lower proportion of acid-soluble phosphorus than soils over quartzose rocks. Forest soils have more total phosphorus and more phosphorus in each of the fractions than savannah soils. The organic phosphorus content is closely related to the organic carbon content. The C/P ratio averages 233 in forest and 247 in savannah, and this is considerably higher than the world average. The N/P ratio of 21·6 in forest and 19·5 in savannah is similarly high. The C/P ratio, unlike the C/N ratio, is not reduced when long-rested savannah soils are cropped. Soluble phosphate applied in permanent fertilizer trials is found in the acid-soluble, the alkali-soluble and organic fractions. In comparison with the amounts of those fractions already present, the acidsoluble fraction increases most and the organic fraction least. Profile samples In thoroughly leached soils the acid-soluble phosphate decreases sharply, and the alkali-soluble inorganic phosphate rather more slowly with depth. In the forest total phosphorus is greatest in the surface, but in the savannah there is no consistent change with depth. The incompletely leached profiles differ in showing a sharp rise in the acid-soluble phosphorus and the total phosphorus in the subsoil. The level of organic phosphorus falls less than expected down the profile, and the C/P ratio, like the C/N ratio, decreases in the subsoil.