The status of phosphorus and other fertility parameters in soils of Puerto Rico

Abstract

There is a need to quantitatively assess the soil fertility status of tropical soils. Descriptive summaries help describe the effectiveness of liming programs, nutritional limitation in soils and the relative risk of off-field nutrient transport. A database of 1,168 soil test results collected from 1989 to 1999 from nearly 400 cultivated farms in Puerto Rico was used. Samples were analyzed for pH, organic matter (Walkley-Black method), extractable phosphorus (P) (Olsen and Bray 1), and exchangeable bases (NH₄OAc method) by a commercial laboratory. Thirty-six percent of the samples had acidity problems (pH <5.5). Twenty-three percent of the samples had low organic matter content (<20 g/kg), and 16% had high category (>40 g/kg) values. Fifty-three and 56% of the samples showed a need to fertilize with magnesium (Mg) and potassium (K), respectively, because they had values below the suggested critical levels of 2.5 cmolc/kg for soil exchangeable Mg and of 0.4 cmol_c/kg for K. On the basis of current soil fertility criteria, P fertilization would be required in 69% of the samples with pH less than 7.3, but only in 28% of the samples with pH greater than or equal to 7.3. Although the soils grouped with pH >7.3 had a greater proportion of samples in the "extremely high" soil test P category, the potential environmental impact may be lessened because the climatic and topographic conditions where these soils occur favor less runoff. Follow-up studies are needed to assess the spatial variability and the temporal dynamics of the nutritional status of soils of Puerto Rico.