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A Plinthaquult of the Aripo Savannas, North Trinidad: I. Properties of the Soil and Chemical Composition of the Natural Vegetation¹

ABSTRACT

Aripo Fine Sand (Plinthaquult) occupies depressed topography in a region in North Trinidad where rainfall is over 250 cm annually; consequently, the area is submerged periodically in the wet season. The profile consists of a surface albic horizon of 25 cm consisting of bleached silt to fine sand with little organic matter; this is underlain by an argillic horizon with characteristic red and yellow mottling. On exposure, the mottled areas harden irreversibly to nodules. The profile is very strongly acid with very low base saturation. Exchangeable Al is correspondingly high. The albic layer has 0.4% free Fe₂O₃ in contrast to over 12 times more in the underlying mottled clay layers. Total P is lower than 80 ppm for the whole profile; only the ochreous exterior of mottles have higher P contents. Except in the bleached layer, Al-P is almost absent and Ca-P is absent throughout the profile. Forms of P associated with Fe make up the bulk of the total P in the soil. Truog's reagent extracted only 2 ppm P in the bleached layers and just traces in the clay enriched part of the profile.

The flora of the area consists of short, coarse, tufted grasses, sedges, herbs, shrubs, and insectivorous and parasitic plants. Dominant plant species were analyzed for total and insoluble ash, N, P, K, Ca, Mg, S, Fe, and Al. Compared to levels of these nutrients present in rice (Oryza sativa L. var. Indica) and sweet potato (Impomoea batatas L. var. 049) grown under favorable conditions, all species of the vegetation had very low amounts of plant nutrients. The greatest difference occurred for P, K and S. All the species were high in Fe content but values fluctuated and only in the Gramineae was there any consistency. Many of the species, especially the Melastomaceae, were very high in Al. Soil and plant analysis indicated a soil of extremely low fertility status which was also reflected in the poorness of the vegetation.

¹ Contribution of the Dept. of Soil Science, University of the West Indies, St. Augustine, Trinidad, and Agronomy Dept., University of Illinois, Urbana.

² Senior Lecturer in Soil Science, UWI, and Associate Professor of Soil Mineralogy, UI, respectively.

Received for publication January 2, 1969. Accepted for publication May 12, 1969.