AbstractThe constituent minerals in the very fine sand, silt, and clay fractions of some of the important soils of Puerto Rico were studied using petrographic and X-ray methods. It was found that these soils could be divided into two general groups: Those that contained appreciable feldspar and those in which feldspars were totally absent or occurred in minute traces. Study of the profile characteristics showed that the feldspar content of the profile allowed a further subdivision into: Those in which the feldspar decreased with depth; and those in which feldspar increased with depth. The feldspars identified consisted almost exclusively of plagioclase, albite, oligoclase, and andesine, these being the principal constituents, with traces of labradorite and very small traces of orthoclase. The clay minerals identified in the soil studied consist of either kaolin, hydrous mica, or illite, one tentatively identified as beidellite, and montmorillonite, with mean cation-exchange capacities of 10.07, 22.30, 31.41, and 48.00 milliequivalents per 100 gm. of soil, respectively. Some soils contained a combination of two or three of these clay minerals. In the upland soils where feldspar occurred, kaolin was the predominating mineral with varying quantities of hydrous mica and montmorillonite. Where no feldspar occurred in the upland soils, the clay mineral was principally kaolin. The inner-plains soils chiefly contained beidellite, regardless of whether feldspar was present or not. Five soils of the terraces and alluvial fans contained feldspar and one did not, and kaolin was the principal clay mineral. The river flood-plain soils all contained feldspar and beidellite was the predominant clay mineral. The coastal-plain soils contained no feldspar and kaolin predominated. The occurrence of the feldspar in a profile seems to have some bearing on the clay minerals formed by the weathering processes. Where feldspars increased with depth, beidellite seemed to be the ultimate clay mineral formed. Where feldspar decreased, the clay mineral formed was kaolin. By comparing the presence or absence of feldspar with the productivity rating as determined by the Soil Survey of Puerto Rico, it was found that soils containing feldspar had a much higher productivity rating than those from which it was absent, or in which it occurred only in small traces—also that soils containing feldspar had a lower clay content than those which did not. If the available potash content using Hegari sorghum was regarded as a plant index, comparisons showed that soils containing feldspar also contained about 2.2 times more available potash than those lacking feldspar. In general, the results obtained indicate that the most productive soils studied were those that contained fairly large quantities of feldspar, had a low clay content, and the clay minerals of which were mixtures of kaolin, hydrous mica, and beidellite and, in some cases, montmorillonite.
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