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Colloidal Properties of Soils from Western Equatorial South America¹

ABSTRACT

Equatorial soils of Ecuador and Peru were studied by chemical, differential thermal, and X-ray spectrographic methods of analysis with the aim of clarifying their classification and fertility properties.

A classification key permits the presentation of the soils and their properties according to major soil groups.

In the humid tropical regions kaolin clays are common, but there is also evidence of juvenile soils. Immaturity is indicated by high contents of exchangeable bases, presence of clays with high cation exchange capacities, evident lack of crystallinity of the clays, and pH values which are relatively low for the existing degree of base saturation.

Dark colored acidic Andean soils possess high phosphate retaining capacities and high exchangeable aluminum. Their C/A capacity ratios are less than 1.

Soils of the drier equatorial regions contain crystalline clay minerals of the 2:1 lattice families.

¹ Contribution from the Department of Agronomy, North Carolina Agricultural Experiment Station. Published with the approval of the Director as paper No. 400 of the Journal Series. Presented before Section II, Soil Science Society of America, State College, Pa., August 30, 1951.

² Research Associate Professor and Associate Professor.