Improving on the Prediction of Cation Exchange Capacity for Highly Weathered and Structurally Contrasting Tropical Soils from Their Fine-Earth Fractions

Abstract

The roles of fine-earth materials in the cation exchange capacity (CEC) of especially homogenous units of the kaolinitic and oxyhydroxidic tropical soils are still unclear. The CEC (pH 7) of some coarse-textured soils from southeastern Nigeria were related to their total sand, coarse sand (CS), fine sand (FS), silt, clay, and organic-matter (OM) contents before and after partitioning the dataset into topsoils and subsoils and into very-low-, low-, and moderate-/high-stability soils. The soil-layer categories showed similar CEC values; the stability categories did not. The CEC increased with decreasing CS but with increasing FS. Silt correlated negatively with the CEC, except in the moderate- to high-stability soils. Conversely, clay and OM generally impacted positively on the CEC. The best-fitting linear CEC function (R², 68%) was attained with FS, clay, and OM with relative contributions of 26, 38, and 36%, respectively. However, more reliable models were attained after partitioning by soil layer (R², 71–76%) and by soil stability (R², 81–86%). Notably FS's contribution to CEC increased while clay's decreased with increasing soil stability. Clay alone satisfactorily modeled the CEC for the very-low-stability soils, whereas silt contributed more than OM to the CEC of the moderate- to high-stability soils. These results provide new evidence about the cation exchange behavior of FS, silt, and clay in structurally contrasting tropical soils.

Keywords:

• Cation exchange capacity
• coarse-textured soils
• kaolinitic soils
• pseudo-fine sand
• soil organic matter
• soil structural stability

Acknowledgment

The sponsorship of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of the Government of Japan is gratefully acknowledged. The authors thank the staff of the Teaching and Research Laboratory of the Department of Soil Science, University of Nigeria, for their assistance in the physicochemical analyses of the samples, especially Ngozi Ojionuka, who assisted with the determination of soil pH and cation exchange capacity.