Comparing the Macro-aggregate Stability of Two Tropical Soils: Clay Soil (Eutric Vertisol) and Sandy Loam Soil (Eutric Leptosol)

摘要

The objective of this work was to compare the macro-aggregate stability of two tropical soils and construe causal relationships between aggregate stability as expressed by both the Coefficient of Vulnerability (Kv ) and Water Stable Aggregates (WSA) with some soil chemical properties. A total of twenty samples were collected, ten from each soil type with two replications were conducted and analyzed. The average values were then used to estimate both parameters. Aggregate stability was measured using both the modified Le Bissonnais wet sieving method and the slaking test that compared the resistance of macro-aggregates to mechanical breakdown. The WSA for the sandy loam soil (E. Leptosol) was about 55.08% as compared to that of the clay soil (E. Vertisol) at 38.07%. Conversely, the Kv for E. Leptosol was about 1.89 and that of the E. Vertisol was about 2.73. The Kruskal-Wallis test for equal medians [(chi)2=9.14, p-value=0.002] revealed that the effect of wet sieving on macro-aggregate stability for both soils was significantly different. Similarly, the pairwise Wilcoxon rank sum test for E. Vertisol (W=30; p-value=0.799) while for E. Leptosol (W=28; p=0.959) within and between group samples of both soils revealed similar effect of wet sieving on macro-aggregate stability. Noticeable differences were also observed in the degree and rate of slaking. Slaking was faster in E. Vertisol than in the E. Leptosol due in part to the comparatively lower levels of calcium and sulfate in the former. Using the Principal Component Analysis (PCA) as shown by the scatter plot further revealed that both sand and clay components as well as phosphorus (P), magnesium (Mg) and calcium (Ca) had positive effects on macro-aggregate stability for the E. Leptosol whereas only calcium and sulfate had both positive effects on macro-aggregate stability for the E. Vertisol. To a larger extent, pH, humus, sulfate (SO4), iron (Fe-III) and sand did not favor the Kv for both soils.