AGGREGATE SIZES AND LAND USE TYPES INFLUENCE THE SOIL AGGREGATE STABILITY OF THE HUMID TROPICAL SOIL ENVIRONMENT

K. S. Are; O. J. Idowu; G. A. Oluwatosin

K. S. Are Institute of Agricultural Research and Training, Obafemi Awolowo University, P.M.B. 5029 Ibadan, Nigeria
O. J. Idowu Extension Plant Sciences Department, New Mexico State University, Las Cruces, New Mexico, USA
G. A. Oluwatosin Institute of Agricultural Research and Training, Obafemi Awolowo University, P.M.B. 5029 Ibadan, Nigeria.

Keywords: Macroaggregates, Management practices, Rainfall simulator, Aggregate size class

Abstract

The impacts of the land use pattern and its management techniques on the size and stability of soil aggregates are still poorly understood. Thus, this study investigated the best aggregate size class suitable for testing soil aggregate stability under different land use types in a humid southwestern Nigeria. Surface soil samples (0–15 cm) were collected under five land use types: continuous arable maize, pasture, coconut, cacao and cassava plantations. The sizes of the tested aggregates were 2–5, 5–10, 10–13, and 13–15 mm. The soil aggregate size classes from the different land use types were subjected to a 5-min. simulated rainfall of 150 mm h^-1. Two sieves with varying aperture sizes (1.0 and 0.5 mm) were used to test these aggregates under the rainfall simulator. The stability of soil aggregates tested on the 0.5 mm sieve did not significantly vary among aggregate size classes, regardless of land use types. However, there were significant variations in the stability of the soil aggregates evaluated on the 1.0 mm sieve between different land use types and aggregate size classes. The order of aggregate stability for soil under pasture (80.4%) > coconut (77.8%) > cocoa (65.9%) > cassava (51.1%) > maize (33.1%). The amount of soil organic carbon in the soils is directly correlated with this trend. The aggregate size classes of 5–10, 10–13 and 13–5 mm did not significantly differ in stability while the 2–5 mm class had significantly lower stability than the other classes. Due to significant high correlations of stability with soil organic carbon, aggregate size classes of 5–10 and 13–15 mm appear to be the best suitable for routine determination of aggregate stability in the study environment.