Composition, fabric, and porosity of an Arenic Haplustalf of Northeast Thailand: relation to penetration resistance.

Sandy uplands of northeast Thailand have subsoils with high resistance to root penetration that varies even though there is no variation in bulk density (Db). The objective of this study was to determine how the resistance to penetration in these soils is related to the size, mineralogy, and arrangement of the elementary particles. We studied the tilled topsoil (Ap horizon), upper subsoil (E horizon), and lower subsoil (Bt horizon) of an Arenic Haplustalf. Results showed similar skeleton grain-size distribution in the three horizons. Results also showed similar Db in the E and Bt horizons, and higher resistance to penetration in the E horizon. Dispersion of the material indicated the absence of a cementing agent between elementary particles. The <2-µm fraction content was 37, 53, and 88 g kg-1 in the Ap, E, and Bt horizons, respectively, and that fraction was about one-third quartz. The clay minerals were kaolinite with a small amount of swelling 2:1 clays in the Ap and Bt horizons. Mercury porosimetry and scanning electron microscopy (SEM) showed a closer arrangement of the sand resulting in less interparticle porosity in the E horizon than in the Bt horizon. The smaller interparticle porosity in the E horizon was compensated for by greater macroporosity resulting from biological activity, thus explaining the similar Db in the E and Bt horizons. Despite similar Db, the greater penetration resistance in the E horizon results from a close packing of sand particles that restricts the displacement of soil particles during penetration by probe or roots.