Physical-hydric properties of Oxisols as influenced by soil structure and clay mineralogy

Abstract

The soil water retention curve (SWRC) and soil hydraulic conductivity (K) are important soil hydraulic properties. In Oxisols, the degree of microaggregation depends on the amount of iron and aluminum oxides, kaolinite and biological activity engaged in modifying the pore size distribution (PSD) of the soil porous system. The objectives of this study were (i) to characterize four Brazilian Oxisols in terms of SWRC and PSD, using undisturbed (structural SWRCs) and disturbed (textural SWRCs) soil samples; (ii) to apply the van Genuchten-Mualem (vGM) model to the prediction of K and (iii) to correlate the classes of porosity and saturated hydraulic conductivity (Ksat) with the physical, chemical and mineralogical properties. Four soils were studied: Xantic Kandiustox, Rhodic Haplustox, Anionic Acrustox and Typic Hapludox. The soil samples were collected from the Bw horizon of each soil. The structural SWRCs were determined by Haine’s funnels and Richards’ chambers, whereas the textural SWRCs were determined by dew point meter. Ksat was evaluated by constant head permeameter and, additionally, soil chemical and mineralogical analyses were prepared. It was concluded that: (i) the four soils showed bimodal PSD; (ii) the four soils presented the maximum frequency of equivalent radius in the class of macropores (structural SWRCs) with rmax varying between 43.37 and 88,98 μm, and in the class of cryptopores (textural SWRCs) with rmax varying between 0.012 and 0.014 µm; (iii) the higher the Ksat, the higher the K predicted by the vGM model over the entire soil water content range of the structural SWRCs, except for Xanthic Kandiustox and Typic Hapludox in which this phenomenon occurred closer to saturation; (iv) the K values estimated by the vGM model from textural SWRCs were very low, varying approximately from 10−25 to 10−11mm h−1 for the four soils; (v) the degree of microaggregation of Rhodic Haplustox and Anionic Acrustox resulting from the high levels of Fe and Al oxides, leading to the formation of greater inter-aggregate space, had an influence on the high Ksat and showed that the mineralogy of the clay fraction was decisive in the physical-hydric behavior of these soils.