Soil porous system changes quantified by analyzing soil water retention curve modifications

Soil water retention curves (SWRCs) relate soil water pressure head (h) to soil water content (θ) and can also be used to find information regarding soil pore distribution. To analyze SWRCs in relation to pore size distribution (PSD), changes due to wetting and drying (W–D) cycles were studied in three different tropical soils (Geric Ferralsol, GF; Eutric Nitosol, EN; Rhodic Ferralsol, RF), using three different treatments: T0, the control with samples not submitted to W–D cycles; T3, samples submitted to three consecutive W–D cycles; T9, samples submitted to nine consecutive W–D cycles. Log-normal PSD equations for each treatment were obtained using the S-theory. For the GF soil, the pressure heads separating structural and matrix domains (hs) were 17.7, 12.2 and 14.7 kPa for T0, T3, and T9, respectively. These values are equivalent to pore radia of 8.4 μm (T0), 12 μm (T3), and 10 μm (T9). For the RF soil, hs values were 8.5 kPa (T0), 20.5 kPa (T3), and 15.1 kPa (T9), equivalent to radia of 18 μm (T0), 7.3 μm (T3), and 9.9 μm (T9); and finally, for the EN soil, hs were 18.1 kPa (T0), 9.1 kPa (T3), and 13.5 kPa (T9), equivalent to radia of 8.2 μm (T0), 16 μm (T3), and 11 μm (T9). It was found that the soil structure presented important changes in PSD due to W–D cycles for all the investigated soils. It was also observed that the W–D cycles increased the Sinf (slope of SWRC) value for the GF soil for all treatments; Sinf did not substantially change in all treatments for the EN soil; Sinf decreased between T0 and T3, and T0 and T9 for the RF soil. According to the S-theory, it is possible to infer that W–D cycles improved the soil structure of GF, made the RF soil structure worse and did not substantially change the EN soil structure.