The effect of aggregate size on water retention and pore structure of two silt loam soils of different genesis

Aggregate size distribution and pore structure affect many soil functions and root growth. The aggregate structure is associated with soil genesis and management practices applied. In this study the effects of various size ranges of aggregates (<0.25, 0.25–0.5, 0.5–1, 1–3, 3–5, and 5–10 mm) and undisturbed soil from the plough layer (0–15 cm) of two types of soils (Haplic Phaeozem and Eutric Fluvisol) of the same silty loam textural group on water retention curves (WRC) and pore size distribution (PSD) were assessed. A greater bulk density and lower humus content characterized the Eutric Fluvisol as compared to the Haplic Phaeozem. The WRC was determined using standard Richards chambers in drying process and expressed as the degree of saturation. Equivalent PSD was derived from the WRC. Resin impregnated sections from the layer of 0–8 cm showed that the Eutric Fluvisol, compared with the Haplic Phaeozem, had coarser pores and aggregates. The degree of saturation in beds of aggregates <0.25, 0.25–0.5 and 0.5–1 mm compared to beds of aggregates 1–3, 3–5 and 5–10 mm was greater at higher values of pressure head for both soils, and for undisturbed soil it was greater for the Haplic Phaeozem than for the Eutric Fluvisol at lower values of pressure head. The inverse relationship was true at higher values of pressure head. The derivative curves of PSD showed that the beds of aggregates and undisturbed soils exhibited multi-peak PSD. The pore radius peaks within the textural (primary) pore system were more defined in beds of aggregates <0.25 mm than in beds of coarser aggregates, whereas in the case of the structural and macropore peaks it was often the reverse. Greater magnitude and narrower shape of the peaks in the undisturbed Haplic Phaeozem compared to the Eutric Fluvisol indicated a more heterogeneous nature of the pore system in the former. The PSD data are discussed in relation to aggregate size distribution and stability of the soil aggregates. The results of this study can be helpful in predicting storage and transmission functions of surface aggregated soils.