CONICET | Buscador de Institutos y Recursos Humanos

Soil volumetric changes have been seldom studied in seasonally ponded soils. Those of the Flooding Pampa (Argentina) have relatively low contents of expansible clay, but noticeable swell - shrink potential. We studied the causes of this discrepancy in a Natraquoll and a Natraqualf of the region. The relationship between the reciprocal of bulk density (i.e. soil specific volume), n, and water content, q, was determined in the laboratory (clod shrinkage curves) and in the field (repeated core sampling). The clods of both Ap horizons showed two distinct (i.e. zero and normal) shrinkage zones, while those of both Bt horizons only showed normal shrinkage that is characterised by equal variations of soil n and soil q (n = d n / d q @ 1) throughout their water content range. These results clearly contrast to those found in the field, where the slope, n, was as high as 1. 47 1.48 at surface horizons, and 1.93 1.98 in Bt horizons. Thus, these volumetric changes can be considered abnormal. The specific volume of air-filled pores did not increase during drying, as usual, but during wetting. This resulted at the maximum swelling point in air-filled porosities as high as 0.24 - 0.34 v/v, and 0.35 v/v in surface and Bt field horizons, respectively. Soil swelling was related to air entrapment, likely under pressure, ahead of the advance of two field wetting fronts. They are caused by: a) fast water table rises during winter-spring periods; and b) surface ponding with rain water; or c) both factors acting simultaneously.

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