| Sumario: | Vertisols and soils with vertisolic characteristics from Entre Ríos province (Argentina) display
a high structural stability when supporting natural vegetation. This is mainly attributed to the
effects of the colloidal fraction, which is characterized by a high proportion of smectitic clays,
exchangeable calcium saturation, and also high organic matter content. However, these soils
are very sensitive to structural decay and show a magnification of degradation symptoms when
cultivated. Thus, intensification of soil agricultural use is the main cause of structure deterio ration. The aim of this work was to evaluate the effect of soil use intensity, with rice as the
main culture on the pore size distribution of a vertisolic soil irrigated with sodic bicarbonate
water. Parameters for assessing structural stability were determined in samples with different
pre-treatments, and, in addition soil properties such as organic matter content, total nitrogen,
soil reaction and exchangeable sodium were routinely analyzed. Soil pore size distribution was
determined using mercury intrusion porosimetry. Increasing rice proportion along the studied rotation caused increased exchangeable sodium and a loss of organic matter content. The for mer result was specifically related to the use of sodic bicarbonate water for irrigation. As a
consequence of the intensification of the rice crop the studied soil exhibited less structural sta bility, thus a weaker pore system. The loss of total porosity was reflected by a decrease in the
percolation level, so that a significant relationship was found between both variables. In addi tion to total porosity decrease, also changes in pore size distributions were observed, meaning
a lost of transmission pores and a relative higher proportion of residual pores.
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