Soil porosity characteristics and water movement under zero tillage in silty soils in Argentinian Pampas

The objective of this study was to identify pore characteristics (quantity, distribution, stability and orientation of pores) that condition water dynamics under continuous zero tillage (ZT) on silty soils of the Argentinian Rolling Pampas. Soil properties were analyzed under continuous chisel plough (CP) and ZT treatments from three trials with different duration and crop sequence. The following soil properties of A horizon were analyzed from 0 to 0.05 and from 0.10 to 0.15 m depths: bulk density, pore size distribution, pore origin distribution, aggregate stability index, infiltration rate and organic matter. Preferred pore orientation was carried out by introducing the sampling cylinders in vertical and horizontal direction and then discriminating by pore size. Total porosity of A horizon under CP was 3.5% higher than under ZT. This could be attributed to macro and mesopores as well as to structural porosity. Aggregates were 30% more stable in ZT than under CP in the top at 0.05 m due to a 21% increase in organic matter. However, infiltration rate was not improved under ZT. Under ZT, there was a tendency of macropores to become orientated in parallel to the soil surface. This horizontal preferential macropore orientation was critical in determining water infiltration. The susceptibility to stratify the structure in the first centimeters of the soil under ZT was attributed to the predominance of soybean in the crop sequence. (c) 2005 Published by Elsevier B.V.