Aggregate Size Distribution in the Row Zone of Tillage Experiments

Abstract

AbstractA sampling procedure was developed to measure the distribution of soil aggregate diameters in the row zone of row‐cropped corn. At each of 31 locations, 4 preplant tillage treatments were used to obtain different soil conditions in the row zone. Undisturbed soil cores were taken from the 0‐ to 3‐ and 3‐ to 6‐inch layer. Dry aggregate‐diameter distribution and dry bulk density (DB) were measured for the 0‐ to 3‐inch layer (DB only in the 3‐ to 6‐inch layer). Within 6 weeks after planting DB increased, but the logarithm of geometric mean diameter (log GMD) and the dispersion of aggregate diameters (σlog d) changed differently depending on tillage treatment and year of study. DB in the 3‐ to 6‐inch layer did not change. Large differences in these measurements were observed among tillage treatments, but within a year of field trials no treatment × location interaction occurred. Some comparisons of measurements were consistent between years and others were not.DB of the 0‐ to 3‐inch layer increased among tillage treatments as σlog d increased, but decreased less as the log GMD increased. A similar relation was shown in the laboratory using mixtures of aggregate‐diameter separates. These changes in DB were mainly due to modification of the interaggregate void space. In the laboratory an increase in weight fraction of water was observed from increasing σlog d. Hence, both log GMD and σlog d are measurable parameters of soil conditions in beds of aggregates, and may help to explain soil water retention and movement, evaporation losses, seed‐soil contact, and root‐soil contact.