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Aggregate Size Distribution in the Row Zone of Tillage Experiments¹

ABSTRACT

A 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 (D_B) were measured for the 0- to 3-inch layer (D_B only in the 3- to 6-inch layer). Within 6 weeks after planting D_B increased, but the logarithm of geometric mean diameter (log GMD) and the dispersion of aggregate diameters (_{log d}) changed differently depending on tillage treatment and year of study. D_B 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 x location interaction occurred. Some comparisons of measurements were consistent between years and others were not.

D_B of the 0- to 3-inch layer increased among tillage treatments as _{log 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 D_B were mainly due to modification of the interaggregate void space. In the laboratory an increase in weight fraction of water was observed from increasing _{log d}. Hence, both log GMD and _{log 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.

¹ Contribution of the Corn Belt Branch of the Soil and Water Conservation Research Division, ARS, USDA, in cooperation with the South Dakota Agr. Exp. Sta. and the Minnesota Agr. Exp. Sta. Presented before joint sessions of Div. S-1 and S-6, Soil Science Society of America, Kansas City, Mo., Nov. 16, 1964.

² Soil Scientists, Soil and Water Conservation Research Division, ARS, USDA, headquarters at Morris, Minn. and Ames, Iowa.

Received for publication March 22, 1965. Accepted for publication May 4, 1965.