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Effect of Prewetting and Incubation of Soil on Aggregate Analysis¹

ABSTRACT

Data are presented for the water stability of air-dry, stored, field soil aggregates as influenced by prewetting treatments made respectively 24 hours and 5 minutes prior to wet sieve analysis. Wetting the samples to moisture equivalent 24 hours prior to wet sieving, and then incubation, increased the water stability of samples significantly over samples wetted to the same moisture percentage 5 minutes before wet sieving. The prewetting treatments, when applied to soils from plots in two crop rotation experiments, placed the soils from the various rotations in different order of water stability, depending on whether the 24-hour or 5-minute prewetting period was used. As an extreme example, the 24-hour prewetting period, resulted in the highest water stability for soil from a corn-oats rotation; whereas the soil from this same rotation, when prewetted for only a 5-minute interval, resulted in the lowest water stability, of the several rotations.

The cause of the increase in aggregation by the two methods was investigated by superimposing a number of additional treatments on the basic wetting treatments. It was found that microbial activity during the 24-hour wet incubation period contributed very little to the increase. The increase was attributed to the hydration of the clay particles with a resulting removal of planes of weakness in the dried aggregates. It is suggested in field studies, that a prewetting treatment be chosen which reinstates the soil to a structural condition that most likely exists, at a time, when the particular physical property of interest of the soils is of concern.

¹ Journal Paper No. J-1767 of the Iowa Agr. Exp. Sta., Ames, Iowa. Project No. 739. Paper presented before Division I, Soil Science Society of America, Cincinnati, Ohio, Nov. 20, 1952.

² Assistaut Professor of Agronomy, Soils.

Received for publication August 24, 1953.