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Physical and Chemical Properties of Soil Aggregates in a Brunizem Soil¹

ABSTRACT

A surface sample of Sharpsburg silty clay loam was separated into aggregate fractions of diameters 4760 to 2380µ, 2380 to 1190µ, + ... +, 37 to 18.5µ by means of rotary sieves and elutriators. A similar separation was made for primary particles. Each fraction of aggregates was analyzed for chemical and physical properties. The physical properties of aggregates were compared with those of primary particles.

Data on water stability of aggregates show that as the diameter of aggregates decreased the stability increased. Particularly, aggregates of diameter 74 to 37µ and 37 to 18.5µ were different from the other fractions. Some of the differences in properties were: higher degree of aggregation, percentage of primary particles, available phosphorus, and exchangeable Ca⁺⁺; lower moisture retention, percentage of organic matter, total nitrogen content, and exchangeable H⁺.

At complete water saturation, aggregates and particles wetted under partial vacuum held greater amounts of water on a volume basis than did aggregates and particles wetted under atmospheric pressure. However, as tension was applied, more water was released from the aggregates and particles wetted under partial vacuum than under atmospheric pressure. At increased tensions, the differences between the two methods of wetting were less marked. At tensions of 0.33 atm. and higher, the moisture contents were nearly identical.

Aggregates treated with VAMA soil additive, under both vacuum-wetting and atmospheric-wetting, released more water at successive tensions than did those without the additive.

Contribution from the Department of Agronomy, Nebraska Agr. Exp. Sta., Lincoln, Nebr. Project was supported by NC-17 Regional Funds on Soil Structure. Published with the approval of the Director as Paper No. 763, Journal Series, Nebraska Agr. Exp. Sta.

² Assistant Professor of Agricultural Engineering, and Professor of Agronomy, respectively.

Received for publication August 21, 1956. Accepted for publication October 23, 1957.