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Some Effects of Aggregate Structure Heterogeneity on Root Growth¹

ABSTRACT

Roots of barley (Hordeum vulgare) seedlings were allowed to grow into soil aggregates about 1 cm in diameter. The aggregates were formed to have densities ranging from 1.4 to 1.8 g cm^-3. The resulting physically different internal structures were used to vary the extent of root proliferation within the aggregate. Aggregates of different densities were mixed so that the roots had equal opportunity to grow into the various physical types of aggregates. Root growth into a given aggregate type was ascertained by tagging certain aggregates with ³²P.

Roots penetrated agregates having dry densities of 1.4 g cm^-3 but were restricted to the periphery of aggregates having a dry density of 1.8 g cm^-3. Root growth within a given aggregate was also affected by the soil strength and pore size distribution within adjacent aggregates. Roots growing in beds of aggregates of uniform density were qualitatively, as well as quantitatively, different from roots growing in beds of aggregates with more than one density. Although not measured directly, the extent of root hair development and the soil volume explored by a given root segment in various physical environments were likely responsible for this difference in root characteristics. The relationship between these data and field observations is given.

¹ Contribution from the North Central Soil Conserv. Res. Center, Corn Belt Branch, Soil & Water Conserv. Res. Div., ARS, USDA, Morris, Minn., in cooperation with the Minn. Agr. Exp. Sta., no. 7177, Scientific Journal Series, and the Iowa Agr. & Home Ec. Exp. Sta.

² Research Soil Scientist, USDA; Assoc. Professor of Agronomy, Iowa State Univ.; Research Soil Scientist, USDA, and Professor, Univ. of Minnesota; and Research Soil Scientist, USDA, and Professor, Univ. of Minnesota; respectively.

Received for publication October 6, 1970. Accepted for publication March 11, 1971.