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Micromorphology of Soil Fabric at Tree Root-Soil Interface¹

ABSTRACT

The influence of tree roots on physical properties and spatial arrangement of adjacent soil materials were studied. Soil fabric in the vicinity of tree root-soil interface was observed microscopically using specially prepared soil thin sections from samples obtained from forest soils by a freezing in situ technique. Micromorphological observations showed that within the 0- to 0.4-mm radial interval from the root-soil interface there was a relative decrease in grains and pores > 20µ. An increase in oriented clay, and percentage elongate grains oriented within 30° tangent to the root surface was observed. These features imply that the matrix adjacent to tree roots (within 1 to 2 mm) contain more fine-grained materials, with a consequent pore-size distribution shift toward smaller pore diameters (20–50µ or less). Microscopic measurement of pore size distribution compared closely to aeration porosity values calculated from bulk density, particle density, and moisture content at 60-cm tension data. Percent pores determined microscopically on soil thin sections without reference to distances from soil-root interfaces agreed closely with values measured at a distance interval of 2.0 to 4.0 mm from root-soil interface. Aeration porosity, measured microscopically in the area of close proximity to actively growing roots at time of sampling, was very low.

These data and inferences suggest that physical properties of the soil contiguous to a tree root may differ appreciably from such factors farther away from the root.

¹ Contribution from Agronomy Department, Ohio Agr. Res. & Develop. Center, State Special Project 151, Journal Paper no. 39–69. Columbus, Ohio, This study was supported by Forest Service, USDA, Contract no. A9 fs-6472; presented before Div. S-5, Soil Science Society of America, New Orleans, La., Nov. 12, 1968.

² Formerly, Research Associate in Agronomy, Ohio State University; now, Assistant Professor of Agronomy, University of Kentucky, and Professors of Agronomy, Ohio State University, respectively.

Received for publication March 20, 1969. Accepted for publication January 15, 1970.