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Electron Microprobe Analysis of Thin Sections of Soil to Observe Loci of Cation Exchange¹

ABSTRACT

To observe the loci of cation exchange in natural soils, (16 cm³) cubes of soil with such micromorphological features as silt and clay varves, isolated clay bodies in a silty matrix, and clay and silt skins on ped surfaces, were leached repeatedly with CsCl. After the soil cubes were saturated with Cs, they were dried and impregnated with a polyester resin.

X-ray images and Cs activity measurements, determined on thin sections with the electron microprobe, demonstrated that Cs sorbed by microstructures enriched with clay was nearly twice that of silty matrices. These observations illustrate that the loci of cation exchange are heterogeneously distributed within the undisturbed soil horizon and are influenced by micromorphological features produced by geologic and pedogenic processes.

Cesium sorbed by weathered biotite sand grains was eight times greater than by clay bodies. The greater exchange capacity of the weathered biotite was attributed to the loss of interlayer potassium and conversion to a vermiculite-like mineral. The smaller exchange capacity of the clay was attributed to the presence of illite, chlorite, and kaolinite, of low exchange capacity, to pedogenic weathering and aluminum interlayer formation in vermiculite and to the presence of 40 to 50% pore space. Although weathered biotite sand grains constituted only 1.6% of the whole soil, they contributed about 15% to the total exchange capacity of Charlton soil.

Contribution from The Connecticut Agr. Exp. Sta., New Haven. Presented before Div. S-2, Soil Science Society of America, New Orleans, La., Nov. 14, 1968. This study was supported in part by the US Atomic Energy Commission under contract AT(30-1)-2955.

² Associate Soil Scientists. The authors are indebted to M. D. McConnell, Research Associate, Geology Dept., Yale University, for preparation of thin sections and assistance with the electron microprobe and to F. C. C. Pedersen for his technical assistance.

Received for publication December 18, 1968. Accepted for publication March 11, 1969.