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Houston Black Clay, the Type Grumusol: II. Mineralogical and Chemical Characterization¹

ABSTRACT

The following analyses are reported on each of the several horizons of five profiles of typical Houston Black clay from widely separated sites in Texas described in Part I: particle size distribution, organic matter, calcium carbonate equivalent, exchange capacity, ethylene glycol retention and pH. In addition X-ray diffraction analyses are reported for the following fractions: silt (0.05 to 0.002 mm.), bulk clay (<2µ), coarse clay (2µ to 0.2µ) and fine clay (<0.2µ). Differential thermal analyses are presented for the <0.2µ fraction.

The average clay (<2µ) content for the 5 profiles varied between the narrow range of 57.7% and 60.8%. Fine clay (<0.2µ) was found to make up as much as 84% of the total clay fraction. The calcium carbonate equivalent varied from a minimum of 5.4% to a maximum of 70.2%. Zones of calcium carbonate enrichment resulting from leaching were not found. Organic matter in the surface horizons (0 to 18 inches) from virgin sites varied from 3.06% to 8.34%. The silt fractions are very largely quartz and calcite. Quartz predominated in the silt fraction from surface horizons, while calcite increased with depth and in three profiles assumed the dominating role in the C horizon. Montmorillonite strongly predominated the clay fraction (<2µ) and based upon X-ray diffraction and exchange capacity studies it occurs in essentially a pure state in the <0.2µ fraction of three profiles. The exchange capacities for the A_1-1 horizons averaged 64 me. while the average for the C₂ horizons was found to be 40 me.

The Houston Black clay has developed from soft calcareous sediments. The mineralogical and chemical measurements indicate that no pronounced weathering of these sediments has occurred.

¹ Contribution from the Texas Agr. Exp. Sta., College Station, Tex., as Technical Article No. 2097. Presented before, a joint meeting of Div. II and Div. V, Soil Science Society of America, St. Paul, Minn., Nov. 11, 1954.

² Assistant Professor of Soil Mineralogy and formerly Professor in charge of Soil Survey, respectively, Agronomy Department, Texas A. and M. College. The authors wish to acknowledge the able technical assistance rendered hy Mr. Eli Whitely.

Received for publication October 22, 1954.