CHAPTER VI.. Physical Composition of Soils, 83.—Clay As a Soil Ingredient,

PHYSICAL COMPOSITION OF SOILS, 83.—Clay as a Soil Ingredient, 83.—Amounts of Colloidal Clay in Soils, 84.—Influence of Fine Powders on Plasticity, 85.—Rock Powder; Sand, Silt and Dust, 86.—Weathering in Humid and Arid Regions, 86.—Sands of the Humid Regions, 86.—Sands of Arid Regions not Sterile, 86.—Physical Analysis of Soils, 88.—Use of Sieves. Limits, 88.—Use of Water for Separating Finest Grain-Sizes, 89.—Elimination of Clay by Subsidence and Centrifugal Method, Hydraulic Elutriation, 90.—Schöne’s Instrument, 90.—Churn Elutriator with Cylindrical Tube, 91.—Figures of Same, 91.—Yoder’s Centrifugal Elutriator, 92.—Number of Grain-sizes Desirable, 93.—Results of such Analyses, 93.—Physical Composition Corresponding to Popular Designations of Soil-Quality. Table, 96.—Number of soil-grains per Gram, 99.—Surface Offered by various Grain-sizes, 99.—Influence of the several Grain-sizes on Soil Texture, 100.—Ferric Hydrate, its Effects on Clay, 100.—Other Substances, 101.—Aluminic Hydrate, 101.—Influence of Granular Sediments upon the Tilling Qualities of Soils, 102.—“Physical” Hardpan, 103.—Putty Soils, 103.—Dust Soils of Washington; Table, Physical Analyses of Fine Earth, 104.—Slow Penetration of Water, 105.—Effects of Coarse Sand, 105.