CHAPTER VII.

DENSITY, PORE-SPACE AND VOLUME-WEIGHT OF SOILS, 107.—Density of Soil Minerals, 107.—No Great Variation, 107.—Volume-weight most Important, 107.—Weight per Acre-foot, 107.—Air-space in Dry Natural Soils. Figure, 108.—May be Filled with Water, 108.—Effects of Tillage. Figures, 109.—Crumb or Flocculated Structure; Cements, 109.—How Nature Tills, 111.—Soils of the Arid Regions; do not Crust, 112.—Changes of Soil-Volume in Wetting and Drying, 112.—Extent of Shrinkage, 113.—Expansion and Contraction of Heavy Clay Soils. Figure, 113.—Contraction of Alkali Soils on Wetting, 114.—“Hog Wallows,” 114.—Physical Analyses of such Soils. Table, 115.—Crumbling of Calcareous Clay Soils on Drying, 116.—Yazoo Bottom, Port Hudson Bluff, 116.—Loamy and Sandy Soils, 117.—Formation of Surface Crusts, Physical Analyses, 117.—Effects of Frost on the Soil; Heaving; Ice-flowers, 118.