Chapter VIII

SOIL AND SUBSOIL; CAUSES AND PROCESSES OF DIFFERENTIATION
HUMUS, 120.—Soil and Subsoil ill-defined, 120.—_The
Organic and Organized Constituents of Soils_, 120.—Humus
in the Surface Soil, 120.—Soil and Subsoil; Causes
of their Differentiation, 121.—Ulmin Substances or
Sour Humus, 122.—Sour Soils, 122.—Cultivation Induces
Acidity, 123.—Humin Substances, 123.—Porosity of Humus,
124.—Physical and Chemical Nature of the Humus Substances.
Table, 124.—Chemical Nature, 125.—Progressive Changes and
Effect on Soils, 126.—The Phases of Humification, Wood to
Anthracite; Table, 127.—Amounts of Humus and Coal formed
from Vegetable Matter, 128.—Figure, From Port Hudson Bluff,
128.—Conditions of Normal Humification, 129.—Eremacausis in
the Arid Regions, 129.—Black Earth of Russia; Kosticheff’s
Table, 130.—Losses of Humus from Cultivation and Fallowing,
131.—Estimation of Humus in Soils; Unreliability of
Combustion Methods, 132.—Grandeau Method, “Matière Noire,”
132.—Amounts of Humus in Soils, 133.—Humates and Ulmates,
134.—Mineral Ingredients in the Humus, 134.—Functions
of the Unhumified Organic Matter, 135.—The Nitrogen
Content of Humus, 135.—Table for Arid and Humid Soils,
136.—Decrease of Nitrogen Content in Humus with Depth,
138.—Table, Russian River Soils, 139.—Influence of the
Original Material upon the Composition of Humus, 139.—Table
of Snyder, 139.—Effect of Humus in rendering Mineral Plant
Food Available, 140.