. Productive soils; the fundamentals of successful soil management and profitable crop production. Soils. 12 SOIL COMPOSITION, CLASSES AND TYPES Silt particles are soil grains medium in size between sand and clay. When moistened, silt has a velvety "feel, '^ but not sticky like clay. Sand, silt and clay as used here refer simply to size of individual soil grains and nothing more. Organic matter in most soils occurs largely as a well-decomposed residue, black or dark in color and coating the soil grains. Fre- quently plant fiber of recent growth can be distinguished. Humus may be defined a
. Productive soils; the fundamentals of successful soil management and profitable crop production. Soils. 12 SOIL COMPOSITION, CLASSES AND TYPES Silt particles are soil grains medium in size between sand and clay. When moistened, silt has a velvety "feel, '^ but not sticky like clay. Sand, silt and clay as used here refer simply to size of individual soil grains and nothing more. Organic matter in most soils occurs largely as a well-decomposed residue, black or dark in color and coating the soil grains. Fre- quently plant fiber of recent growth can be distinguished. Humus may be defined as a black, waxy, complex substance Coarse sondj tedium SaU Fine sandi Silt \ne silfi . Coahse cjlay. "&: Very Fine day FlQ. 5.—^Diagram illustrating the sizes of soil gr^^ins The coarsest particles grade into the finest coating the soil grains, and which is derived from partial decay of organic matter. All humus is organic matter, but all organic matter is not humus. The term humus is sometimes used as a general term meaning organic matter. In these chapters we shall use the term ''organic matter" since that is more readily imderstood. Soil is a Complex Medium.—^What we call soil is something more than a mere mixture of sand, silt, clay and organic matter. It is a composite, the framework of which is mineral matter. Aside from organic matter it contains: (a) Wal&r (moisture) which in reahty is a dilute solution con- taining weak acids and small amounts of any soluble substance found in the soil. (6) Soi7 organisms—bacteria, fungi and worms. These have. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Weir, Wilbert Walter, 1882-. Philadelphia London, J. B. Lippincott company
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