FREE ESSAY ON SOIL

SOIL

Soil, superficial covering of most of the earth's land area; an aggregation of
unconsolidated mineral and organic particles produced by the combined action of wind,
water, and organic decay. Soils vary widely from place to place. The chemical composition
and physical structure of the soil at any given location are determined by the kind of
geologic material from which it originates, by the vegetation cover, by the length of
time that the soil has been weathered, by the topography, and by artificial changes
caused by human activities. In nature, changes in soil are gradual, except those
resulting from natural disasters. The cultivation of land, however, deprives soil of its
natural vegetational covering and of much of its protection against erosion by water and
wind, permitting more rapid changes. Agriculturists have had to develop methods of
preventing harmful alteration of soil resulting from cultivation and of rebuilding soil
already detrimentally altered.
Although knowledge of the gross texture of soil is important to engineers who construct
buildings, roads, and other structures on the earth's surface, all the specific
properties of soil are of great concern to agriculturists. For agriculturists, knowledge
of the mineral and organic components of soils, aeration and water-holding capacity, as
well as many other aspects of soil structure, is necessary for the production of
successful crops. The soil requirements of different plants vary widely, and no
generalizations can be made concerning an ideal soil for the growth of all plants. Many
plants, such as sugarcane, require moist soils that would be deemed poorly drained by
wheatgrowers. Characteristics suitable for successful growth of crops are not necessarily
inherent in the soil itself; some of these characteristics are created by successful soil
management.
Nature of Soil 
The primary components of soil are (1) undissolved inorganic or nonliving components
produced by the weathering and breakdown of surface rocks; (2) soluble nutrients used by
plants; (3) various forms of organic matter, both living and dead; and (4) gases and
water required by plants and subterranean organisms.
The physical nature of the soil is determined by the proportions of particles of various
sizes. Inorganic particles in soil range in size from fairly large pieces of stone and
gravel to extremely small particles less than 1/40,000 cm (less than 1/100,000 in) in
breadth. Large soil particles, such as sand and gravel, are mostly inactive chemically,
but small inorganic particles, the chief components of fine clays, serve also as a
reservoir from which nutrients are drawn by plant roots. The size and nature of these
tiny inorganic particles also largely determine the ability of a given soil to store
water, which is vital to all plant growth processes.
The organic fraction of soil is composed of undecayed plant and animal debris, together
with variable amounts of an amorphous organic material called humus. The organic fraction
makes up 2 to 5 percent of the surface soil for many soils in humid regions, but may be
less than 0.5 percent in arid soils or more than 95 percent in peat soils.
The liquid component of soils, called by scientists the soil solution, is largely water
containing a number of mineral substances in solution, as well as comparatively large
amounts of dissolved oxygen and carbon dioxide. The soil solution is highly complex and
is not well understood scientifically; it is primarily important as the medium through
which nutrient materials reach and are absorbed by the roots of plants. When the soil
solution is defective in one or more of the nutrient elements needed for plant growth,
the soil is infertile.
The gases contained in soil are chiefly oxygen, nitrogen, and carbon dioxide. The first
of these gases is important for plant metabolism because its presence is necessary for
the growth of the various soil bacteria and other organisms causing the decomposition of
organic materials. The presence of oxygen is also vital for plant growth, in that its
absorption by the roots of plants is necessary to their metabolic processes.
Types of Soils 
Soils exhibit marked variation in appearance, fertility, and chemical characteristics
depending on the mineral and plant materials from which they have been formed. Color is
one of the simplest criteria for judging varieties of soil. The general but not
invariable rule is that dark soils are more fertile than lighter-colored soils. Darkness
in soil color usually results from the presence of a large amount of humus in the soil.
Sometimes, however, soils that are dark brown or black owe their hue to mineral materials
or to excessive dampness; in such cases, dark color is not an index of fertility. Soils
having a red or reddish-brown hue usually contain a large proportion of iron oxides
(derived from the parent rocks) that have not been subjected to excessive humidity. Red
color in a soil, therefore, is generally an indication that the soil is well drained, not
excessively humid, and fertile. This generalization is particularly true in the
southeastern U.S. but is not always true in other parts of the wor