Soil is formed by the weathering of rocks into particles and humus from decaying plants and animals. Different types of soil like clay, sand, and loam contain various sizes of rock particles and amounts of humus that determine their suitability for agriculture. Modern farming utilizes machinery, fertilizers, irrigation, and crop rotation to replenish nutrients in soil and maximize its productivity, supporting a high standard of living.

1. SOIL AND AGRICULTURE
The basis of man’s existence on earth is the soil under his feet. The soil makes possible
the growth of plants; these are eaten by man, or by animals which, in turn, become sources of our
food supply. Every bit of food which the human race consumes, with the exception of small
amount derived from the sea, comes directly or indirectly out of ground. In addition, the soil
yields other important products, such as lumber, cotton, tobacco, etc. it is evident, then, that the
study of the soil and of the methods used to cultivate it is of the greatest importance. Farming has
rightly been termed the basic industry of mankind.
Down to comparatively recent times, farming was handicapped by primitive methods and
by and ignorance of fundamental principles. Modern science, however, over has overcome these
conditions and has made a tremendous contribution to increase the productivity of agriculture.
The farmer of today is aided by elaborate and efficient machines. He makes use of scientifically
prepared fertilizers, of irrigation, and of specially developed varieties of seeds. In addition,
science has taught the farmer to employ methods by which he may protect the fertility of his soil.
The productivity of our farm is one of the basic reasons for the comparatively high standard of
living prevailing in the United States.
Soil and Characteristics
What is soil? In the previous chapter we learned how to forces of water, wind and ice, together
with certain chemical agents, have combined to reduce rock to a powder form. The fine product
of this process of weathering is soil. When the ordinary soil was examined with magnifying
glass, it is found to be composed of hard, lustrous, irregularly shaped particles that are dark, soft
and non-lustrous. The first are tiny fragments of rock; the second are bits of decayed plant and
animal matter, called humus. In addition to these two materials, a soil well adapted for farming
must contain water, air and microscopic organisms known as bacteria.
Types of soil. Soils are classified in accordance with the size of their particles of rock, and with
their content of humus.
1. Gravel soil is made up of coarse rock particles and practically no humus. It does not
hold water readily, and is unsuited for agriculture.
2. Sandy soil consists of medium-sized rock particles with little or no humus. Although it
can hold only a relatively small amount of water, it has certain definite advantages, and
with proper cultivation, may be made to yield a good return of certain crops, such as rye.
3. Clay Soil contains extremely fine rocks particles and some humus. One serious
disadvantage of this type of soil that water can penetrate it only very slowly. In addition,
it tends to form mud, which cakes, thus excluding air.
4. Loam consists of rock particles of various size, together with an abundance of humus.
In color, it is a rich dark-brown, almost black. Loam is the soil that characterizes the
world’s most productive farming areas. Gravel loam and sand loam contain more of the
coarser rock particles, while clay loam and silt loam are made up mostly of the finer
particles. Each variety is particularly well adapted for certain crops.
How Soils Hold Water. Water is known to adhere to the surface of soil particles in the form of a
thin film. The more particles there are in a given quantity of soil, the larger the total surface, and
consequently the more water retained by that soil. Weight for weight, the total surface presented
by clay is enormously greater than that presented by gravel, and therefore clay holds more water

2. than does gravel. Sand and common soil, as shown by the experiment, are between these two
extremes in their ability to hold water.
Although clay soil holds water so well, it is not necessarily the best type of soil. Too
much water in the soil may result in shutting out the air. Furthermore, clay soil, since it dries
slowly in the spring, tends to be a cold soil, and is therefore unsuited for early planting. On the
other hand, a sandy soil, being drier, is known as a warm soil, and affords ideal conditions for an
early spring garden.
Elements Needed by Plants. There are dozen or more chemical elements which growing plants
need as food substances. Of these, one or two are obtained from the air, but the rest must be
provided by the soil. The most important of such substances are nitrogen, phosphorus,
potassium, calcium, and magnesium. Large quantities of these elements are removed from the
ground by plants, and they must be replaced if the soil is to retain its fertility. The methods most
commonly used for this purpose are: (1) the addition of decaying organic matter (manure); (2)
the addition of commercial fertilizer; and (3) the rotation of crops.
Phrase
Basis of man’s existence
Scientifically prepared fertilizers
High standard of living prevailing
Hard, lustrous, irregularly shaped particles
Tiny fragments of rocks
Bits of decayed plant and animal matter
A soil well adapted for farming
Medium size rock particles
Rich dark-brown