these different layers is very important to agriculture. Soils
which have a brown hardpan stratum underneath the surface
are quite different from those having a clay or sand under-
neath. These layers serve as a basis of classification and nam-
ing of soils.
 Among the other important soil variations are differences in
available plant food elements or fertilizer nutrients such as
nitrates, phosphates, potash and others. These variations are
inherent properties of the soil and further explains why some
areas are productive and others are not. Many plants grow
normally without the aid of man, but most of our economic
plants require special cultivation and care for satisfactory pro-
duction. Such crops as corn, cotton, sugar cane, beans, cab-
bage, celery and tomatoes require a certain amount of inter-
tillage, sometimes referred to as cultivation, while oats, barley,
rye, clover and such crops, planting is the essential operation.
Tree crops do not require as much tillage as annuals.

 Crops which require inter-tillage or cultivation are usually
hard on the soil. When the land is plowed and changed from
its natural condition it is more subject to erosion and leaching
losses. Cultivation increases the rate of availability of the plant
food elements, thereby increasing the productive power of the
soil. But it also opens the way for erosion losses as may be
seen on the slopes of many farms in the South. (See Plates
II, III, and IV)

 EARLY FARMERS USED POOR SOIL CONSERVATION
 MEASURES
 During the early history of America, most crops were grown
without a great deal of regard for soil conservation. Land was
cleared and cropped until the surface soil had become depleted
and often eroded; then new land was acquired and the process
repeated. This type of farming has resulted in the loss of so
much of the soil, largely through erosion processes, until at
the present time over fifty per cent of the cultivated land in
the United States has been affected by erosion losses. It has
been conservatively estimated that over fifty million acres in