eluding soybeans, velvet beans, crotalaria, cowpeas, and annual
'varieties of lespedeza) ; peanuts when pastured, perennial grasses
such as centipede, Dallis, Bermuda, and carpet grass; winter
cover crops, such as rye, barley, oats, and small grain mixtures.
Furthermore tree crops planted since 1934 are designated as
soil conserving crops.

 SOIL IMPROVING CROPS
 Experience has taught that some plants may even improve
the soil by preventing leaching or erosion losses or by securing
through the roots atmospheric nitrogen. Such crops as cow
peas, velvet beans, crotalaria, beggar weed and clover, are gen-
erally known as leguminous plants. They are often grown as
cover crops and when they, or any other crops are grown especial-
ly for the purpose of improving or conserving the soil, without
being grazed or sold, they are designated as soil building crops.

 SOIL EROSION
 Although losses through leaching processes and the sale of
crops reduce the natural productivity of the soil, by far the
greatest soil loss is erosion. The annual erosion losses of plant
food in the United States has been conservatively estimated at
four hundred million dollars ($400,000,000) annually. (3) It is
further estimated that the aggregate loss in terms of plant food
soluble and potentially soluble is sixty times that of the nutrients
added in fertilizers. (4) (See Plates II, III,. and IV.)
 While erosion may be caused by either wind or water, that of
water is more general and more severe than that by wind. In
both cases the losses usually begin when the native vegetation,
either grass or trees, is removed. (See Plates III, and IV.)
Wind erosion is most severe in arid regions where the native
sod has been broken as in the Dakotas and other western states.
From a national standpoint the grass sod is more important
for agriculture than cultivated crops in such regions. Breaking
the sod reduces the binding properties of the soil and permits
severe wind erosion and even water erosion during certain
periods of heavy rainfall.
('). Circular 33, Soil Conservation Service-U.S.D.A.
(4). United States Department of Agriculture. Yearbook 1934.