74 BULLETIN 355, U. S. DEPARTMENT OF AGRICULTURE. 
that a considerable part of this black humus is of a very resistant 
character, and after the more decomposable portion has been used up 
by a few years' cropping, the nitrogen does not become available 
rapidly enough to supply the needs of growing crops. Under these 
conditions nitrogen must be supplied by the growing of legumes, 
the use of barnyard manure, or in some other way. The amount of 
lime occurring in these soils is also quite variable. As a rule, soils 
which were formed in standing bodies of water contain a fair amount 
of this material, secreted by shell animals and deposited as the clay 
formed, and also derived from streams running into such bodies of 
water, which very commonly carry more or less lime. Nevertheless, 
clay soils of this character are often found which are very low in 
lime carbonate, or are even acid, so that lime must be used. 
Erosion (Ref. No. 2, pp. 50-54; 3, p. 14). — The erosion of soil is a 
cause of much loss of fertility, and on hillsides, especially of clay soils, 
it often nearly ruins the fields eroded. Sandy soils are not so readily 
eroded as clay, because the coarser texture permits the water, except 
in beating rains or on frozen ground, to pass down into the soil instead 
of running off the surface. The most practical means of lessening or 
preventing erosion are: (1) Keeping a high content of decaying vege- 
table matter in the soil, (2) the maintenance of a grass sod where 
practicable, (3) the use of channels having a slight grade, keeping 
grass growing in the bottom where possible, (4) subdrainage, and (5) 
terracing. A high content of decaying vegetable matter in clay soils 
causes a texture of increased water-holding capacity, and thus less 
water will have to run off the surface. Land which is so steep as to 
give trouble from erosion should be kept in grass as much as possible. 
It is often possible to grow one intertilled crop on hillsides without 
danger so as to permit of a rotation, though a second or third year in 
succession of tilled crops would be followed by serious difficulty. 
Hillsides should sometimes be laid out in narrow plow lands along the 
slope and carefully planned so that the dead furrows when cleaned out 
may be used as channels with very slight fall to conduct the water 
along the hillside to well-grassed or otherwise well-protected main 
ditches extending up and down the slope. Deep plowing, which will 
increase the amount of water a soil may hold from a heavy shower, 
will lessen the amount which must run off the surface and conse- 
quently lessen erosion. The same principle may be still further 
followed by placing tile for subsurface drainage on springy hillsides, 
the soil of which would otherwise be kept saturated so near the 
surface that the water from rain must run off the surface, thus caus- 
ing erosion. The extreme method of preventing erosion is through 
the use of terraces, which are sometimes necessary on steep sidehills, 
especially in the South and other sections where the rainfall is very 
heavy. 
