74 BULLETIN 355, U. S. DEPARTMENT OF AGEICULTUEE. 



tliat 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 supphed 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 shght 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 satm^ated 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. 



