66 



being clared of the natural growth, large crops of wheat, maize, 

 tobacco, and cotton, may now be seen furrowed with gullies as with 

 the wrinkles of age, and abandoned to brush and briers. 



A surface layer of good agricultural soil 6 inches deep resulting from 

 the slow and gradual disintegration and decay of rocks and accumula- 

 tion of humus may have required hundreds of years for its natural 

 formation, and yet it is liable to be washed away in a single storm. 



This excessive erosion, or washing, of lands may be prevented, and 

 the already gullied fields may be recovered, and steep slopes of loose 

 material may be held and prevented from washing- - 



(1.) By chemical means, in the application of manures and fertilizers 

 and in the accumulation of organic matter, which change the texture of 

 the soil and make it more porous and more absorbent of water, so that 

 there is less to run off over the suiface. 



(2.) By means of cultivation and underdrainage, which prevent ero- 

 sion by distributing the surface flow over the ground and increase 

 the amount carried off by underdrainage. 



(S.J By reforestation, or the planting of trees, which act mechanically 

 to prevent washing. 



( 4- .) By grass and similar vegetation, which bind the soil grains and 

 prevent their washing away. 



The erosion of a soil is caused by the wearing of the rain and snow 

 waters which can not penetrate into the soil fast enough to be carried 

 away by underdrainage, and which, by reason of the slope or contour 

 of the land, run off over the surface carrying al ng particles of sand 

 and clay. When this water accumulates in a depression in the fields 

 the force of the torrent may be sufficient to cut out a great gully in a 

 short space of time. 



The extent of washing to which the soil is exposed depends upon the 

 quantity of rainfall in a given time, the slope or contour of the surface, 

 the textnre of the soil, the vegetative covering of the surface, and the 

 kind and condition of cultivation. A soil composed chiefly of moder- 

 ately coarse grains of sand, and having good underdrainage, will 

 absorb the heaviest rainfall without much danger of surface erosion. 

 A clay soil, on the other hand, into which the water can not percolate 

 with anything like the rapidity of the precipitation, will be]washed and 

 gullied by the torrent of water which must flow over the surface. 



CHEMICAL RELATIONS OF THE SOIL TO SURFACE WASHING. 



It has been repeatedly shown by experiments and by the experience 

 of farmers that a soil, as a rule, absorbs water more readily as the con- 

 tent of organic matter and humus increases. Surface erosion can, 

 therefore, be largely prevented by such a system of cultivation and 

 cropping as will introduce as large a quantity of organic matter into 

 the soil as possible. A very old method of recovering washed and gul- 

 lied lands is to place straw in the furrows while ploughing, the straw not 

 only acting mechanically to hold the soil in place and prevent surface 

 erosion, but also in a very efficient way to increase the quantity of 

 humus, thus making the soil hold large quantities of water which 

 otherwise would have passed off over the surface. In this simple way 

 fields which have been badly washed and gullied and entirely aban- 

 doned may be recovered and made highly productive. 



