SOIL IMPROVEMENT 163 



several times as fast for clear water as for muddy water. 

 Since the soil is porous clear water will enter rapidly but the 

 silt of muddy water clogs the pores and makes the soil im- 

 pervious. The rain water becomes muddy when the drops 

 hit the bare soil, but if they hit vegetation which breaks them 

 into a fine spray they will not disturb the soil. A good lawn 

 is almost perfect in this respect because of the porous soil and 

 the covering of both growing grass and a mulch of the lawn 

 clippings. This should prevent run-off except in the winter 

 and when the rainfall reaches cloudburst proportions. Soil 

 erosion experiments have shown that a pasture may retain 

 90 per cent of the rainfall with no loss of soil while a like area 

 in cultivation may retain only 50 per cent of the rainfall, and 

 many tons of soil may be carried from an acre each year. 

 Water run-off and the loss of soil depend on the amount of 

 vegetative cover, on the rate and amount of rainfall, on the 

 kind of soil, and on the slope. 



The slope is important because it influences the length of 

 time the water will stand on the soil in order to allow for 

 percolation. If the slope is greater than 6 per cent it should 

 be terraced, if that is practical, in order to have a high per- 

 centage of the area nearly level. If the terraces can be built 

 as contours the water may be carried from one terrace to the 

 next at the ends with a minimum of fall. 



If a part of the area is cultivated, it should be done in 

 strips with lawn between the strips. If this practice is fol- 

 lowed the muddy water of the cultivated area will flow into 

 the grass and lose its mud. This practice is recommended 

 to farmers under the term strip-cropping. 



REFERENCES 



Hall, A. D., The Soil, 4th Edition, Murry, London, 1931. 



Soil Conservation Literature of the United States Department of Agriculture. 



Soils and Men, Yearbook of Agriculture, United States Department of Agriculture, 1938. 



