MAINTAINING SOIL FERTILITY 85 



when lands are put into cultivation, the loss is great. According 

 to King, 1 the Mississippi River carries annually toward the Gulf 

 sufficient soil material to cover seventy-two sections of land with 

 a deposit four feet deep. Whenever the subsoil is exposed by 

 washing away the surface soil, the value of the land for crop 

 production is greatly decreased. This is a common cause of 

 decreased productiveness on rolling lands. 



Some lands erode much more rapidly than others, even with 

 the same topography. Coarse sandy soils and heavy clays are 

 least subject to erosion, while loams and those soils containing 



FIG. 38. Tile drainage reduces erosion 



A dam of earth is built across the gulley. The soil carried by the water is deposited, and 

 the clear water is carried away by tile 



much silt and fine sand are subject to the greatest erosion. 

 In the Southern states, where the soil does not freeze during 

 the winter, erosion is much greater than in the Northern states. 

 In the South it is common to plow the land and to plant crops 

 across, rather than up and down, the slopes. This is known as 

 contour farming. Sometimes, where the slopes are quite steep, 

 the land is thrown up into terraces (Fig. 37). As soon as a gully, 

 or ditch, begins to form, it should be made to fill again by 

 throwing in straw or brush, or by constructing a ditch, to 

 check the flow of water and to catch the soil that is being carried 

 away (Fig. 38). Sowing grasses in ditches is another common 

 method of preventing erosion. 



1 The Soil, p. 48. 



