232 AGRICULTURE 



good tilth. On the other hand, a good tilth is equally hard 

 to maintain on soils that easily burn out in a drought, or 

 that for any reason are not good reservoirs of water. 



5. Erosion of the Soil 



Note the color of the soil on some cultivated hilltop 

 and on the lower ground at the foot. 



1. Why is the color on the hilltop lighter? Why is 

 the crop yield less? Are there any slopes near by with 

 gullies washed out ? 



2. Have you seen sheets of soft earth which have been 

 carried by the wash from heavy rains and spread over low 

 ground or along the courses of streams? Where does this 

 soil come from ? What is its texture, fine or coarse ? What 

 is the structure of this overflow soil, heavy and dense, or 

 friable? Is such soil fertile? Must this fertility be lost 

 from the ground from which the wash came? 



3. Have you ever seen banks of soil built by the car- 

 rying of soil particles in the wind? Great masses of wind- 

 built soil, called loess, are to be found in Illinois, Wisconsin, 

 Iowa, eastern Kansas and Nebraska, and northern Mis- 

 souri. These loessial soils consist chiefly of silt. 



4. Which loses more soil from the action of the wind, 

 hilltops or valleys? In certain western plains regions as 

 much as an inch of top soil is sometimes removed in a single 

 dust storm lasting twelve hours. 



Nature of erosion. By erosion is meant the wearing 

 or carrying away of soil by the action of running water and 

 the wind. Erosion by running water takes place in some 

 degree on all slopes ; the hills are gradually but constantly 

 being carried into the valleys. But it is on the steep hillsides 

 where the velocity of the water is greatest that most damage 

 is done. The reason for this is easily seen when it is re- 

 membered that the transporting power of water increases 



