NATURE OF SOIL 309 



Soil structure and tilth. You have seen some seed 

 beds that were granular or full of lumps, with very little 

 fine earth for packing about the seeds. Others are finely 

 pulverized, and favorable for plant growth. The condition 

 presented by the soil with reference to plant growth is 

 called its tilth. 



Tilth depends chiefly on soil structure. Dense heavy 

 soils that have a tendency to form into lumps when plowed, 

 or that easily bake after rain, make it difficult to maintain a 

 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 for twelve hours. 



