SOILS 123 



milk, wool, and so on and three-fourths may be returned to 

 the land in the excrements if saved without loss. 



(c) When grown on soils of normal productive capacity, 

 legumes secure about two-thirds of their total nitrogen from 

 the air and one-third from the soil. 



(d) Clover and other biennial or perennial legumes have 

 about two-thirds of their total nitrogen in the tops and one- 

 third in the roots, while the roots of cow-peas and other 

 annual legumes contain only about one-tenth of their total 

 nitrogen. 



(e) Hay made from our common legumes contains about 

 40 pounds of nitrogen per ton. 



(f ) Average farm manure contains 10 pounds of nitrogen 

 per ton. 



Question: How many tons of average farm manure must 

 be applied to a 40-acre field in order to provide as much 

 nitrogen as would be added to the soil by plowing under 2 1/2 

 tons of clover per acre? Answer 400 tons. 



PRACTICAL EXERCISES AND HOME PROJECTS 



1. Field trip study of soil formation. Early in the study 

 of soils the instructor should take the class to places previ- 

 ously noted where there are good examples of soil formation 

 by weathering, by plant action, by animals, and by other 

 agencies discussed in the chapter. Require the pupils to 

 take notes on observations made. 



2. Comparison of surface soil and subsoil. Go to a field 

 and get three soil samples as follows: 



(a) Scrape away the plant growth and surface trash, 

 take a sample of soil just below this, and seal it air-tight in 

 a small jar. 



(b) Dig or bore down to a depth of six inches and take 

 another sample of soil. 



