The Composition and Use of Fertilizers. 21 3 



Need of Humus in Soils. — In soils under constant cropping, 

 there is usually a gradual loss of humus, unless special means 

 are employed to add humus-making materials to the soil. Peat 

 and muck soils are apt to contain an excess of humus. Sandy 

 soils are most apt to be deficient in humus. Clay soils lose their 

 humus less rapidly than sandy soils under like conditions. 



Supplying Humus to Soils. — Humus-forming materials may be 

 furnished the soil in the following ways : By crop roots remain- 

 ing in the soil, by plowing under green crops or stubble, by addi- 

 tion to soil of animal and vegetable materials, such as farm 

 manures, straw, refuse, cottonseed meal, dried blood, etc. The 

 value of humus as a direct source of plant food and as a means 

 of making available the insoluble potash and phosphoric acid of 

 the soil varies according to the kind of material from which it 

 comes. Humus from materials rich in nitrogen is more valuable 

 than when containing less nitrogen. In supplying humus to 

 soils, it should be remembered that the presence of lime in the 

 soil is one of the conditions required for the production of humus 

 of value. 



Amount of Plant Food in Soil. — The proportion of plant food 

 even in a fertile soil is comparatively small. One thousand 

 pounds of a good soil may contain: 



Phosphoric acid, li pounds. 



Nitrogen, 1-| pounds. 



Potash, 2 pounds. 



Some soils contain larger quantities than these. But when 

 we consider the total amount of plant food in one acre of soil, the 

 amounts appear large. 



While the weight of soil in an acre of different kinds of land 

 varies, we may take the average weight of dry soil in one acre to 

 the depth of nine inches as approximating about 3,000,000 to 

 3 500,000 pounds. One acre of soil containing the proportions 

 of plant food given above would, therefore, contain the follow- 

 ing aggregate amounts: 



Nitrogen, 4,500 pounds. 



Phosphoric acid, 4,500 pounds. 



Potash, 6,000 pounds. 



A large portion of the plant food in the soil is not available. 

 The character of the soil affects very considerably the available 

 condition of the plant food. For example, a sandy soil is ren- 

 dered fertile by a smaller amount of plant food than is a clay 



