190 NITROGEN, PHOSPHORUS AND POTASSIUM 



Substances Contributing to the Supply of Available Plant- 

 food Elements. — When the pioneer farmer tilled the virgin soil, he 

 reaped bountiful harvests of corn and grain — not for one year only, 

 but for many years. What contributed to the crop needs of 

 nitrogen and mineral elements? Three substances, viz.: 



(a) A small amount of soluble salts in the soil. 



(&) Organic matter. 



(c) Mineral soil particles. 



As time went on, crop yields fell off, and the farmer began to 

 realize that soils can become '^exhausted'' or "worn out.'' From 

 times immemorial tillers of the soil have been advised to keep up 

 the "strength" of the land by adding substances to it. Herein 

 lies the theory of fertilizers. The process of adding fertilizing 

 elements to the soil or rendering available the elements present 

 in the soil is fertilization. The substances commonly used to add 

 fertilizing elements to the soil, or to render available those already 

 there 'are: 



(a) Vegetation and crop residue, as roots, stubble, straw, etc. 



(&) Green crops plowed under (green manuring). 



(c) Commercial fertiUzers. 



id) Manure. 



Vegetation produced the organic matter found in virgin soils. 

 Its value is well known. All plant residue such as roots, stubble, 

 etc., aids materially in maintaining the organic matter. Thus leaf 

 mold, grass, etc., should always be plowed under wherever possible, 

 and not burned. 



The further discussion in this chapter will be under three main 

 headings: (1) Green Manuring; (2) Commercial Fertilizers, and 

 (3) Manures. 



GEEEN MANURING 



Green Manuring and Its Benefits.— Green manuring is the 

 plowing under of green crops for soil improvement. The benefits 

 to be derived through this practice are : The organic matter may be 

 maintained; nitrogen is added to soils, in case of legumes; the 

 available supply of nitrogen and mineral elements is increased; 

 soil structure is improved; the development of good tilth through 

 tillage is made easier; and soils become less difficult to 

 work. Some green manuring crops may also serve as catch 

 and cover crops. 



Legumes are the best crops to use whenever possible. In 



