ACID-FORMING ELEMENTS 65 



grain crops where the phosphates accumulate in the 

 seed, and are sold from the farm. 



76. Nitrogen. — This element is present in soils in 

 various forms. As a mineral constituent it is combined 

 with oxygen and the base-forming elements as potassium, 

 sodium, or calcium, forming nitrates and nitrites, which, 

 on account of their solubility, are never found in ordi- 

 nary soils in large amounts. Nitrogen is present 

 mainly in organic combinations, being associated with 

 carbon, hydrogen, and oxygen as one of the elements 

 forming the organic matter of soils. Nitrogen may 

 also be present in small amounts in the form of am- 

 monia, or of ammonium salts, derived from rain 

 water and from the decay of vegetable and animal 

 matter. While nitrogen is present in the air, in a free 

 state, in large amounts, it can be appropriated indirectly 

 as food in this form by only a limited number of plants. 

 For ordinary agricultural crops, particularly the cere- 

 als, nitrogen must be supplied through the soil as com- 

 bined nitrogen. This element is the most expensive 

 and is liable to be the most deficient of any of the ele- 

 ments of plant food. No other element takes such an 

 important part in agriculture or in life processes. 



77. Oxygen. — Oxygen is combined with both the 

 acid- and base-forming elements and is present in 

 nearly all of the compounds of the soil. It has been 

 estimated that about one-half of the crust of the earth 

 is composed of oxygen, which is found in large pro- 



