62 SOILS AND FERTILIZERS 



potassium, sodium, or calcium, forming nitrates and 

 nitrites, which, on account of their solubility, are 

 never found in average soils in large amounts. Nitro- 

 gen 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 ammonia, or of ammonium salts, derived from 

 rain water and from the decay of vegetable- and ani- 

 mal 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 num- 

 ber of plants. For ordinary agricultural crops, par- 

 ticularly the cereals, nitrogen must be supplied 

 through the soil as combined nitrogen. This element 

 is the most expensive and is liable to be the most de- 

 ficient of any of the elements of plant food. No other 

 element takes such an important part in agriculture or 

 in life processes. 



78, 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 

 amounts combined with silicon, forming silica. That 

 which is held in chemical combination in the soil 

 takes no part in the formation of plant tissue. In ad- 

 dition to being present in the soil, oxygen constitutes 

 eight-ninths of the weight of water and about one-fifth 

 of the weight of air. It also forms about 50 per cent. 



