NITROGEN COMPOUNDS OF THE SOIL 1 09 



129. Nitrates and Nitrites. The amount of nitro- 

 gen in the form of nitrates and nitrites, varies from 

 mere traces to 150 pounds per acre. Calcium nitrate 

 is the usual form met with, especially in soils which 

 are sufficiently supplied with calcium carbonate to 

 allow nitrification to progress rapidly. Nitrates and 

 nitrites are the most valuable forms of nitrogen for 

 plant food. Both are produced from the organic 

 nitrogen of the soil. A nitrate is a compound com- 

 posed of a base element as sodium, potassium, or cal- 

 cium, combined with nitrogen and oxygen. A nitrite 

 contains less oxygen than a nitrate. 



Potassium nitrate, KNO 3> sodium nitrate, NaNO 3 , 

 and calcium nitrate, Ca(NO 3 ) 2 , are the nitrates which 

 are of most importance in agriculture. The nitrites, 

 as potassium nitrite, KNO 2 , are present to a less 

 extent than the nitrates. Nitrates and nitrites are 

 found in surface well waters contaminated with 

 animal and vegetable matter. Many well waters 

 possess some material value as a fertilizer on account 

 of the nitrates, nitrites, and decaying animal and 

 vegetable matters which they contain. 



130. Ammonium Compounds of the Soil. The 



amount of ammonium compounds in a soil is usually 

 less than the amount of nitrates and nitrites. The 

 sources of the ammonium compounds are, rain-water 

 and the organic matter of the soil. The ammonium 

 compounds are all soluble and readily undergo fixa- 

 tion. See Section 207. They cannot accumulate in 

 arable soils, because of nitrification. They are usually 



