SOILS 109 



b. Plants can get nitrogen only as a compound from the 

 soil. 



c. Nitrogen is a free gas in the air. 



d. There are about seventy million pounds of nitrogen 

 over each acre. 



e. Nitrogen in the combined form is unstable and easily 

 lost. 



f . The nitrogen supply in most soils is low. 



g. Nitrogen compounds in the soil are mainly in organic 

 matter and extend only a few inches below the surface. 



h. Nitrogen is the one element of plant-food that is most 

 easily lost and wasted, and is often the limiting element in 

 maximum crop production. 



i. The growing crops draw heavily upon the nitrogen sup- 

 ply in the soil. 



j. If it were possible to exhaust the supply in the average 

 soil, it would be entirely used up by thirty-two 100-bushel 

 crops of corn. 



k. Considerable nitrogen is lost by percolation of drain- 

 age water. 



1. Many nitrogen compounds are easily lost by leaching. 



Nitrogen is obtained for agricultural purposes from the 

 following sources: rainfall, snowfall, and electrical storms; 

 bacteria are the natural means by which nitrogen is supplied 

 to the soil. Fish, blood, tankage, cottonseed, sodium nitrate, 

 calcium nitrate, and ammonium sulphate are commercial forms 

 of nitrogen. Farm manure, green manures, such as inocu- 

 lated legumes and crop residues, are the natural economic 



