156 Bacteria in Relation to Country Life 



nitrogen per acre to that depth. Smaller, but still con- 

 siderable, quantities of nitrogen are found in the subsoil. 

 It has been estimated that in some of our rich prairie 

 soils there are present in every acre of ground 25,000 

 pounds of nitrogen to a depth of three or four feet. All 

 this nitrogen exists in the humus in the decaying 

 tissues of plants that once grew in the soil. 



The interesting question arises as to the origin of 

 all this nitrogen. We know that the phosphorus and 

 potash, as well as all the other mineral ingredients of 

 plant-food, are derived from the rocks of which the soil 

 is made. We know, also, that the rocks of the earth's 

 surface do not, as a rule, contain any compounds of 

 nitrogen. We must conclude, therefore, that all of the 

 combined nitrogen in our soils is derived from the nitro- 

 gen in the air. Soil bacteria have played, throughout 

 many ages, a predominating role in the accumulation 

 of humus-nitrogen. The activities of other soil bacteria, 

 that concern the various changes that the humus- 

 nitrogen undergoes, are varied and numerous. 



Nitrogen compounds in the soil-humus. The nitro- 

 gen compounds in the soil-humus are complex. They are 

 akin to the protein substances of food, and, as such, 

 cannot be utilized by the crops. They must first be 

 broken down and changed into simple substances, 

 ammonia, nitrites and nitrates. The breaking down of 

 the nitrogen compounds in the humus is accomplished 

 by bacteria and fungi in the soil. For them is reserved 

 the task of providing building-material for the tissues of 

 higher plants. These destructive activities of the soil 

 bacteria mean so much to cultivated crops that it be- 



