68 DRY FARMING 



and water vapor, the residue, humus, becoming a part 

 of the soil mass. 



47. Preparation of Available Plant-food. The grow- 

 ing plant has need of an ever-present supply of plant 

 food that is soluble and may be taken up readily by the 

 roots. The bacteria are always at work preparing this 

 food, and should their activity be stopped for any con- 

 siderable period of time, the plant would suffer and die 

 of starvation. Though concerned in the liberation of 

 all plant-food elements, the relations of the bacteria to 

 nitrogen are especially important. 



48. The Nitrogen Cycle. Farm crops depend upon 

 the soil for their nitrogen, and this nitrogen must be in 

 the form of nitrates to be utilized by plants. (Legumes 

 are an exception and when inoculated, they may draw 

 upon the nitrogen gas of the air). Chile salt-peter 

 (sodium nitrate), a common fertilizer, represents such 

 a form which is available to plants. The nitrate is ab- 

 sorbed by the roots and is built up into protein in the 

 plant. The latter may be eaten by an animal, the pro- 

 tein then becoming a part of the animal body or may be 

 voided in the manure. The original bit of nitrate may, 

 therefore, exist in altered form as a part of the prairie 

 sod, of the wheat stubble, or of the animal flesh or man- 

 ure. If returned to the soil and plowed under, before it 

 can again be used by plants, it must go through a pre- 

 paratory process. First the plant or animal remains 

 must undergo decay or decomposition, the nitrogen be- 

 ing converted into ammonia. A great many kinds of 

 bacteria in the soil may take part in this process, the 

 end product in every case being ammonia. Certain 

 bacteria in the soil (the nitrous acid bacteria) seize upon 

 this ammonia and convert it into nitrite, a nitrogen com- 



