76 THE ESSENTIALS OF AGRICULTURE 



set free through decay. It is therefore largely through the 

 decay of organic matter in the soil that the phosphorus and 

 potassium are made available to plants. 



82. How nitrogen is made available. The supply of nitrogen 

 is found largely in the organic matter of the soil. The nitrogen of 

 organic matter cannot be used by plants until the organic matter 

 undergoes decay. This decay is brought about by soil organisms 



mainly by bacteria. In the decay of organic matter, one of the 

 first compounds formed is ammonia, the odor of which is noticed 

 where stable manure is decaying. This combines with carbonic 

 acid to form ammonium carbonate. The bacteria which set the 

 ammonia free are called ammonifying bacteria, and the process 

 is known as ammonification. As soon as this group of bacteria 

 has acted upon organic matter and ammonium carbonate is 

 formed, other bacteria, known as nitrifying bacteria, act upon 

 this ammonium carbonate. One group of these nitrifying bac- 

 teria produces nitrous acid, then a second group transforms the 

 nitrous acid into nitric acid. Both the soil and the manure 

 contain substances which combine with the nitric acid to form 

 nitrates, such as calcium nitrate or sodium nitrate. This proc- 

 ess by which the ammonium carbonate is transformed into 

 nitrates is called nitrification. It is in the form of nitrates, 

 which compounds are soluble in water, that plants take up most 

 of their nitrogen. 



83. Conditions that favor nitrification. The decay of organic 

 matter and the transformation of nitrogen from the complex 

 organic compounds to soluble nitrates are dependent upon 

 various conditions. First, a reasonable amount of heat is neces- 

 sary, since these processes do not go on rapidly in cold soil. 

 Second, oxygen is necessary. This is supplied by the air within 

 the soil. In soils which are full of water, so that the air is ex- 

 cluded, there is little or no decay of organic matter. In fact, 

 under such conditions a group of bacteria known as denitrify- 

 ing bacteria may break down the nitrates already in the soil 

 and render the nitrogen unavailable to plants. This is just 

 the opposite process from the one the farmer desires. Third, 



