22 FERTILITY AXD FERTILIZER PIIXTS 



The organisms we are most interested in are the bacteria 

 (minnte plants) because of their beneficial effect in crop jiro- 

 duction. 



The number of bacteria in the soil depends upon its physical 

 condition. Water-logged soils, sandy soils, acid soils, and soils 

 low in organic matter contain very few and sometimes no bac- 

 teria. Soils rich in humus, contain sometimes as high as loo,- 

 000,000 bacteria per gram,^ while the average well cultivated soil 

 contains 1,000,000 to 5,000,000 per gram. The cold winters 

 of the north decrease the number of bacteria but these multiply 

 during spring and summer.'^' 



Nitrification. — Certain bacteria have the power of converting 

 the organic nitrogen present in animal and vegetable matter into 

 ammonia. No doubt you are all familiar with the ammonia 

 smell around fermenting manure. This is the result of the action 

 of bacteria. The same action that takes place in the manure 

 heap occurs in the soil when organic matter is present. W'hen 

 the ammonia is formed another kind of bacteria seizes it and 

 changes the ammonia into nitrous acid or nitrites, and this latter 

 compound is in turn attacked by another organism and con- 

 verted into nitric acid or nitrates. In this latter form it is readi- 

 ly dissolved by the soil water and available as plant food. There 

 is a continual cycle of the forms of nitrogen. The plant uses 

 the nitrogen in the form of nitrates, converts it into organic 

 nitrogen, and wdien the plant dies it may be returned t(^ the soil 

 to go through the same process again. 



Manure or other organic matter helps nitrification. ^'^ Keeping 

 the soil well open so that a liberal supply of air may ]:)ermeate it 

 has a beneficial effect on nitrification. The more porous the soil 

 the deeper nitrification occurs. 



Denitrification. — There are some bacteria that set free nitrogen. 

 These bacteria exert a reducing action rather than an oxidizing 

 one. Some reduce nitrates (nitric acid) to nitrites C nitrous oxide) 

 and ammonia. Others reduce nitrates to nitrites and then to 

 free gaseous nitrogen. It has been found that there arc more 



1 One ounce -- 28.35 grams. 



