Plant Food in the Soil 83 



organic matter, or matter that has once formed the bodies 

 of animals or of plants, also brings ammonia to the soil. 

 But it has been found that the roots of ordinary plants do 

 not use ammonia. When the organic matter decays in 

 the soil, however, the decay is caused by certain low forms 

 of plant life known as bacteria, which hasten the decay by 

 their growth and through this decay release the ammonia. 

 Then other forms of these bacteria, which thrive in a soil 

 abounding in organic decay or humus, feed upon the 

 ammonia and the final result is the formation of nitric acid 

 in the soil through their oxidation of the nitrogen. Then 

 this nitric acid at once combines with whatever alkaline 

 base may be present, usually hme or potash, and we have 

 a nitrate which plants can use at once, and which if not 

 used at once is soon washed from the soil by reason of its 

 great solubility. Careful experiments have shown that 

 our green-leaved plants use nitrogen only after it has 

 been brought to the condition of a nitrate, and as this 

 process of reducing organic matter and releasing its am- 

 monia and converting it into nitric acid is carried on by 

 the minute plants in the soil known as bacteria, it is evi- 

 dent that to maintain this useful life in the soil there must 

 be in the soil a due amount of organic decay or humus. 

 The office of nitrogen in the plant seems to be the promo- 

 tion of vital activity tending to rapid growth. But this 

 growth can be maintained and matured properly only 

 when the nitrogen has associated with it a due percentage 

 of the ash elements which plants get from the soil. Prob- 

 ably the most important of these is phosphorus. 



We have shown that we cannot use nitrogen as a pure 

 element in the feeding of plants. In like manner we can- 



