SOIL FERTILITY. 1 9 



Living organisms represent the fourth factor in soil fertility. 

 Ground rock is not soil; it needs to be mingled with decaying 

 plant material. It needs more, — it needs to be teeming with life. 

 Destroy the life from a productive soil and you greatly lessen 

 its fertility. A rich garden soil baked in the oven will not grow 

 plants so well as before. This factor of soil fertility leads us to 

 a field in which our knowledge is limited. These organisms or 

 bacteria are too minute to be seen except with a powerful micro- 

 scope, and even then only the trained bacteriologist can interpret 

 what he finds. We are dependent largely upon his word for 

 results. Deficient as our knowledge is, we know that these 

 organisms do play an important part in plant growth. They 

 even assist in the breaking down of rock and the formation of 

 some of the mineral compounds in the soil. It is in connection 

 with the great nitrogen problem, however, that they play their 

 most important part. A load of stable-manure or rotted straw 

 spread upon the fields contains some nitrogen but very little of 

 this is in a form in which plants can use it. It must undergo 

 several changes before reaching that stage. First, it is seized 

 upon by one class of bacteria which break it down or decom- 

 pose it, changing it to the form of ammonia. In this stage 

 plants may utilize it to some extent, though it is generally 

 believed that but little of the nitrogen is taken in this form. 

 Another class of organisms must take this ammonia and add to 

 it an atom of oxygen, changing it to the form of nitrous acid. 

 Still it is unavailable, and a third kind must take this nitrous 

 acid and add still more oxygen, changing it to the form of nitric 

 acid. This can then combine with the lime or potash or the 

 sods of the soil to form a nitrate, which is the condition in which 

 the plant can readily take it up. Notice that this process is 

 largely one of the addition of oxygen. This means that there 

 must be air in the soil, and emphasizes the importance of good 

 texture which admits the air. With the air shut out another 

 class of organisms may seize upon nitrogen already in the form 

 of nitrates and carry it back to unavailable forms, even to free 

 nitrogen where it may escape into the air. 



With the best of management some nitrogen will be lost. 

 With bad management these losses will be heavy. Experiments 

 in Minnesota have shown that four consecutive crops of wheat 



