1 8 BACTERIA AND SOIL FERTILITY 



Changes in temperature cause small cracks to appear even in 

 the more resistant rocks. Bacteria being of microscopic size soon 

 penetrate these small crevices. Here they produce carbonic and 

 other acids v^^hich gradually dissolve av^ay the rock. The organ- 

 isms act upon even the most minute fragments, reducing them 

 continually to smaller and smaller particles. If one will pick 

 from a fertile soil a bit of rock, it will be found coated with a 

 fine film of these minute plants. They have drawn on it for their 

 food, and at the same time they have rendered it soluble so the 

 wheat, alfalfa, or corn plant can feed upon it. Bacteria con- 

 tinue their work long after the rocks have been changed to soil. 

 Each day they liberate a little more plant-food for the growth 

 of the crop during that day. During the year they are able in a 

 fertile field to liberate enough plant- food for the production of a 

 good crop. When manure is applied to a soil it not only supplies 

 food for the growing plant, but it also carries to the soil bene- 

 ficial bacteria and supplies those already there with food. These 

 in turn liberate a little more iron, sulfur, potassium, or phosphorus, 

 and the farmer reaps from his soil a bigger and a better crop. 



One of the essentials for plant production, and the one which 

 is usually in the soil in the smallest quantities, is nitrogen. This, 

 unless it is applied to the soil in the form of the expensive fer- 

 tilizer, nitrates, must be prepared for the plant by bacteria. They 

 are the true kitchen maids of the soil, and the farmer finds his 

 crops measured directly by the speed with which they prepare the 

 meal. If they are active and prepare the right food, other things 

 being favorable, there will be a good crop. But if they fail in 

 their work, though everything else may be ideal, yet there is no 

 crop. 



An examination of soils has shown that those which are most 

 fertile contain the greatest number of beneficial bacteria. If 

 the soil is rich in plant residues and has the right quantity of 

 water and sufiicient heat many bacteria will be found there changT 

 ing the plant residues into gases and acids which act upon insolu- 

 ble substances and render them soluble. One group of bacteria 

 changes the plant and animal proteins into ammonia, and one can 



