130 SOILS AND FERTILIZERS 



attacking the insoluble substances and rendering them more 

 soluble. The phase of this subject that is of most apparent 

 agricultural importance is the effect of microorganisms 

 on the very difficultly soluble rock or bone phosphoric acid, 

 converting it into phosphoric acid available to plants. 

 In laboratory experiments with pure cultures of bacteria 

 these changes have been found to occur. There has also 

 been found to take place a reverse process by which the more 

 easily soluble phosphoric acid is converted into the less 

 soluble one. There is, at present, no way by which man can 

 control this operation in the soil. It has been held that the 

 presence of a large quantity of organic matter will make the 

 phosphoric acid of rock readily available. The results of 

 experiments with raw rock phosphate and farm manure do 

 not always confirm this idea. Under some conditions the 

 dominant process may be the conversion of difficultly soluble 

 into readily soluble phosphoric acid, while under other 

 conditions the reverse may take place. 



165. Decomposition of non-nitrogenous organic matter. 

 — There is much organic matter on the surface or in the 

 plowed soil that contains no nitrogen. The cell walls of 

 plants, and the sugars, starch and fats of plants contain no 

 nitrogen. These substances are broken down by bacteria, 

 passing through different stages among which acids occur, 

 and finally being resolved into carbon dioxide and water. 

 We have seen that the plant uses carbon dioxide as food 

 material, and we may now understand the cycle through 

 which the carbon of this gas goes. Plants absorb carbon 

 dioxide through their leaves, decompose it and use the carbon 

 in their tissues. After the plant is dead, the tissues decom- 

 pose and carbon dioxide is again formed and passes into the 

 air. Just as higher plants live and grow by using carbon 

 from carbon dioxide, so bacteria live and grow by using the 

 carbon of plant tissues. 



