322 ANNUAL REPORT OF THE Off. Doc. 



VII. THE CHEMICAL CHANGES TKODUCED BY BACTERIA 



IN THE SOIL. 



The Elaboration of Plant Food. 



The processes going on in the soil by -svliich plant food in its crude 

 state is prepared for the use of the growing crop are two, (1) the 

 decay of organic matter and (2) the disintegration and disolution of 

 mineral matter. They will be accordingly considered in turn. 



1. The Decay of Organic Matier. 



Organic matter, whether of animal or vegetable origin, when 

 freshly incorporated with the soil, undergoes a partial and incomplete 

 process of decay resulting in the production of a dark material known 

 as humus. 



The amount of humus in soils may vary from one per cent, in the 

 soils of the arid region of the West to as high as five per cent, in 

 black prairie loams. 



The original supply of humus in virgin soils becomesi a constant 

 source of plant food through its slow but constant decay. Long 

 continued cropping and tillage produces in time, however, a ''burning- 

 out," which means that the humus content of the soil is being 

 gradually reduced. Thus according to Snyder,8a virgin soil with 

 four per cent, of humus will, after twenty years of grain cropping, 

 show a reduction to 2.5 per cent, of the same material. 



2. Forms of Organic Matter in the Soil. 



Organic matter becomes incorporated with the soil largely in the 

 form of vegetable materials such as fallen leaves, sticks, seeds, 

 straw, stubble, sod, the roots of various plants, green crops turned 

 under, etc. 



Such vegetable matter of whatever kind is composed of a frame- 

 work of cells constituting its woody portion. The material constitu- 

 ting the walls of these cells is, for want of a better term, designated 

 as cellulose, since the latter substance in one form or another pre- 

 dominates. Within the cavities of the cells, partially or completely 

 filling them, may be found certain organic substances of which the 

 m.ost important are: (1) proteid matter, including protoplasm, proteid 

 granules, aleurone and gluten; (2) carbohydrates, including starch, 

 grape sugar, cane sugar, vegetable mucilage, gums; (3) fats and oils. 

 Besides these are small quantities ef a great variety of other sub- 

 stances, as glucosides, tannin, alkaloids, essential oils, resins, bal- 

 sams, turpentine, coloring matter, etc. Notwithstanding the great va- 

 riety of plant products, the principal materials, forming practically 



