306 ANNUAL REPORT OP THE Off. Doc. 



all these and many other forms of crude plant food are in themselves 

 • of no use to the plant until the elements therein are put into such a 

 shape as to be taken up into the juices of the plant through the 

 absorbing rootlets. Furthermore, as we have intimated, this work 

 of digesting the crude plant food of the soil is continually being car- 

 ried on by myriads of microscopic organisms present in every normal 

 soil. Through their agency nourishment is gradually and continual- 

 ly being supplied to growing crops as rapidly as their needs demand^ 

 and there results a beautiful and w'onderful relationship and balance 

 between the life of the highest and lowest of the plant creation. The 

 one is dependent upon the other, and independently neither can nor- 

 mally exist. 



Such is the general relationship existing between soil micro-organ- 

 isms and plant growth. 



We are thus led to understand the importance of the study of Soil 

 Bacteriology to general agriculture. The more detailed exposition 

 of the subject, together with the relation of its principles to practice, 

 will be outlined in the pages which follow. 



I. THE ELEMENTS AND SOURCES OF PLANT FOOD. 



Ninety-three to ninety-six per cent, of the dry weight of agricul- 

 tural plants is organic matter, and is composed mainly of the four 

 elements: carbon, hydrogen, oxygen and nitrogen. The remainder is 

 inorganic or mineral matter which is recovered for the most part in 

 the ash when the plant is burned. 



The elements found in the organic portion occur in approximate 

 proportions as follows: Carbon 45 per cent., oxygen 49 per cent, 

 hydrogen 6 per cent. Besides these, nitrogen may exist in amounts 

 varying from 0.5 to 1.0 per cent, of the whole. 



The green parts of all plants, but particularly the leaves, in- 

 hale and exhale atmospheric air. In the latter is ordinarily con- 

 tained about four parts of carbon-dioxide for every 10,000 parts of air. 

 Carbon-dioxide is composed of the elements carbon and oxygen in the 

 proportion of one part of the former to two of the latter. It is this 

 compound which furnishes to the plant all of the carbon and a por- 

 tion of ihe oxygen. 



The roots absorb water, and conduct it to the stem, whence it is 

 carried to the leaves. Water contains the elements hydrogen and 

 oxygen in the proportion of two parts of the former to one of the 

 latter. Water furnishes all of the hydrogen and a portion of the 

 oxygen. In other words, the two compounds, carbon dioxide and 

 water, are brought together in the leaves and a chemical reaction 



