EXAMINATION OF SOIL. 445' 



animal, composed of the most complex chemicg,l compounds, 

 serve as food for this microscopic world which in turn con- 

 verts the elernents present into compounds which are suit- 

 able for the maintenance, of higher plant, and thus of higher 

 animal life. 



The bacterial changes going on in the soil are usually 

 designated as those of fermentation or putrefaction. Either 

 term implies the tearing down by bacteria or other organs 

 isms, of complex matter and transforming this into simpler- 

 forms. While a small number of bacteria may be consid- 

 ered as the most relentless foes of man, animals and even 

 of plants, yet the vast army of these organisms constitute 

 man's best friend. 



The changes induced by bacteria, as usually observed, 

 are analytic or reducing in character. Some of the organ- 

 isms in the soil, however, give rise to important oxidation 

 chahges, such as is seen in the conversion of ammonia into 

 nitrous and nitric acids. The so-called nitrifying bacteria 

 are found widely distributed in the soil and in water. Under 

 certain special conditions their action is so pronounced as- 

 to give rise to vast quantities of their characteristic pro- 

 duct. The salt-peter of India or potassium nitrate, and the- 

 Chili salt-peter or sodium nitrate, as indicated heretofore, 

 are most valuable commercial products which result from, 

 the action of certain bacteria upon animal excreta. 



Another interesting group of soil bacteria, already re- 

 ferred to (p. 110), is met with in the characteristic nodules 

 on the roots of leguminous plants. These bacteria have 

 undoubtedly the power of assimilating the free nitrogen of 

 the air and of transmitting it to the growing plant.. 

 Strange to say, the higher plant in this case is practically 

 dependent upon these parasitic bacteria for its existence.. 

 It may grow in sterile soil but the growth in that case is 

 poor and dwarfed, and presents a striking contrast to the: 

 plant growing in soil to which these organisms have been, 

 added. 



