BACTERIA, OR SOIL FLORA 21 



very careful note of this fact. Much is lost by neglecting to 

 remedy such a state of matters. The amount of organic matter 

 present in the soil has, of course, a direct relation to the number 

 of bacteria then present. Within limits, the more organic 

 matter, the more bacteria present. 



It has been computed that the bacteria in one acre of land 

 give off 165 Ib. of carbonic acid each day. Probably, in tropical 

 countries, the amount is much greater. This goes, in part, to 



FlG. 4. Illustration of various Nitrogenous Bacteria, magnified 

 many thousand times. A, two species, living free in soil 

 fixing atmospheric nitrogen ; B, a species making nitrite 

 from ammonia in humus, from a Java soil ; C, a species 

 making nitrate from nitrite ; D, two species found in the 

 nodules of leguminous plants, fixing atmospheric nitrogen. 



supply stores of carbonic acid for the green leaves of plants. 

 In part also it renders soluble the silicates and aluminates in 

 the soil, which contain minute quantities of valuable salts. 



On the roots of all leguminous plants there are to be found 

 swellings, nodules or gatherings. (See illustration of bacterial 

 nodules on the roots of mimosa.) In 1888 Hellriegel and 

 Wilfarth proved that these contained many bacteria, and that 

 these bacteria obtained supplies of nitrogen direct from the air 

 and furnished it to plants in an elaborated form. It was known 



