94 PHARMACEUTICAL BACTERIOLOGY. 



renews the productiveness. As already indicated, soil exhaustion means 

 that the crop plants in a few years use up a high percentage of the available 

 foodstuffs. By proper tillage the moisture retaining power of the soil is 

 increased and air is introduced, conditions which are favorable to the develop- 

 ment of soil bacteria which have the power of converting a new supply of 

 unavailable plant food into available plant foods. When a fertilizer, as 

 manure or guano, is added to the soil, it is first attacked by myriads of rotting 

 bacteria, which convert some of the insoluble organic manure compounds 

 into soluble compounds, known as peptones and albumoses. These are in 

 turn converted into ammonia by other microbes, and the ammonia is con- 

 verted into nitric acid by the so-called nitrifying bacteria. The nitric acid 

 at once combines with potash and lime in the soil, forming potassium nitrate 

 and calcium nitrate, in which form these substances are available as plant 

 foods. 



Eminent scientists declare that certain bacteria of the intestinal tract are 

 absolutely essential to life. Those bacteria constantly associated with the 

 roots of plants presumably play a very important part in the life history of 

 these plants. The mutually beneficial biological relationships or associa- 

 tions (mutualistic symbiosis) between bacteria and animals and between 

 bacteria and plants are very numerous. In fact the antagonistic (parasitic) 

 or objectionable associations are a decided minority. The recent investi- 

 gations along this line have revealed some very interesting life conditions, 

 as will be more fully explained in the discussion of industrial bacteria. 



In green manuring, microbes and higher fungi cause the starch, sugar, 

 and cellulose of the plants used for this purpose to undergo fermentation; 

 organic acids are liberated which render the insoluble soil phosphates (of 

 calcium) soluble. That is, the insoluble basic phosphates are converted 

 into neutral phosphates, which are soluble. Carbon dioxide, another very 

 important bacterial product, combines with potash to form carbonates, and 

 these in turn acf upon the silica in the soil, forming the potash zeolites 

 (hydrates of silica). Certain microbes, lower hyphal fungi and soil algae, 

 have the power of chemically binding the free nitrogen of the air, thus ren- 

 dering this abundant element available as plant food. 



By means of thorough soil cultivation and the systematic use of fertilizers 

 we simply encourage the development of the particular microbes that will 

 set free or render available the food substances required by the crop plants 

 under cultivation. Agricultural bacteriology is beginning to make practical 

 use of certain plant food forming microbes. Of these the free nitrogen- 

 binding microbes are most promising from the standpoint of practical com- 

 mercial utility, and have received much attention in recent years. The 

 more important species are: Rhizobium mutabile, Bacillus ellenbachiensis 

 Caron, Azotobacter chroococcum; Bacillus subtilis, Bacillus calif orniensis, and 



