234 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



of these agents is due to the destruction of larger forms of life 

 which feed on the bacteria and so keep down their number. 



Effect of Additions on Bacterial Changes. Fertilizers, lime, 1 

 stable manure, and other additions to the soil affect the bacteria 

 of the soil. In some soils, on fertilization with sodium nitrate, 

 bacteria develop, which change nitrates into ammonia and pro- 

 tein, and which may affect decidedly the utilization of the nitro- 

 gen by plants. 



Azotobacteria appear to be dependent on the presence of lime 

 and magnesium carbonates ; so much so that it has been suggested 

 that this dependence may be utilized for the detection of the need 

 of soils for lime. Liming the soil may thus increase the growth 

 of nitrogen-fixing bacteria. 



Active plant food may also affect the bacterial relations. Some 

 bacteria have greater power for securing their mineral nutrients 

 than others. Additions of plant food to the soil may increase 

 the number and activity of certain kinds of bacteria. The bacterial 

 life of soils deficient in active plant food may also be low. 



Weeds may have an effect upon the bacterial relations of the 

 soil. Cuizeit, by estimating the nitrifying power of soils seeded 

 to oats alone, and to wild mustard, found that the mustard de- 

 creased the nitrifying power of the soil, and the differences per- 

 sisted the following year. He concluded that the unfavorable 

 effect of weeds such as wild mustard was due not only to unfavor- 

 able effects on general conditions of growth, but also to their 

 unfavorable effects on the bacterial content of the soil. 



Fixation of Phosphoric Acid and Potash by the Soil. When a 

 solution of potassium chloride is brought in contact with a soil, 

 and afterwards subjected to analysis, it is found that a portion 

 of the potash has disappeared from solution. This phenomenon 

 is called absorption, or fixation. Phosphoric acid, organic matter 

 and other bodies, likewise disappear from solution. 



Fixation may be studied by bringing a weighed quantity of soil 

 in contact with a definite quantity of a solution of known com- 

 position, for a definite time, shaking from time to time and then 

 1 Brown, Iowa Research Bulletin No. 2. 



