IO2 SOILS AND FERTILIZERS 



ticularly sensitive, while many weeds have such strong 

 power of endurance that they thrive in the presence of 

 acids. Weeds frequently reflect the character of a soil 

 as to acidity, in the same way that an alkali soil is 

 indicated by the plants produced. The acid and alka- 

 line compounds of the soil greatly influence the bacterial 

 flora. In the presence of strong acids or alkalis, many 

 of the bacterial changes necessary for the elaboration of 

 plant food fail to take place. 



THE ORGANIC COMPOUNDS OF SOILS 



111. Sources of the Organic Compounds of Soils. 

 The organic compounds of soils are composed of the 

 elements carbon, hydrogen, oxygen, and nitrogen. 

 When vegetable and animal material undergoes decay 

 in contact with the soil, compounds, as carbon dioxide, 

 water, ammonia, 28 organic acids, and various derivatives 

 are formed, while some of the organic acids unite with 

 the minerals of the soil to form humates. Micro- 

 organisms take an important part in the decay of ani- 

 mal and vegetable matter and the production of organic 

 compounds. In some soils, the organic compounds of 

 plants, as cellulose, proteids, and carbohydrates, are 

 present, while in others they . have undergone partial 

 oxidation. Some authorities claim that a portion of the 

 initial organic matter of soils is the result of the work- 

 ings of carbon assimilating micro-organisms. The main 

 source of the soil's organic compounds, however, is the 



