CHAPTER VII 

 THE SOIL: INORGANIC MATTER 



In the previous chapter the discussion of organic matter 

 in the soil brought out the fact that it was the immediate 

 source of nitrogen for plants. The other necessary elements 

 which are derived from the soil come, originally at least, 

 from the mineral particles or inorganic portion of the soil. 

 The organic matter in its decomposition furnishes acids which 

 are important agents in the solution of mineral particles. 

 Other factors in the changes of mineral particles are the 

 gases present in the soil. 



133. Soil Gases. — The pore spaces of a soil are filled, 

 part of the time with gases, and part of the time with water. 

 The latter condition happens only after a rain, and in a soil 

 of good structure does not last very long. The water running 

 down into the country drainage is followed by atmospheric 

 gases. From the decomposing organic matter gases are 

 added to those already present, and at the same time some 

 gases are withdrawn by absorption from the soil atmosphere. 

 Certain chemical reactions also involve changes in the 

 composition of soil atmosphere. It has been found that 

 nitrogen varies but little, existing in the soil in about the 

 same proportion that it does in the air, namely 78 per cent. 

 (Section 113). 



Oxygen varies from 10 to 20 per cent., whereas in the 

 air it runs rather constantly at 21 per cent. This difference 

 is due to changing rates of oxidation resulting from bacterial 

 action on the organic matter, as well as to ordinary chemical 

 oxidation of minerals in the soil. The amount of carbon 

 dioxide varies inversely with the oxygen content, running 

 from about 11 to 1 per cent. As oxygen is used up in oxidiz- 

 ing organic matter, carbon dioxide is evolved. Ordinarily the 

 disappearance of oxygen causes the appearance of an equal 

 volume of carbon dioxide. Intermolecular or anaerobic 

 oxidation is not sufficient in ordinary soils to cause much 



