SOIL AIR 257 



ing would occur, no minerals would break down, and no solu- 

 tion would be possible. Oxidation must go on rapidly and 

 continuously in tlie normal soil, not only for chemical but for 

 biological reasons as well. By it the organic matter that 

 would soon accumulate to the exclusion of higher plant life is 

 disposed of, and its nutrient materials are brought into a 

 condition in which they may be absorbed by roots. The 

 presence of oxygen is essential either directly or indirectly 

 to the organisms that facilitate decomposition. Through such 

 a process, roots of past crops, as well as other organic matter 

 that has been plowed under, are rapidly changed in the soil. 

 The processes of decay give rise to products, chiefly carbon 

 dioxide, that are solvents of mineral matter, and leave the 



nitrogen and ash constituents more or less available for plant 

 use. 



Oxygen is also necessary for the germination of seeds and 

 the growth of roots. These phenomena, although not involv- 

 ing the removal of large quantities of oxygen, are entirely de- 

 pendent on its presence in considerable amounts. 



134. Volume of the soil air.— The amount of air in soils 

 is determined by their physical properties, the variability in 

 any particular soil being due to certain changes to which such 

 a soil is normally subject from time to time. The factors 

 that influence the volume of air in soil are: (1) texture; (2) 

 structure; (3) organic matter; and (4) moisture content. 



It is a well recognized fact that the finer the texture, the 



better the granulation and the larger the amount of organic 



matter, the greater is the amount of pore space. Since about 



the same proportion of the pore space is filled with water in 



every soil when it is in optimum condition for crop growth, 



it is obvious that with finer texture, better granulation and 



increased organic matter, there will be a greater amount of 



air present. 



■Rnssell, B. J., and Appleyard, A,, The Influence of Soil Conditions 

 on the Decomposition of Organic Matter in the Soil; Jour. Agr. Sei., 

 Vol. YIII, Part 3, pp. 385-417, 1917. 



