[Chap. XXX ROOTS: PROCESSES AND SOIL RELATIONS 317 



Oxygen diffuses into soil from the atmosphere and is also carried into 

 it in solution in rain water. Within the soil, oxygen is combined with 

 various reduced compounds by soil organisms and is consumed in the 

 respiration of roots and countless numbers of small plants and animals 

 that inhabit the soil. 



Carbon dioxide, because of its high solubility in water, enters the 

 soils in rain water especially if that water percolates through a layer of 

 leaf litter, duff, and humus at the soil surface. It also diffuses into the 

 soil air and soil solution from the respiration of roots and soil organisms. 

 Soil air may contain as much as 5 to 15 per cent of carbon dioxide. 



In temperate regions soils may undergo rather rapid temperature 

 changes at the soil surface, where in the summer time fully exposed areas 

 may be heated as high as 120° to 140^ F., or even higher. Temperature 

 fluctuations gradually decrease from the surface downward until the 

 temperature is more or less stabilized at about the average annual tem- 

 perature of the locality. 



Fig. 131. Profile of a cultivated soil (a) with the roots of a carrot in place; 

 the profile of a mature soil (b) that developed beneath a northern coniferous 

 forest; and (c) the profile under a northern mixed prairie. One-foot intervals are 

 indicated at the side of each picture. Photos, (a) from H. C. Thompson, Cornell 

 Univ.; (b) and (c) from C. E. Kellogg, United States Department of Agriculture. 



