PHYSIOGRAPHIC FACTORS 161 



tation supported by that soil in the given climate. This point 

 should be further emphasized. Newly formed soil material has no 

 profile and bears no resemblance to the mature soil of the region. 

 It cannot support the vegetation that grows on a mature soil, but 

 the plants that can grow upon it contribute to its development, 

 probably most effectively through their decomposition products, 

 and so, in time, the resulting soil changes permit other plants to 

 grow. There results, sometimes over a long period, a succession 

 of edaphically controlled vegetation types leading ultimately to a 

 climatically controlled community. Paralleling the plant succes- 

 sion are changes in the soil— called soil development— -which are 

 primarily possible because of the plants and which lead to the 

 mature profile, also controlled by climate. Soil development and 

 vegetational development are intimately related and together are 

 controlled by climate. 



SOIL WATER* 



Soil water probably affects plant growth much more commonly 

 than any other soil factor. It follows, therefore, that a basic under- 

 standing of what causes differences in amounts and availability of 

 soil moisture and what such differences may mean to a plant is 

 ecologically necessary. 



Classification of Soil Water.— A simple, arbitrary system of 

 classification that divides soil water into four general categories is 

 sufficient for most ecological purposes. 



1. Gravitational water occupies the larger pores of the soil 

 and drains away under the influence of gravity. For a 

 short time after a heavy rain or irrigation, the soil may be 

 completely saturated with water, the air in it having been 

 displaced from the noncapillary pore spaces between the 

 particles. Under the influence of gravity, the free water 

 soon percolates downward through the soil toward the 

 water table unless prevented by some barrier, such as a 

 hardpan or other impermeable layer. Within two or three 

 days after a rain, all the gravitational water usually drains 

 out of at least the upper horizons of the soil, and the pore 



*Much of this section is adapted from a review by Kramer, 149 which in- 

 cludes an extensive bibliography. 



