PLANT ASSOCIATIONS: ECOLOGICAL FACTORS 173 



126. Soil — The soil factor is not merely important to 

 consider in connection with those plants directly related 

 to the soil, but is a factor for all plants, as it determines 

 the substances which the water contains. There are two 

 things to be considered in connection with the soil, namely, 

 its chemical composition and its physical properties. Per- 

 haps the physical properties are more important from the 

 standpoint of soil-related plants than the chemical com- 

 position, although both the chemical and physical nature 

 of the soil are so bound up together that they need not be 

 considered separately here. The physical properties of the 

 soil, which are important to plants, are chiefly those which 

 relate to the water supply. It is always important to de- 

 termine how receptive a soil is. Does it take in water 

 easily or not ? It is also necessary to determine how re- 

 tentive it is ; it may receive water readily, but it may not 

 retain it. 



For convenience in ordinary field work with plants, 

 soils may be divided roughly into six classes : (1) rock, 

 which means solid uncrumbled rock, upon which certain 

 plants are able to grow ; (2) sand, which has small water 

 capacity, that is, it may receive water readily enough, but 

 does not retain it ; (3) lime soil ; (4) clay, which has great 

 water capacity ; (5) humus, which is rich in the products 

 of plant and animal decay ; (6) salt soil, in which the water 

 contains certain salts, and is generally spoken of as alka- 

 line. These divisions in a rough way indicate both the 

 structure of the soil and its chemical composition. Not 

 only should the kinds of soil on an area be determined, 

 but their depth is an important consideration. It is 

 very common to find one of these soils overlying another 

 one, and this relation between the two will have a very 

 important effect. For instance, if a sand soil is found 

 lying over a clay soil, the result will be that the sand soil 

 will retain far more water than it would alone. If a humus 

 soil in one area overlies a sand soil, and in another area 



