PLANT SOCIETIES: ECOLOGICAL FACTORS 
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 various 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 
