320 INTRODUCTION TO BOTANY 
material, with a minimum amount of decayed material from 
plant and animal life. If the water level is below such soils, 
the water runs away and there is left in the soil little available 
moisture and little available organic matter. In sandy soils 
the rock has been broken by weathering processes, sometimes 
aided by the action of plant life, until the rock particles are 
small. The coarseness of the sand depends upon the extent 
to which the rock particles are broken. They may have been 
crushed and worn into pieces so small that clay is formed and 
the separate grains can be seen only by magnification. 
Water may be in the soil either in the spaces between soil 
particles or adhering to the particles. As the soil becomes 
finer its ability to retain water increases. This is due to two 
facts: first, that water adheres to the surface of the soil parti- 
cles; second, that the larger the number of particles within 
a given volume the greater is the surface exposed.! Also, soil 
with fine-grained sand will hold the products of decayed plant 
and animal life better than rocky soils. Soils are classified 
into many kinds, according to the size and nature of the rock 
material and the nature of the plant and animal material con- 
tained. The leading kinds are gravelly svils, containing small 
pebbles which usually show by their form and sometimes by 
their markings the kind of treatment they have undergone ; 
sandy soils, in which the rock material is more uniform and 
has gone farther in its reduction ; clay soils, whose particles are 
so small and fit together so compactly that the rock origin 
is not very evident; praty soils, containing comparatively little 
rock material but much more of the products of partial decay 
of plant and animal bodies. There are all possible gradations 
between these different kinds of soils. The chemical and 
physical nature of the rock-and-humus content of soils has 
much to do with their relation to plant life. 
1 This may be shown by calculating the surfaces of a cube ten inches in 
diameter, then cutting it into one-inch cubes and calculating the surfaces of 
these, and then comparing the surface of the original cube with the sum of 
the surfaces of those made from it. 
