820 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. 1 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 soils, 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 ; peaty 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 surface 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. 



