TYPES OF SOILS 33 



side, to measure an inch. Clay is slow to absorb water, and its 

 reluctance in this- respect causes it to gully badly in heavy 

 storms. It is equally slow to give up moisture once absorbed. 

 The particles of wet clay cling together with great tenacity, 

 and as they dry they form a compact mass traversed by 

 numerous cracks, due to the shrinking of the mass as the 

 water evaporates. The total surface of the particles of clay 

 to which the water clings is very great. When thoroughly wet 

 it is able to hold much water, often as much as 40 per cent. 

 In wet weather, therefore, it may contain too much water for 

 good crops, while in dry weather it may bake and become too 

 hard for the roots of plants to penetrate. Owing to its great 

 water content, it warms slowly, but it cools as slowly, and in 

 autumn the vegetation lasts longest on the clays. Because of 

 the smallness of the particles composing clay, the air spaces are 

 correspondingly small. In consequence the air cannot move 

 through it freely and it is always poorly aerated in spite of 

 the fact that it contains a greater amount of pore space than 

 sand. Clay is the source of considerable plant food, mostly 

 potash, and it also fixes other plant foods that may be in the 

 soil. The farmer calls clay a cold and heavy soil, but the 

 heaviness refers to the difficulty with which it is worked, and 

 not to its weight, for it is much lighter than sand. 



Sand consists of hard separate grains. These have little 

 tendency to stick together, even when wet, though in certain 

 positions, as on seabeaches, they may form a firm, hard sur- 

 face when saturated with water. Sand does not bake nor 

 crack, and in drying returns to the loosely granular form. It 

 absorbs water readily and as readily gives it up. It often con- 

 tains less than 5 per cent of water. Plants, however, can get 

 more of the contained moisture from sand than from clay. 

 Sand does not gully as badly as clay because it so rapidly 

 absorbs water. It warms up quickly and cools much more 

 rapidly than clay. Owing to its large air spaces, air moves 



