TILLAGE AND CULTIVATION 



63 



small crumbs. Each of these is composed of a number 



of soil particles, between which there are air spaces and 



each of which has a layer of moisture surrounding it. 



Such a soil crumb, about one 



hundred times the natural 



size, is shown in figure 34. 



The white spaces indicate the 



air bubbles, the dark spaces 



the water surrounding each 



grain. The water holds the 



grains together, and from 



these grains the root hairs 



FIG. 34. A soil crumb greatly 

 (866 page 13) get moisture enlarged to show the parti- 



and also plant food. When 



this water dries away, the crumb falls to pieces if 

 the soil is very sandy. But if there is some clay, 

 as in loams, the clay forms a kind of cement and holds 

 the crumbs in shape, so that the air can still pene- 

 trate easily. When land is plowed with the turning 

 plow in the right condition of moisture, the furrow 

 slice (the portion that is turned over) will also fall in 

 loose crumbs. The furrow slice will be higher than the 

 unplowed land, and lie loosely. This is shown in 

 figure 35, where S is the unplowed land and fs is the 

 loose furrow slice. 



This desirable crumb structure of the soil can, of 

 course, be destroyed by tramping, especially when the 

 land is wet; also by heavy, beating rains, and by 



