238 A TEXTBOOK OF BOTANY [Ch. V, 6 



than six primary constituents, viz. pulverized rock, water, 

 air, humus, dissolved substances, and micro-organisms. 

 These are by no means intermingled without order, but have 

 relations to one another which result incidentally in a 

 kind of crude structure. 



Pulverized Rock. This constitutes the great bulk, 

 fully 90 per cent, of ordinary soils. It is derived from the solid 

 crust of the earth either by chemical decay of the rock or 

 else by mechanical attrition. Attrition occurs by force of 

 moving ice, as in glaciers (which have ground the surfaces of 

 most northern countries), or else of running water, as in 

 rivers, which forever are grinding the bowlders in their beds 

 to fine silt. Thus we find every gradation, from great 

 bowlders down through gravel and sand to silt and the finest 

 clay. Under the microscope any soil presents the aspect 

 of rough-angular fragments of rock, variously colored, and 

 more or less crystalline. The weight and mutual pressure 

 of these rock particles provide the resistance needful in the 

 anchorage function of roots, while their irregularity in size 

 and shape, forbidding a tight packing together, insures the 

 open irregular spaces through which water and air can 

 circulate in the soil. These features are well shown in our 

 generalized drawing (Fig. 169). 



Water. This comes second in abundance though first in 

 importance of the soil constituents. It furnishes the en- 

 tire supply to ordinary plants, which can take none through 

 their leaves or stems. It comes into the soil either direct 

 from the rain or else by way of capillary movement up from 

 lower levels. It is sometimes so plentiful as to saturate a soil, 

 that is, fill its spaces completely, as occurs temporarily in all 

 soils after drenching rains and permanently in bogs and 

 swamps. Such a standing, or hydrostatic, condition of the 

 water is not beneficial to ordinary plants, because, while 

 supplying far more than they need, it displaces the air essen- 

 tial to the respiration of the roots. As this too plentiful 

 water drains or dries away, however, the larger spaces be- 



