24 BOTANY: PRINCIPLES AND PROBLEMS 



Soils vary much in physical texture, chemical composition, 

 depth, origin, richness, and other respects, but all are normall}^ 

 made up of a mixture of distinct components, each of which has 

 its particular influence upon the life of plants. These com- 

 ponents are rock-particles, water, air, humus, dissolved sub- 

 stances, and organisms (Fig. 11). 



Rock Particles. — The bulk and the basic material of a soil 

 is composed of small, angular particles which have been formed 

 by disintegration of rock. These make up 90 per cent of the 

 weight of ordinary good soil, furnish the necessary anchorage for 

 the plant, and, through the substances dissolved from their 

 surfaces, contribute to the supply of available 

 nutrient materials. The particles vary greatly 

 in size, from those of fine clay to those of coarse 

 gravel. They also differ in shape and in chemical 

 composition according to the type of rock pro- 

 ducing them. The irregularity of contour which 

 iiG. 12.— Soil- these particles display makes it impossible for 



crumbs or floccules. ^ r- ^ v 



Much enlarged. them to fit very closely together, and a consider- 

 able amount of space (pore-space) is thus left 

 between them which may be occupied by air or by water. In 

 soils which are in good condition for the growth of ordinary 

 plants the particles cohere in groups to form crumbs or floccules 

 (Fig. 12), the component grains of which are held together by 

 water-films or by such a cementing substance as clay. One 

 important purpose of tillage is to impart this crumb structure or 

 flocculation to a soil. At the soil surface, by the direct action of 

 the rain or by other means, these crumbs may be broken into 

 their constituent particles, which then pack closely together 

 and on drying harden into a firm, clay-like crust. 



Water. — Water is of vital importance to plants in many ways. 

 It constitutes the great bulk of their bodily material; it enters 

 into the manufacture of food; it assists in maintaining the plump- 

 ness and rigidity of the tissues; and, in its capacity as a solvent, 

 it serves as the general medium in which most physiological 

 processes are carried on. 



The chief source of soil water, and in most cases the only one, 

 is the rain which falls upon the soil surface. Various fates 

 await this water (Fig. 13). A considerable part of it may not 

 enter the soil at all if the surface is hard or the rainfall heavy, 

 but may drain away instead. This run-off is lost to plants, and 





