154 ROOTS 



that it can not be driven off except by the application of heat. 

 The driest of "air dry" soils still contain considerable hygroscopic 

 water, as shown by their loss in weight when they are further 

 dried in an oven. 



Plants depend mainly upon capillary water, although in some 

 cases, especially in soils with high hygroscopic power, some hygro- 

 scopic water may be available to the plant. When soils are satu- 

 rated, as after heavy rains or in bogs and swamps, there is more 

 water present than the plant needs, and, besides, the air which 

 roots must have is driven from the soil pores. Experiments have 

 shown that, in general, a soil is best adapted to plant growth 

 when the water present is not more than 60 per cent of the amount 

 required for saturation, or, in other words, when about two-fifths 

 of the pores are open for the circulation of air. 



The forces which resist the pulling away of hygroscopic and 

 capillary water from the soil particles tend to keep the water 

 equally distributed. Thus as water is lost from the pores in the 

 surface soil, either by evaporation or root absorption, it is replaced 

 by water moving up from below through the force of capillarity. 

 Consequently, as a root absorbs, the movement of water toward 

 it from all around enables it to obtain water from regions several 

 feet away. In fact, capillarity has been known to raise water to 

 a height of 10 feet in one kind of soil. Again, in hygroscopic 

 water the thin films, which are like stretched rubber around the 

 soil particles, are connected where the soil particles touch, and to 

 compensate the greater water loss one film may have over others, 

 there is such a movement of water between the films that all for 

 a considerable distance around share in the loss. Thus, due to 

 the forces of capillarity and the surface tension of hygroscopic 

 films, soil water tends to move to the point where it is being 

 absorbed. It is now clear why the soil becomes so evenly dry 

 around a plant. 



The air in soils is necessary to supply oxygen to roots and 

 soil organisms and to oxidize the poisonous substances excreted 

 by roots or formed through the deca,y of organic matter. Through 

 oxidation the harmful effects of the poisonous substances are 

 destroyed. Soils are tilled largely to keep them porous so that 

 ail can circulate through them. 



Humus is organic matter partially decomposed, and consists 

 of leaves, stems, roots, bodies of animals, etc., decaying in the 



