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 1s 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. 
Air in the soil is necessary for the respiration of roots and 
micro-organisms. It is also of use in oxidizing poisonous sub- 
stances which result from the decay of organic matter in the soil, 
so that their poisonous effects on roots are destroyed. 
Humus consists of organic matter in a state of decomposition. 
When only partially decayed as in some bogs where it accumu- 
lates in large quantities, it forms peat. It gives to soils the dark 
color which is characteristic of good soils, such as loams where it 
