24 BULLETIN 355, U. S. DEPARTMENT OF AGRICULTURE. 
12. Give some influences which cause a circulation of air in soils. 
13. Why is air circulation in soil important? 
14. Discuss the different factors which have to do with good tilth. 
LESSON IV. THE WATER SUPPLY OF THE SOIL. 
The soil is a reservoir which, stores a part of the water supplied to it 
by rain and irrigation, giving it up again to meet the needs of growing 
plants. 
Water-holding capacity of soils (Ref. Nos. 2, pp. 157-162; or 3, pp. 
210-218; 10, pp. 119-122).— If the surface soil of a field is thoroughly 
saturated with water for some time, most farm-crop plants stop 
growing, because the small amount of oxygen dissolved in the water 
will not suffice for the needs of the plants and further supplies can not 
penetrate the saturated soil. If land with a porous subsurface or an 
underdrainage system be examined after a thorough soaking with 
rain, it will be found that the water remaining is held in the form of 
films surrounding the individual soil grains and the smaller clusters 
of soil particles. The excess of water which has drained away under 
these conditions is called drainage or gravitational water. (Ref. No. 
10, pp. 104, 105.) That which remains is called capillary or film 
water. (Ref. No. 10, p. 106.) 
Since capillary water exists as a film surrounding the soil grains and 
therefore depends on the area of these particles, fine-textured soils 
can hold more water than coarse-textured soils. Moreover, this 
capillary water in the soil not only forms films around the soil grains, 
but these films are continuous from the surface downward in such a 
way that the moisture in the subsurface soil forms a weight on the 
films above, just as the lower links in a chain hanging by one end pro- 
duce the weight supported by the upper links. The result of this is 
that the films near the surface in the soil are stretched by the capillary 
moisture below, so that a soil layer which is a number of feet above 
saturated soil can hold less capillary water than a layer only a few 
inches above saturated soil. The amount of capillary moisture held 
by the soil after a heavy rain depends, therefore, not only on the 
texture of the soil, but on the distance to the saturated subsurface 
soil on the ground-water table. The thickness of these films also 
varies with the temperature of the water. Films of warm water are 
drawn out considerably thinner than those of cold water. As a 
result of this principle, as soils get warmer during the summer, the 
quantity of capillary water diminishes. 
Organic matter and water-holding capacity (Ref. No. 3, p. 218). — 
Vegetable matter in the soil in various stages of decomposition has a 
strong power to absorb and hold water. In a well-advanced stage of 
decay, as with muck and humus, organic matter can hold several 
times its own weight of water and very much more than the mineral 
