24 BULLETIN 355, U. S. DEPAETMENT OF AGEICULTUKE. 



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 supphed to it 

 by rain and irrigation, giving it up again to meet the needs of growing 

 plants. 



Water-holding capacity of soils (Rei. 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 wiU 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 caUed 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 capiUary 

 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 satm'ated 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 fihns also 

 varieis 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 weU-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 



