114 SUCCESSFUL FARMING 



increase per acre in the dry matter in corn amounting to 1300 pounds in 

 one week in July. On the basis of requiring 300 pounds of water for 

 each pound of dry matter, the consumption of water by the growing corn 

 in one week would equal 1.72 inches of rain. This, of course, is for a single 

 week in the height of the growing season, but it shows the large amount of 

 rainfall required to meet fully the needs of a large and rapidly growing 

 crop. It should emphasize the importance of storing in the soil the largest 

 possible amount of available water to tide over periods of deficiency 

 in rainfall. 



Forms of Soil Water. Water exists in the soil in three forms: (1) 

 gravitational water, or that which is free to move through the soil under 

 the influence of gravity; (2) capillary water, or that which is held against 

 the force of gravity by capillary power or, as it is sometimes called, sur- 

 face tension; (3) hygroscopic water, or that which adheres to the soil 

 particles so firmly that it will not be given off, even when the soil becomes 

 dry. Not all of the water in the soil is available for plants. Very few of 

 our economic plants use any of the gravitational water of the soil, except 

 as it may rise by capillarity and be used from the capillary store which 

 it replenishes. It is also certain that plants cannot benefit from the 

 hygroscopic water of the soil, because they are unable to get it from the 

 soil particles by which it is so tenaciously held in this form. The capil- 

 lary water is, therefore, the one form that is of importance in plant 

 growth. The relative amounts of the three forms of water in the soil 

 depend on a number of factors. 



The amount of pore space in soils ranges from 35 to 60 per cent of 

 the volume of the soil. When there is no underdrainage and a super- 

 abundance of rain this space may become fully occupied with water to 

 the exclusion of air. The soil is then said to be saturated. If rains cease 

 and underdrainage is established, the gravitational water will escape by 

 means of the drainage channels. The amount which will escape in this 

 way is determined chiefly by the texture of the soil and the percentage 

 of pore space in it. The larger the pore space, the greater the amount of 

 water that will escape in this way; the finer the texture of the soil, the 

 larger the amount held by capillarity and the less the amount that will 

 escape by drainage. 



Capillary Water. This is the important portion of the soil water 

 supply. It is the form on which plants wholly depend for their water 

 supply. Plants cannot exhaust from the soil all of the capillary water, 

 because a portion of it will be too tenaciously held by the soil particles to 

 be removed by the plant root hairs. The optimum, or most favorable 

 percentage of water in the soil for plants, differs for different crops. Such 

 crops as corn and potatoes do best with a moderate percentage of water 

 in the soil, which gives opportunity for plenty of air. Such plants as 

 timothy, redtop and other grasses do best when the percentage of water 

 in the soil is somewhat higher. Field experiments have shown that when 



