134 SUCCESSFUL FARMING 



organic matter. Soils of fine texture and those having plenty of organic 

 matter hold the largest amount of capillary water, and are able to with- 

 stand periods of drought better than those with a lesser capacity. 



Plant roots move toward the water supply in the soil, and as they 

 withdraw water from the soil particles, water moves to those points by 

 capillary action to replace that removed. The rate of capillary move- 

 ment is slowest in soils of fine texture, and is most rapid in sandy soils. 

 The distance through which capillary power acts on the other hand is 

 least in sandy soils, and greatest in soils of fine texture. We find, there- 

 fore, that plant roots are most extensive in sandy soils and extend to 

 greater depths in search of a water supply. 



Gravitational Water. — Since gravitational water is but little used by 

 plants, it becomes a menace in soils more often than a benefit. Over large 

 areas of comparatively level land where there is an abundant rainfall, it 

 often becomes necessary to remove the gravitational water by means of 

 various forms of drainage. The movement of gravitational water within 

 the soil depends chiefly on the texture and structure of the soil. The 

 amount that needs to be removed under agricultural conditions depends 

 chiefly on the rainfall of the region and the amount that escapes over the 

 surface of the land. The depth to which this gravitational water should 

 be removed will be determined chiefly by the character of crops to be 

 grown. Seldom is it advisable to place underdrains for this purpose at 

 a depth of less than three feet. For deep-rooted crops, such as alfalfa 

 and orchard fruits, four feet and sometimes more is advisable. 



While this form of water may be injurious to upland plants, when 

 it exists at a depth of from four to six feet below the surface it does no 

 harm and serves as a reservoir from which water may be drawn by cap- 

 illarity to meet the losses above by evaporation and plant removal. 



Hygroscopic Water. — The water which is held by the soil when a 

 thin layer is spread out and allowed to become air dry is called hygro- 

 scopic moisture. When this soil is placed in an oven and heated to the 

 temperature of boiling water for several hours, it loses its hygroscopic 

 water and becomes water free. The amount of this form of water held 

 by soils varies directly with the texture of the soil and may amount to 

 as much as 16.5 per cent in case of clay, while in a muck soil it may be 

 as high as 50 per cent. The percentage of hygroscopic water will also be 

 influenced by the temperature and humidity of the air with which it comes 

 in contact. 



Water Affects Temperature of the Soil. — A requisite degree of warmth 

 in the soil is essential to physical, chemical and biological processes that 

 make for soil fertility. Warmth is essential to the germination of seeds 

 and growth of plants. The chief source of warmth in the soil is from the 

 sun. The rapidity with which a soil warms under the influence of the sun 

 depends more largely on its water content than on any other factor. 

 One pound of water requires four times as much heat to increase its tern- 



