THE FORMS OF SOIL- WATER 175 



too loose surface, may accomplish the same effect but by an 

 opposite process. 



At certain seasons of the year capillarity should be im- 

 peded near the surface, as it continually carries valuable 

 water upward to be lost by evaporation. This movement may 

 be checked somewhat by producing on the soil surface, by 

 appropriate tillage, a layer of dry, loose soil. This layer, called 

 a soil-mulch, resists wetting because of its dryness, while at 

 the same time it affords but little surface and few angle inter- 

 stices for effective capillary pull. Moisture also moves very 

 slowly from a moist, cool soil to a dry, warm one. 1 Thus it is 

 that a farmer, in order to meet immediate or future plant 

 needs, may alter and control capillary movement by careful 

 attention to physical conditions, especially those at the sur- 

 face where evaporation is always active. 



93. Gravitational water and its movement. — As soon 

 as the capillary capacity of a soil column is satisfied, further 

 addition of moisture will cause the appearance of free water 

 in the air spaces. By the attraction of gravity, this water 

 moves forward through the soil at a rate varying with con- 

 ditions. In general, the flow is governed by four factors — 

 pressure, temperature, texture, and structure. An under- 

 standing of the operation of these forces is important, since 

 the rapid elimination of free water from the soil is necessary 

 for normal plant growth. 



It is very evident that any pressure exerted on a water 

 column will alter the rate of flow. Under normal conditions 

 pressure may arise from two sources, atmospheric pressure 

 and the weight of the water column. Changes in barometric 

 pressure are communicated to gravitational water through a 

 movement of the soil-air. As the mercury column rises more 

 air is forced into the soil and the pressure on the soil-water 



1 Bouyoucos, G. J v Effect of Temperature on Movement of Water 

 Vapor and Capillary Moisture in Soils; Jour. Agr. Kes., Vol. V, No. 4, 

 pp. 141-172, Oct., 1915. 



