THE FORMS OF SOIL WATER 233 



point is approached when the soil is in the best physical 

 condition for crop growth. Tillage operations in general, 

 tile drainage, and the addition of lime and organic matter, 

 operate toward this result by their granulating tend- 

 encies; while rolling, by compacting a too loose surface, 

 may accomplish the same effect but by an opposite process. 

 At certain seasons of the year capillarity must be 

 impeded near the surface, as it continually pumps val- 

 uable water upward to be lost by evaporation. This 

 movement may be checked by producing on the soil sur- 

 face, by appropriate tillage, a layer of dry, loose soil. 

 This layer, called a soil mulch, affords much resistance 

 to wetting because of its dryness, while at the same time 

 it affords but little surface and few angle interstices for 

 effective capillary pull. Thus it is that a farmer, in order 

 to meet his immediate or future needs, may alter and 

 control capillary movement by careful attention to phys- 

 ical conditions, especially those at the surface where 

 evaporation is always active. 



157. Gravitational water. — 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 

 downward through the earth at a rate varying with soil 

 and climatic conditions. In general the flow is governed by 

 four factors — pressure, temperature, texture, and structure. 

 An understanding of the operation of these forces is im- 

 portant, since the rapid elimination of free water from the 

 soil is necessary for optimum plant growth. The actual 

 procedure, however, is considered under the head of " Land 

 Drainage," a distinct phase of soil management in itself. 



158. Pressure and the movement of gravity water. — 

 It is very evident that any pressure exerted on a water 



