166 THE STUDY OF PLANT COMMUNITIES ' Chapter VII 



tion of the surface must reduce loss by evaporation since it pre- 

 vents capillary movement. It is now known that, unless the water 

 table is very near the surface, capillary rise is negligible under any 

 circumstances. This being true, the dust mulch, or cultivated sur- 

 face, has little to support it. In fact, if the surface capillary water 

 is not connected with the water table, as is frequently true under 

 irrigation, cultivation for a mulch probably increases the loss of 

 water. Organic mulches seem to be more effective in reducing 

 water loss, probably because they shade the soil and reduce its 

 temperatures, increase the distance of diffusion from soil to air, 

 and protect the soil from the drying effects of wind. 



Water lost to the atmosphere through transpiration far exceeds 

 that lost by evaporation. Whereas evaporation seems to be effective 

 only in the surface soil, plants remove water from considerable 

 depths. Studies of orchard soils in different parts of the country 

 indicate that all readily available water may be removed to a depth 

 of three to six feet in three to six weeks, depending upon atmos- 

 pheric conditions and the kind of soil. Sandy soils, of course, are 

 exhausted more quickly than clayey soils. The relative losses by 

 evaporation and transpiration are illustrated by experiments, 262 in 

 which water was lost from a bare soil surface in a tank at the rate 

 of 4.7 pounds per square foot during one growing season, while a 

 four-year-old prune tree removed water from a similar tank at the 

 rate of 416 pounds per square foot of soil surface. An acre of de- 

 ciduous fruit near Davis, California, used eight acre-inches of 

 water in six weeks in midsummer. Corn grown in Kansas requires 

 some fifty-four gallons of water per plant to mature. If this were 

 applied at one time, as by irrigation, it would cover a cornfield to 

 a depth of about twelve to fifteen inches. Plants growing natural- 

 ly have similar requirements. The knowledge that transpiration is 

 the chief means of reducing capillary water in the soil has led to a 

 consideration 142 of what kinds of plants on watersheds will least 

 reduce the supply of water by transpiration and still prevent ero- 

 sion. 



Soil Moisture Constants.— To compare the moisture character- 

 istics of soils or to discuss them with respect to plants, quantita- 

 tive expressions of hydro-physical properties are a necessity. These 

 properties, determined under fixed conditions, are called constants. 



