112 IRRIGATION PRACTICE 



contact with a comparatively small area of the soil-water 

 film. As water is drawn into the plant, there must be a 

 flow of water toward the point of contact between the 

 active roots and the soil-moisture film. If the film is 

 thick, the water will move with some freedom and the 

 plant, in a given time and with the expenditure of a given 

 amount of energy, will absorb a larger quantity of water 

 than would be possible if the film were thin and offered 

 greater resistance to the moving water. The same prin- 

 ciple has been shown to hold generally, as when water 

 evaporates directly from the surface of the soil. The per- 

 centage of water in the soil is a fair measure of the thick- 

 ness of the soil-water film, and the rate of loss of water 

 from the soil increases, therefore, as the initial percentage 

 of moisture in the soil increases. This is the same as saying 

 that the more water contained by the soil to a given depth, 

 the more is lost in a given time by plant- and sun-action. 

 This important law seems to imply that plants are not 

 able to regulate the quantity of water taken up by roots; 

 but rather that, assuming all other factors to be uniform, 

 the rate of transpiration varies only with the ease with 

 which water may be obtained. If this be true, plants 

 may easily waste water if too much is found in the zone 

 of root-growth; unless, indeed, the rate of growth is 

 proportional to the use of water a condition which does 

 not exist. Here is evidently, then, because of the inability 

 of the plant to regulate its consumption of soil moisture, a 

 danger which the farmer must carefully heed. While the 

 plant cannot possibly be prevented from taking more 

 water from moist than from dry soils, yet, the farmer 

 may so reduce the percentage of soil moisture that the 

 plant is not always absorbing water at its maximum 

 capacity. Manifestly, in spite of all that can be done. 



