252 SOILS: PROPERTIES AND MANAGEMENT 



ratio to be 300, the amount of water actually used by 

 the plant would amount to 600 tons to the acre, or about 

 5.2 inches of rainfall. This does not include the evapora- 

 tion that is continually going on from the soil surface, 

 which might very easily amount to as much more. More- 

 over, this draft on the soil water is not a uniform one, but 

 increases gradually as the crop develops, until at heading 

 time great quantities must be supplied in a short period. 

 The necessity of moisture conservation in order to meet 

 the plant requirements and preserve its normal develop- 

 ment, even in humid regions, becomes obvious. 



175. Role of capillarity in the supplying of the plant 

 with water. — A query arises at this point regarding the 

 mode by which this immense quantity of water is supplied 

 to the plant. The plant rootlets, especially their absorb- 

 ing surfaces, are few in number as compared with the 

 interstitial angles that contain most of the water retained 

 in the soil. How, then, does the plant avail itself of 

 water not in immediate contact with its rootlets? This 

 question has been anticipated in the discussion concern- 

 ing the capillary equilibrium which tends to occur in all 

 soils. As soon as the rootlet begins to absorb at one 

 point, the film in that interstitial angle (see Fig. 36) is 

 thinned. A considerable convexity of the water surface 

 occurs at that point, resulting in a great outward pull 

 which causes the water to move in all directions toward 

 that point. Thus, a feeding rootlet, by absorbing some 

 of the soil solution with which it is in contact, creates a 

 condition of instability which results in considerable 

 film movement. It can therefore be said that capillarity 

 is the important factor in any soil in supplying the plant 

 with proper quantities of moisture. 



Many of our early investigators have overestimated 



