194 NATURE AND PROPERTIES OF SOILS 



considerable film movement. It can, therefore, be said that 

 capillarity is an important factor in any soil in supplying 

 the plant with proper quantities of moisture. 



Many of the early investigators have over-estimated the 

 distances through which this adjustment may be effective in 

 properly supplying the plant. It must always be kept clearly 

 in mind that it is the rate of water supply that is the con- 

 trolling factor. Therefore, capillarity, although it may act 

 through a distance of eight or ten feet if time enough be al- 

 lowed, may actually be of immediate practical importance 

 through only a few inches as far as the crop is concerned. No 

 extended data are available as to this particular phase, but the 

 knowledge of capillary movement indicates that capillarity of 

 the soil is of greatest importance in a restricted zone immedi- 

 ately around the surface of each absorbing root. 1 



103. Why plants wilt. — As has already been indicated, 

 water may be of little use to a plant because of distance, since 

 capillary action may not move the water rapidly enough 

 for normal needs. Water near at hand may be unavailable 

 through the obstruction of capillarity, friction in this case 

 being the cause. As the rootlet thins the interstitial film at 

 any point, the surface tension equilibrium is disturbed and 

 water moves toward the absorbing surface. This movement is 

 rapid enough for plant needs until the film channels on the 

 particles become thin. As such a condition approaches, fric- 

 tion increases rapidly, cutting down the capillary movement 

 to such an extent as to interfere with the normal functions 

 of the plant. 



Wilting occurs, therefore, merely because the soil is unable 

 to move the water rapidly enough for crop needs. As the 

 friction increases very rapidly after the point of lento-capil- 

 larity is reached, the wilting coefficient is a figure somewhat 



x Burr, W. W., The Storage and Use of Soil Moisture; Nebr. Agr. Exp. 

 Sta., Ees. Bui. 5, 1914. 



Miller, E. C, Comparative Study of the Boot System and Leaf 

 Areas of Corn and Sorghums; Jour. Agr. Ees., Vol. VI, No. 9, pp. 311- 

 331, 1916. 



