22 IRRIGATION PRACTICE 



21. Irrigated soils are unsaturated. In all proper 

 irrigation practice, the farmer is dealing with unsaturated 

 soils. Many of the best results of irrigation are due to 

 this condition; and the irrigation practices to be out- 

 lined refer largely to soils which are not fully saturated 

 with water. 



Water should ordinarily be applied to crops when the 

 water in the soil has reached the point of lento-capillarity 

 as described in the preceding chapter; that is, the point 

 below which capillary movements become very slow. 

 When the soil water has been exhausted to this point, 

 the plant is obliged to expend an unnecessary amount of 

 energy in securing water, and the soil should be irrigated. 

 Experienced, practical irrigators declare that irrigation 

 is necessary about the time this point has been reached. 

 It may usually be recognized by a flabbiness and a slight 

 change of color in the leaves and stalks of the plants. 



The percentage of moisture in the soil which corres- 

 ponds to this point of slow capillary motion varies accord- 

 ing to the fineness of the soil. In coarse soils it may be 

 below 10 per cent; in fine clayey soils, 20 per cent or 

 more. For average loam soils it is in the neighborhood 

 of 12 or 13 per cent. 



The maximum capacity for capillary soil moisture 

 also varies with the soil. In coarse sandy soils it fre- 

 quently falls to 12, or even 10, per cent; in fine clayey 

 soils it rises to 30 or 40, or more, per cent. In average 

 loam soils it is not far from 24 per cent. 



The water actually used in a wise system of irrigation 

 varies, then, between the maximum capillary saturation 

 and the point of lento-capillarity, which, in an ordinary 

 loam soil, is a variation of from 24 to 12 per cent. This 

 must be supplied from time to time by irrigation; but, 



