14 IRRIGATION PRACTICE 



the other 19.83 per cent, and the third 51 per cent of iron 

 hydrate. The hygroscopic coefficient of the first was 

 4.92 per cent; of the second, 15.40 per cent, and of the 

 third, 19.66 per cent. At the Utah Station it was found 

 that on the dry-farms, in the fall, after the baking heat 

 of summer and before the autumn rains, the soil moisture 

 remaining was in proportion to the clay contained by 

 the soils. 



The hygroscopic moisture is held very firmly by the 

 soil, and it is very doubtful if it has any direct value for 

 plants. The part clinging around the soil grains probably 

 has no such value, but it is possible that the colloidal 

 soil constituents often containing much water may be 

 made to give up some of their water to the growing plant. 

 King has suggested that in seasons of drought the hygro- 

 scopic water may evaporate at one point in the soil and be 

 condensed elsewhere upon the root-hairs in search of water. 

 The chief agricultural interest of hygroscopic soil mois- 

 ture is that upon it and possibly in part by it, is held the 

 water which really can be used by plants. 



16. The wilting coefficent. Water added to a soil, 

 the hygroscopic coefficient of which has been satisfied, 

 simply thickens the soil-moisture film or more com- 

 pletely saturates the colloidal soil constituents. The 

 first water thus added above hygroscopic saturation is 

 also held very firmly and is of little or no direct value to 

 plants. As more water is added, however, and the film 

 is thickened around the soil grains, the outer layers of the 

 film water are held with less and less force, and a point 

 is at last reached above which plants can use the soil 

 moisture in growth, although it may be with some diffi- 

 culty. Whenever the soil moisture in a cropped field is 

 reduced to this point, the plants growing on the soil 



