SOIL MOISTURE 17 



Whenever the soil moisture is above the lento-capil- 

 lary point, plants secure their water with the least expendi- 

 ture of energy, and it should therefore be the purpose 

 of irrigation to maintain the moisture in the soil above 

 this point, at least during the periods of most rapid plant- 

 growth. In practice, this is usually done, except in sea- 

 sons of water-shortage. The practical irrigator recog- 

 nizes the need of irrigation by a faint change in the color 

 and rigidity of the plants possibly a condition prelim- 

 inary to wilting. When this occurs, the soil moisture is 

 ordinarily just above or below the lento-capillary point. 



During two summers, on the experimental fields of 

 the Utah Experiment Station, the moisture in the soil 

 was determined immediately before each of several hun- 

 dred irrigations. In the first year, the percentage of soil 

 moisture was 13.17; in the second, 13. In every case, 

 the practical irrigator declared the field in need of irriga- 

 tion. The lento-capillary point was determined for this 

 soil to be about 12.68 per cent, or almost identical with 

 the percentage of soil moisture at which irrigation was 

 declared advisable. 



18. Maximum capillary capacity. As the 'soil-mois- 

 ture film is thickened by the further additions of water 

 above the lento-capillary point, the force with which the 

 outer layers of water is held becomes weaker and weaker. 

 At last a point is reached above which no more water 

 can be taken up. When this thickness of the film is 

 reached, new additions of water simply slide off the film 

 and are drawn away by gravity. This is the point of 

 maximum capillary capacity. 



19. Free water. Water added to a soil above the 

 maximum capillary capacity is called free water. It 

 moves slowly downward through the pores and tubes of 



B 



