SOIL AS WATER RESERVOIR 29 



much to lessen the danger of injury from water-logged 

 and alkali lands. 



In ordinary practice, lands should not be irrigated 

 until the crop has reduced the soil-moisture content 

 nearly to the lento-capillary point, and a little lower in 

 the upper layers. Then only enough water should be 

 applied to supply the zone of crop-action, say 10 or 12 

 feet. This quantity varies, in different soils, but seldom 

 exceeds a depth of 6 inches at an irrigation. 



24. Field moisture capacity. The law of distribu- 

 tion of water in soils makes it clear that the average 

 percentage of water held in a soil to a depth of say 10 feet, 

 after even a heavy irrigation, is far below the maximum 

 capillary water capacity. Under the conditions prevail- 

 ing in irrigated districts, except when over-irrigation is 

 practised, the top foot or often the top layer contains 

 only the maximum capillary percentage of moisture. 

 The percentage becomes steadily smaller with increasing 

 depth until, at 8 to 15 feet, it is very little above the 

 point of slow capillary motion. This is especially well 

 brought out in the spring, in districts where the precipita- 

 tion conies chiefly in the winter time. In early spring, 

 after the water from the winter snows and rains has 

 soaked into the soil and distributed itself, it was 

 found that, in the Utah experiments, a soil with a maxi- 

 mum capillary capacity of 25 per cent invariably con- 

 tained, to a depth of 8 feet, an average of 18 or 19 per 

 cent of moisture. Crawley observed similarly that certain 

 Hawaiian soils of a maximum capillary capacity of 32 to 

 39 per cent contained in the field only 22 to 29 per cent 

 of moisture. The percentage of moisture held in field 

 soils to a depth of 8 to 10 feet, when the top foot is 

 saturated, may be called the field water capacity of a 



