108 DRY-FARMING 



the soil grains is very minute. King has calculated 

 that a film 275 millionths of an inch thick, clinging 

 around the soil particles, is equivalent to 14.24 per 

 cent of water in a heavy clay ; 7.2 per cent in a loam ; 

 5.21 per cent in a sandy loam, and 1.41 per cent in 

 a sandy soil. 



It is important to know the largest amount of 

 water that soils can hold in a capillary condition, 

 for upon it depend, in a measure, the possibilities 

 of crop production under dry-farming conditions. 

 King states that the largest amount of capillary 

 water that can he held in sandy loams varies from 

 17.65 per cent to 10.67 jier cent; in clay loams from 

 22.67 percent to 18.16 ]X'r cent, and in humus soils 

 (which are practically unknown in dry-farm sections) 

 from 44.72 ]3er cent to 21.29 per cent. These results 

 were not obtained under dry-farm conditions and 

 nmst be confirmed by investigations of arid soils. 



The water that falls upon dry-farms is very 

 seldom sufficient in quantity to reach the standing 

 water-table, and it is necessary, therefore, to deter- 

 mine the largest percentage of water that a soil 

 can hold imder the influence of gravity down to a 

 depth of 8 or 10 fec^t — the depth to which the roots 

 penetrate and in which root action is distinctly felt. 

 This is somewhat difficult to determine because the 

 many conflicting factors acting upon the soil-water 

 are seldom in equilibrium. Moreover, a consider- 

 able time must usually elajjse before the rain-water 



