98 TILLAGE— MOVEMENTS OF SOIL WATER [chap. 



subsoil water which is constantly withdrawn from below. 

 But when the land is cultivated the capillary channels 

 are broken, water cannot rise into the loose layer of 

 surface soil, which in the main is separated from the 

 firm soil below by large spaces across which water 

 cannot rise; hence the surface soil can become dry, 

 because it is cut off from the subsoil water, which in its 

 turn is preserved for use later. The drying of the 

 surface soil which ensues, through its severance from 

 the water-yielding subsoil, is of the greatest possible 

 importance in obtaining a tilth. At a certain stage 

 the soil can be dragged and will fall in pieces, but if 

 it be not detached from the subsoil it will either remain 

 persistently wet, so that it cannot be harrowed down, 

 or if it be forced to dry under the action of wind and 

 sun, it will set very hard and " steely," should it contain 

 any admixture of clay. The sudden forced drying of 

 strong land always produces hard and intractable clods, 

 which may defy all the efforts of the cultivator during 

 the rest of the season, unless a fortunate succession of 

 weather enable him to begin to make his tilth over 

 again. 



It may be thought that the amount of water lifted 

 by surface tension cannot be so large as to result in 

 any serious losses to the subsoil store, but in soils of 

 suitable texture enough can certainly be raised to keep 

 the crop alive during periods of drought. In some of 

 King's experiments with a cylinder full of very fine 

 sand, he found that the evaporating surface lost daily 

 an amount of water equal to 0-46 inch if the per- 

 manent water level were 1 foot below, 0-405 if the 

 water had to be lifted 2 feet, and 0-18 inch if the 

 water had to rise 4 feet to the evaporating surface. 

 When the sand was replaced by a clay loam, the lift 

 of water to the surface was somewhat less, but in all 



