148 PRINCIPLES OF RURAL ECONOMICS 



crops can be grown and a living made on lands which were 

 formerly thought to be too dry for profitable cultivation. 



Perhaps the most important factor in the successful cultiva- 

 tion of lands formerly thought to be too dry is that of maintain- 

 ing a " dust mulch." It is found that one great source of loss 

 of moisture is evaporation. The moisture which sinks into the 

 ground when it rains tends to rise to the surface by capillary 

 attraction, just as the oil rises in a lamp wick. If it rises quite 

 to the surface, it is evaporated and carried off by the wind, just 

 as the oil in the lamp is burned off. By keeping the surface 

 soil constantly stirred and loosened up, the moisture is pre- 

 vented from rising quite to the surface. The loosened surface 

 soil being less compact, and the capillary ducts being broken, 

 the water does riot rise through this layer so readily. The 

 moisture therefore tends to remain in the subsoil, being pro- 

 tected from the air by this layer of loose dirt as by a blanket. 

 By practicing this simple method of preventing the waste of 

 moisture by evaporation, it has been found possible to grow crops 

 on land which was formerly thought to be too dry for that 

 purpose. One danger, however, is that the wind will not only 

 carry away the moisture but will blow the dust mulch itself 

 entirely off the land. 



A still further extension of the tillable area is made possible 

 by the system of alternating crops and fallows, combined with 

 that of maintaining a dust mulch. A growing crop itself ex- 

 tracts a great deal of moisture from the soil, where it is ab- 

 sorbed by the roots, carried up into the plant, and given off by 

 evaporation from the leaves or blades. Where the land does 

 not receive moisture enough to grow a crop every year, even 

 with the system of constant cultivation and maintenance of the 

 dust mulch, it is frequently possible to grow a crop every second 

 year, allowing the land to lie fallow, but constantly stirring it, 

 however, on the alternate year. Under this system a part of 



