IRRIGATION AND COVER CROPS 



179 



to moisture supply in the soil consists in opening the soil to rain, 

 or to irrigation, and in subsequently closing it to evaporation. These 

 are the principles which were recognized and applied in California 

 half a century ago and are now enjoying somewhat sensational 

 renaissance in the "dry farming" agitation in the interior of the 

 United States. 



A Negative Declaration. It is interesting that current practice 

 affords full demonstration of the foregoing claims both positively 

 and negatively. The negative argument in favor of moisture con- 

 servation by clean summer cultivation is found in the fact that 

 growers in regions of heaviest rainfall approve the growth of cover 

 crops, like clover, after the trees reach bearing age, and also that 

 others employ scant summer cultivation, or cultivation for a short 

 period only. The idea of these growers is that such practices relieve 

 the soil of excessive moisture, either by the growth of the cover 

 crop or by facilitating surface evaporation, and so prevent the tree 

 from being stimulated to too large wood growth, or maintaining 

 growth so late in the season as to enter the frost period in too 

 active a condition and with new wood not properly matured. Quite 

 in contrast with this is the practice, which is gaining ground in the 

 hottest parts of the irrigated region, of growing alfalfa as a cover 

 crop for the purpose of shading the soil and thus reducing soil tem- 

 perature and, perhaps, of avoiding the ill effects of the reflection of 

 burning sun heat from a smooth surface of light-colored soil, or 

 the ill effect of "burning out of humus" by clean summer culture. 

 In such cases more irrigation is needed to supply enough water for 

 the growth of both trees and cover crop. But at present these ex- 

 ceptions are of rare occurrence. 



Cultivation Not Determined by Irrigation. The adoption of a 

 policy of clean cultivation in the dry season is not conditioned upon 

 the amount of moisture available either by rainfall or irrigation. It 

 is pursued both where irrigation is practiced and where it is not, 

 and also where the rainfall is greatest and where it is least. It pre- 

 vails in the humid region where rainfall may rise to 60 inches or 

 more and in the arid region where it may not exceed one-tenth as 

 much. As a matter of fact, there does not appear to be a good fruit 

 soil so deep and retentive that it can retain enough even of a very 

 heavy rainfall to effect good tree growth and fruit bearing if it is 

 forced to sustain the loss by evaporation from a compact surface 

 during the long dry season following. There may be, it is true, soils 

 weak in capillary, in which water can not rise from a great depth 

 and in which deep rooting plants may find ample water in the sub- 

 soil, providing it is held there by impervious underlying strata. 

 There are many more instances where loss by natural drainage is 

 added to loss by evaporation. But, disregarding exceptions, the 

 loss of moisture by both drainage and evaporation during the dry 



