160 CALIFORNIA FRUITS: HOW TO GROW THEM 



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 exceptions 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 prevails 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 re- 

 tentive 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 follow- 

 ing. 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 subsoil, 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 dur- 

 ing the dry season is so great that the soil to a depth of several feet 

 loses practically all the water which is available for plant growth, and 

 the trees fail or become unprofitable. Loss by drainage can not, prac- 

 tically, be prevented, but loss by evaporation can be so reduced that trees 

 and vines will be adequately supplied in spite of the loss by drainage. 

 Because, therefore, the soil can not retain enough water in its natural 

 state, no matter how much it may receive, clean summer cultivation, 

 involving quite complete and more or less frequent stirring to the sur- 

 face to the depth of 5 or 6 inches, as discussed in Chapter XIII, is the 

 almost universal practice, irrespective of local rainfall or of irrigation. 



Cultivation, However, Determines Success of Irrigation. The 



prevailing motive for cultivation in the dry-summer region is moisture 

 retention. In this respect good surface tilth is so effective that, though 



