CALIFORNIA FRUITS: HOW TO GROW THEM 



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, 

 practically, 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 of the surface to the depth of 5 or 6 inches, as dis- 

 cussed in Chapter XIII, is the almost universal practice, irre- 

 spective of local rainfall or of irrigation. 



Cultivation, However, Determines Success of Irrigation. The 



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

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

 though enough moisture can not be retained without it, so much 

 can be retained with it that, even where irrigation or rainfall is 

 moderate in amount, it may serve all purposes of the tree or vine. 

 Thus cultivation enters into the fruit-growers' practice in the 

 region under consideration, not to make large rainfall effective as it 

 does in some parts of the region, but to make moderate rainfall 

 effective, or to make small irrigation effective, by increasing the 

 duty of water which is applied. It becomes not only a ruling con- 

 sideration in the effectiveness of a certain amount of rainfall, as 

 has already been suggested in another connection, but it also deter- 

 mines the success of irrigation and the amount of water required ; 

 for, although it was an early and rude practice to rely upon irri- 

 gation to support uncultivated fruit trees and to irrigate more and 

 more frequently as the ground became harder from its use, this 

 policy has now no standing in commercial fruit growing. Not only 

 was it wasteful of water, but it was otherwise detrimental to the 

 thrift of trees. 



Cultivation and Irrigation Work for Soil Improvement. Thor- 

 ough cultivation, both in winter and summer, has other very impor- 

 tant ends in view. It opens the soil and promotes aeration; it en- 

 courages deeper rooting and thus encourages the tree to take pos- 

 session of a greater soil mass both for moisture and other plant 

 food. It is part of the very valuable policy of increasing humus by 

 plowing under the natural growth of weeds or specially sown 

 legumes, which is discussed in Chapter XIV. This affords oppor- 

 tunity to use water, beyond the amount the trees require, for soil 

 improvement. 



WHEN TO IRRIGATE 



When to irrigate is governed by local conditions and the needs 

 of different fruits, and can not be stated in general rules. There 

 are, however, some principles involved which may be hinted at. 



