WHEN TO IRRIGATE 165 



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 discussed in Chapter XIII, 

 is the almost universal practice, irrespective of local rainfall or of 

 irrigation except where irrigation water is so abundant that it can 

 be used to grow summer crops of legumes for plowing under as well 

 as to supply the moisture requirements of the trees. 



Cultivation, However, Determines Success of Irrigation. The 



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

 ture 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 culti- 

 vation 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 consideration in the 

 effectiveness of a certain amount of rainfall, as has already been 

 suggested in another connection, but it also determines the success 

 of irrigation and the amount of water required ; for, although it was 

 an early and crude practice to rely upon irrigation to support un- 

 cultivated fruit trees and to irrigate more and more frequently as 

 the ground became harder from its use, this policy has now no stand- 

 ing in commercial fruit growing. Not only was it wasteful of water, 

 but it was otherwise detrimental to the thrift of trees. 



Tillage and Irrigation Work for Soil Improvement. Rational 

 tillage, both in winter and summer, has other very important ends 

 in view. It opens the soil and promotes aeration ; it encourages 

 deeper rooting and thus encourages the tree to take possession of a 

 greater soil mass both for moisture and other plant food. It is part 

 of the very valuable policy of increasing organic matter by plowing 

 under the natural growth of weeds or specially sown legumes. This 

 affords opportunity to use water, beyond the amount the trees re- 

 quire, 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. 



Winter Irrigation. On lands with sufficient depth of fairly re- 

 tentive soil, the grower may artificially supplement a scanty rainfall 



