WHEN TO USE IRRIGATION 161 



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 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 re- 

 quired; for, although it was an early and crude practice to rely upon 

 irrigation 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 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 humus by plowing under the 

 natural growth of weeds or specially sown legumes, which is discussed 

 in Chapter XIV. This affords opportunity 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, how- 

 ever, 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 by 

 thoroughly soaking the land by winter irrigation and then by careful 

 summer cultivation he will be able to conserve enough water in the soil 

 to carry deciduous fruit trees or vines through bearing and autumn bud 

 formation without further water supply. But there are other situa- 

 tions in which no amount of winter irrigation nor rainfall will suffice 

 for these ends. There are foothill orchard areas in which the winter 

 rainfall is two or three times as great as in the valley situations where 

 fruit is successfully grown without irrigation, and yet water must be 

 applied in summer on those foothills or the fruit would be unmarket- 

 able and the trees in distress. The forty or more inches of rainfall 

 falling on a shallow soil underlaid by sloping bedrock in some cases 

 nearly sluices the cultivated soil from its foothold, and yet the over- 

 saturation in winter avails nothing for summer growth, because most 

 diligent cultivation can not retain moisture enough in shallow soil thus 

 situated to sustain bearing trees in good crops of full-sized fruit. The 

 same is true of valley soils underlaid by hardpan. In such cases winter 

 irrigation could add nothing but distress to the soil over-soaked by rain- 

 fall, and summer irrigation, well-timed and adequate, is the secret of 



