162 



CALIFORNIA FRUITS: HOW TO GROW THEM 



They thus require more water than do deciduous trees. There is 

 the same relation between irrigation and rainfall with citrus, as with 

 deciduous fruit trees, but the degree of relation is different. Many 

 trials have shown that it is practically impossible to grow satisfac- 

 tory citrus fruits without irrigation, unless there be underflow, 

 and this is often attended by the usual difficulties of high ground 

 water and may be undesirable. There is no combination of heavy 

 rainfall, or winter irrigation, and soil retentiveness which will supply 

 the summer and autumn thirst of the orange or lemon in California. 

 Irrigation, too, must be maintained both summer and winter 

 wherever the rainfall is not well distributed and adequate. In the 

 chief citrus regions of the State rainfall is seldom adequate except 

 during January and February, and not always then. Under such 

 conditions an estimate of the average requirements of citrus fruit 

 trees in bearing would be about 20 inches of irrigation, irrespective 

 of rainfall, although there are localities of larger rainfall and more 

 retentive soils where crops of these fruits can be made with 10 

 inches used at just the right time. 



RELATIONS OF SOIL TO IRRIGATION 



As already stated, the desirability of irrigation is unquestionably, 

 in many cases, conditioned upon soil depth and character. This 

 relation has received careful attention from soil physicists, and an 

 understanding of it involves problems of plant growth and the move- 

 ment of water in soils, the leading facts of which are available in 

 popular form.* 



Analysis of such phenomena can not be undertaken in this con- 

 nection, but a few striking contrasts in existing practice are very 

 suggestive. 



On the famous river-bank fruit land of the Sacramento Valley, 

 with loams of great depth and good retentiveness, and with an aver- 

 age rainfall of approximately 20 inches, irrigation is resorted to only 

 in years of minimum rainfall, when the precipitation is perhaps only 

 about half the average. At nearly the same level, as already cited, 

 where the soil is shallow and overlies hardpan, irregular irrigation 

 is required. But still more marked contrast is found in the foothills 

 within sight of these valley fruit lands, where with twice the average 

 rainfall irrigation must begin early in the summer and continue until 

 autumn is well advanced, because, first, the slope is so rapid that 

 much rainfall is lost by run off ; second, the soil is too shallow above 

 bedrock to hold much water. Even here, however, there conies 

 in a local variation of measurable effect. When the soil lies upon 

 vertical plates of bedrock much water is retained between them, 

 and is capable of being reached by tree roots, while soil lying upon 

 flat plates of rock has no such subterranean reservoir. In the foot- 

 hill region there also occurs exceptional exposure from slopes facing 



f a rtn* R?lat tu nS f S u ils * to . climat *> U. S. Dept. AKT., Weather Bureau Bui. 3. Water as a 



m the Krowth of plants. Yearbook U. S. Dept. of A K r.. 1894, p. 165. Some inter- 



: stm K soil problems. Yearbook U. S. Dept. A K r., 1897. p. 429. The movement and re- 



^,?r P TT Wa | er n n . SO A S ' Y ?? - ok U r $?*# ARr " 1898 ' - 399 - The mechanics of soil 

 moisture, U. S. Dept. Aj?r., Division of Soils, Bui. 10. 



