158 CALIFORNIA FRUITS: HOW TO GROW THEM 



the chief citrus regions of the State rainfall is seldom adequate ex- 

 cept 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, irre- 

 spective 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. 



RELATION OF SOIL TO IRRIGATION 



As already stated, the desirability of irrigation is unquestion- 

 ably, 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 movement of water in soils, the leading facts of which are avail- 

 able 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 aver- 

 age 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 bed rock to hold much water. Even here, however, there 

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

 upon vertical plates of bed rock 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 

 foothill region there also occurs exceptional exposure from slopes 

 facing the midsummer sun in an atmosphere whose dryness is but 

 slightly ameliorated by the influence of air currents from the coast. 



In the valley and foothill contrast, just cited, the unirrigated 

 valley looks up to the irrigated foothills. There are also places 

 where unirrigated hillslopes look down upon irrigated valleys. The 

 uplands of San Diego County are nearer the coast than those above 

 the Sacramento Valley. They, too, have a rainfall usually ample 

 for deciduous fruits suited to their elevation. Their rolling plateaus 

 of deep soil, free from excessive heat and evaporation which occur 

 on the highlands farther inland and 500 miles farther north, produce 

 very successfully without irrigation. In this region, however, the 



*Relations of soils to climate, U. S. Dept. Agr., Weather Bureau Bui. 3. Water as a 

 factor in the growth of plants, Yearbook U. S. Dept. of Agr., 1894, p. 165. Some interesting 

 soil problems, Yearbook U. S. Dept. Agr., 1897, p. 429. The movement and retention of water 

 in soils, Yearbook U. S. Dept. Agr., 1898, p. 399. The mechanics of soil moisture, U. S. Dept. 

 Agr., Division of Soils, Bui. 10. 



