

IRRIGATION AND SOILS + 



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. 



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 

 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 

 average 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 hard-pan, 

 regular 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 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 



* 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. 



