CULTIVATION AND IRRIGATION 159 



rainfall in the valleys below is often less than the needs of even the 

 deciduous fruit trees, and waters flowing from mountain snows 

 through a region of unirrigated uplands must be used to irrigate 

 them. 



Still another striking contrast, and one involving another and 

 wholly different factor, is found in the San Joaquin Valley. Near 

 Visalia 2 feet above river bottom and 4 feet above the surrounding 

 plains, there is a large area of deep alluvial soil with much decayed 

 vegetable matter. The land is moistened by underflow from the 

 river, and, though the rainfall is but 7]/2 inches, deciduous fruits are 

 grown without irrigation. In the same county, and only 18 miles 

 distant, there are areas of rich loam mixed with granitic sand 16 to 

 18 feet deep. In this locality, though the rainfall is 11*4 inches, irri- 

 gation is practiced freely, as the loss of moisture in summer is very 

 great. 



RELATION OF TILLAGE TO IRRIGATION 



Tillage, particularly during the dry season of the year, under 

 some conditions, directly determines the need of irrigation, and is 

 to a certain extent, as the popular phrase goes, a substitute for irri- 

 gation. Under all conditions surface tillage by promoting con- 

 servation of soil moisture, is determinative of the actual duty of 

 water, whether it be from rainfall or irrigation. The effect of fre- 

 quent surface tillage has been accurately determined by investiga- 

 tion and experiment, both in humid and arid regions. These experi- 

 ments fully support the view taught by the experience of about half 

 a century in California, in accordance with which thorough tillage 

 has been so widely practiced in the arid sections as an essential to 

 successful fruit growing 1 . 



As already maintained in Chapter XIII, the relations of tillage to 

 soil moisture include both reception and conservation. For the re- 

 ception of moisture, deep work with the plow, and sometimes with 

 the subsoiler also, is almost indispensable. To retain this moisture 

 and to prevent, as far as possible, its escape into the thirsty air of 

 the arid region by surface evaporation, less depth and more thor- 

 ough surface pulverization are required. Recent practice has been tend- 

 ing toward deeper summer cultivation, so that, as previously claimed, 5 

 or 6 inches of loose, finely divided soil is now obtained where formerly 

 half that depth was considered adequate. It has also been shown 

 that frequent stirring of this fine surface layer checks evaporation, 

 even when no water is applied to compact the surface or where no 

 weeds grow to draw upon the soil moisture. In a word, the aim 

 of tillage in the arid region, so far as it relates to moisture supply 

 in the soil consists in opening the soil to rain, or to irrigation, and 

 in subsequently closing it to evaporation. These are the principles 

 which were recognized and applied in California half a century agt> 

 and are now enjoying somewhat sensational renaissance in the "dry 

 farming" agitation in the interior of the United States. 



A Negative Declaration. It is interesting that current practice 

 affords full demonstration of the foregoing claims both positively 



