CULTIVATION AND IRRIGATION 163 



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 

 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^ inches, deciduous fruits are 

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

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

 18 feet deep. In this locality, though the rainfall is 11/4 inches, 

 irrigation is practised 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 conserva- 

 tion of soil moisture, is determinative of the actual duty of water, 

 whether it be from rainfall or irrigation. The effect of frequent 

 surface tillage has been accurately determined by investigation and 

 experiment, both in humid and arid regions. These experiments 

 fully support the view taught by the experience of more than half 

 a century in California, in accordance with which thorough tillage 

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

 successful fruit growing. 



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 thorough 

 surface pulverization are required. Recent practice has been tending 

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

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



