178 



CALIFORNIA FRUITS: HOW TO GROW THEM 



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. Xear 

 Visalia, 2 feet above river bottom and 4 feet above the surround- 

 ing 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 7J/2 inches, decid- 

 uous 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 rain- 

 fall is 11J/4 inches, irrigation 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 irrigation. Under all conditions surface tillage by promoting 

 conservation 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 investi- 

 gation and experiment, both in humid and. arid regions. These 

 experiments 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. 



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

 soil moisture include both reception and conservation. For the 

 reception 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'deepersummercultivation,sothat.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 





