CHAPTER XIII 

 CULTIVATION 



It was demonstrated very early in California experience in 

 fruit growing, that "clean culture" is generally the proper treat- 

 ment for trees and vines during the proper season, at least. 

 Though the frequent stirring* of the soil and the eradication of 

 grass and weeds have been advocated by certain horticulturists 

 for generations and have recently been demonstrated to be desirable 

 by careful comparative experiments, it has nowhere secured such 

 w r ide adherence as in California. It may even be held to be an 

 essential to successful growth of tree and vine in most soils and 

 situations in California, and the several advantages of clean culture 

 are intensified under our conditions. 



Chief of these advantages is the maintenance of the soil in a 

 condition favoring root growth, and the main feature of this con- 

 dition is the retention of the moisture, though regulation of summer 

 temperature in the soil is also involved. Where moisture-retention 

 is not the chief concern, because of ample irrigation facilities, 

 and the moderation of soil temperature is of greater moment, a 

 summer-growing cover crop may be of benefit to the trees. In 

 irrigaied districts of excessive heat and dry air this policy may 

 prevail, but it will be only the exception to the rule of clean culture. 



Retaining Moisture by Cultivation. It is a familiar fact that 

 water will rise in a tube of exceeding small diameter very much 

 higher than the surface of the body of water in which the tube is 

 held upright. The water rises by capillary attraction. A compact 

 soil has extending through it, minute spaces, formed by the partial 

 contact of its particles, which facilitate the rise of water from 

 moist layers below, in accordance with the same principle which 

 causes the water to rise in the capillary tube. This movement 

 is constantly going on in firm soil, and as fast as the top layer 

 is robbed of its moisture by evaporation, the water rises from 

 below and it too is evaporated. During the long, dry s.ummer, 

 the water rises and is evaporated from a depth of several feet 

 in some soils, and the earth, beneath the baking sun heat, becomes 

 "dry as a brick." 



When a soil is broken up by cultivation, capillarity is tempo- 

 rarily destroyed through the disturbed layer, because the particles 

 are so separated that the mutual connection of the minute inter- 

 spaces no longer exists. But if it be roughly broken up, so that 

 the disturbed layer takes the form of coarse clods, the air has free 

 access to the upper surface of the firm soil beneath them, in which 



141 



