CHAPTER XIII 



CULTIVATION 



It was demonstrated very early in California experience in fruit 

 growing, that "clean culture" is generally the proper treatment for 

 trees and vines during the growing season, at least. Though the fre- 

 quent stirring of the soil and eradication of grass and weeds have been 

 advocated by certain horticulturalists for generations and have recently 

 been demonstrated to be desirable by careful comparative experiments 

 it has nowhere secured such wide 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 condi- 

 tion favoring root growth, and the main feature of this condition 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 of greater moment, a summer-growing cover crop may 

 be of benefit to the trees. In irrigated 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 accord- 

 ance 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 summer, 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 temporarily 

 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 can 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 the capillary condition 

 still exists, and evaporation proceeds in the same way, though in a 

 somewhat less degree, as if there had been no cultivation. It becomes 

 evident, then, that the pulverization of the disturbed layer must be so 

 complete that the particles are separated and capillarity destroyed, and, 

 farther, that the free access of air to the lower point, where capillarity 



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