CHAPTER XTII 



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 

 frequent stirring of the soil and 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 wide adherence as in Cali- 

 fornia. It may even be held to be an essential to successful growth 

 of tree and vine in most soils and situations of California, and the 

 several advantages of clean culture are intensified under our con- 

 ditions. 



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

 dition favoring root growth, and the main feature of this condition 

 is the retention of the moisture, though aeration and regulation of 

 summer temperature in the soil are also involved. Where moisture- 

 retention is not the chief concern, because of ample irrigation facili- 

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

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

 gated districts of excessive heat and dry air this policy is successful, 

 but it may always be only the exception to the rule of clean culture 

 for the greater part of our fruit-growing areas. 



Retaining Moisture by Cultivation. It is a familiar fact that 

 water will rise in a tube of exceedingly 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 facilitates 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 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 be only roughly broken up, so that the dis- 

 turbed 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 culti- 

 vation. It becomes evident, then, that the pulverization of the dis- 

 turbed layer must be so complete that the particles are separated 



