274 CITRUS FRUITS AND THEIR CULTURE. 



In most cases, the fibrous feeding roots in old groves 

 form a solid mass and these extend quite close to the sur- 

 face. In such cases the cultivation should be shallow, 

 but if a proper system of cultivation has been carried on 

 from the first, the roots will not be too close to the surface. 

 This mat of roots is usually disturbed to some extent, 

 but considering the fact that commercial fertilizers are 

 used extensively, it is not good policy to cultivate deeply 

 and break them up too much. Shallow cultivation should 

 be the rule, a rule to which there may, of course, be ex- 

 ceptions. 



California and Arizona. On the heavier soils of 

 California and Arizona the plow can be used to advan- 

 tage, and, in fact, as an implement of cultivation, it is 

 well nigh indispensable. 



Irrigation is an essential part of citrus culture in 

 all western districts and after each irrigation the ground 

 must be cultivated. Frequently the mistake has been 

 made of cultivating at exactly the same depth year after 

 year. The result has been the formation of a hard, im- 

 pervious layer just below the top layer of cultivated soil. 

 This layer of hard earth has been termed "hard-pan," 

 "irrigation hard-pan," and "plow-sole." Through it the 

 water cannot penetrate. As a result none reaches the 

 lower roots and the trees suffer. 



To break up this hard-pan, the sub-soil plow has fre- 

 quently been brought into operation, and while good re- 

 sults have followed in some cases, much damage has been 

 done in others. Frequently the difficulty may be over- 

 come by slightly increasing the depth of cultivation after 

 each irrigation, until the cultivator or plow finally makes 

 its way down through the hard-pan. 



