588 Thirty-second Annual Report 



proved, Mediterranean Sweet, Ruby Blood, Joppa, and Lue Gim 

 Gong oranges; Tahiti lime; and Sampson tangelo. 



THE EFFECT OF FERTILIZERS AND COVER CROPS ON TREE GROWTH 



AND YIELD 



Results secured during the past two years failed to show 

 any material difference in tree growth and yield as influenced 

 by the use of different kinds of commercial fertilizers. How- 

 ever, a very marked effect of the previous summer's cover crop 

 on the foliage of four-year-old grapefruit trees was observed 

 during the past fall and winter. Parts of the orchard where 

 cowpeas had been turned under showed a distinctly green color; 

 whereas the foliage of other parts of the orchard where no 

 cowpeas had been plowed under was decidedly yellowish in ap- 

 pearance. The experiment indicates that leguminous cover 

 crops have a beneficial effect on the growth of citrus trees, not 

 found in the use of stable manure or commercial fertilizers. 

 The orchard in which the test was conducted had been liberally 

 fertilized with stable manure annually, previous to the use of 

 the summer cover crops. 



The excellent growth of leguminous cover crops on virgin 

 soil, between the rows of young citrus trees suggests the possi- 

 bility of building up and maintaining the fertility of orchard 

 soils on the Yuma Mesa without the use of more expensive fer- 

 tilizers. In view of the extremely light character of the soil 

 of this district, this is an important matter. Sour clover (Meli- 

 lotus indica), common vetch, and hairy vetch, planted on No- 

 vember 3, made a growth of twelve to eighteen inches by the 

 end of May. It might be noted further that the value of inocu- 

 lating leguminous orchard cover crops in this district was 

 clearly demonstrated by the remarkably greater growth of in- 

 oculated clovers and vetches as compared with the growth of 

 similar crops planted without treatment. The leguminous crops 

 that were not inoculated were an absolute failure. 



An experiment was recently started on the Yuma Mesa 

 to determine whether there is any advantage in attempting to 

 build up orchard soils through the use of cover crops before 

 the trees are planted. As a first step in the operation, a sum- 

 mer cover crop of cowpeas was planted on June 21. This will 

 be followed by a winter legume crop. 



