230 THE MONTHLY BULLETIN. 



cover crops, bean straw, alfalfa hay, manure, and the like would be 

 equally satisfactory. 



This series of experiments coupled with others at the Experimeut 

 Station seems to indicate that nitrogen from organic sources is to be 

 preferred in citrus fertilization. The reason for this, if we are correct 

 in the observation, can not now be explained. 



An intensely interesting feature of these experiments is in their 

 comparison with the trees in another experiment, conducted at the 

 Station, primarily on a test of citrus stocks. This stock experiment is 

 in two blocks, one block adjoining the upper end of the fertilizer block, 

 and the other the lower end. The soil is comparatively the same over 

 the entire area. The trees of the stock experiment include Washington 

 navels, Valencias, and Eureka lemons, and were planted the same 

 spring as the fertilizer experiment. All of the trees in the stock experi- 

 ment are fertilized by a cover crop of vetch or MelilotiLs. grown during 

 the winter and turned under in the spring, with a fertilization of stable 

 manure and raw phosphate rock (last year 5 cubic feet of manure and 

 12 pounds of raw phosphate rock per tree). In the summer period the 

 cultivation of these plats is practically the same as the cultivation of 

 the plats of the fertilizer experiment. The difference in the treatment 

 of these plats rests primarily in the fact that a cover crop is groAvn in 

 the winter, thus stopping the cultivation for about five months during 

 the fall and winter and keeping the ground covered with a green 

 growth; while the plats of the fertilizer experiment are given clean 

 cultivation throughout the year. 



The trees on the two blocks of the stock experiment are in general 

 larger, more thrifty, and are yielding much better than those on any 

 plat in the fertilizer experiment. There is furthermore almost no indi- 

 cation of mottling on either of the two blocks of the stock experiment. 

 The superiority of the trees under this treatment to those under any 

 treatment in the regular fertilizer experimeut is very marked. The 

 indications are that the effect of the cover crop on the soil, entirely 

 aside from the nitrogen that it adds, is very beneficial. One can 

 scarcely escape the conviction that the continuous clean cultivation 

 used on the regular fertilizer plats is injurious. This is very significant 

 in view of the fact that about 70 to 80 per cent of the California 

 groves are regularly given clean culture. In only a very small pro- 

 portion of the groves are winter cover crops grown, and even where a 

 cover crop is grown, it is frequently a non-legume, such as rye or 

 barley. 



It would seem that in general one of the factors of fundamental 

 importance in citrus groves is to increase the amount of organic matter 

 in the soil, and this can probal)ly best be done by the regular use of a 

 winter cover crop, such as Melilotus or purple vetch, and the applica- 

 tions of manure, alfalfa hay, bean straw and similar materials as 

 fertilizers. 



