Aug. 7, 1916 Mottle-Leaf of Citrus Trees 723 



occurs with a view to determining the extent of its correlation with soil 

 conditions. The results of these investigations form the subject of 

 this paper. 



FIELD SURVEY 



As a basis for the investigation a field survey was made of about 175 

 orange and lemon groves near Riverside, Redlands, Highland, and 

 Rialto, Cal. ; a few groves were also examined in the Ontario, Pomona, 

 and Azusa districts. 



Ten or twelve representative trees were selected from each grove 

 examined, usually a 10-acre block. The percentage of mottled leaves 

 on each tree was determined by two men working independently. Soil 

 samples were also taken near the same trees in i-foot sections to a depth 

 of 3 feet, the samples for a given foot section being combined. The 

 fertilizer treatment of each grove and the cultivation and irrigation 

 methods employed were also ascertained as accurately as the records of 

 each grower would permit. 



The soil samples representing each grove were promptly air-dried, and 

 the organic carbon, "humus," total nitrogen, mineral carbonates, and 

 bicarbonates determined in each sample. The moisture equivalent of 

 each sample was also determined in order to compare the moisture 

 retentiveness of the soils. 



The limiting of the sampling to a depth of 3 feet was based upon the 

 results of numerous observations, which showed that the feeding roots of 

 orange and lemon trees do not as a rule extend much beyond this depth. 

 The taproots, of course, go deeper when the soil conditions permit; but 

 the feeding root system spreads out laterally near the surface, and this 

 lateral feeding system does not ordinarily fully occupy the ground even 

 to a depth of 3 feet. Excavations in the districts examined showed 

 repeatedly that the main feeding root system was found usually from 

 within a few inches of the surface to a depth of 18 to 24 inches. 1 Soil- 

 moisture determinations in orange groves, to be presented in another 

 paper, also showed that when the upper 3-foot layer dried out below 

 the wilting coefficient, the tree could not get enough water to keep from 

 wilting, even with available moisture immediately below this layer. 



EXTRANEOUS FACTORS COMPLICATING THE CORRELATION OE MOTTLING 



WITH SOIL CONDITIONS 



The study of the correlation between the degree of mottling and the 

 general soil conditions of the groves is complicated by a number of 

 extraneous factors. One of these is the kind of stock on which the 

 selected buds are grafted. Other conditions being the same, a tree top 

 on sour-orange stock is likely to show more mottling than one on sweet- 



1 Striking exceptions are, however, occasionally met with. Dr. H. J. Webber, Director of the University 

 of California Citrus Experiment Station, informs the writers that in the Claremont section he has observed 

 fine fibrous roots at a depth of 14 feet. 



