Aug. 7. 1916 Mottle-Leaf of Citrus Trees 733 



Mention has already been made of the fact that the appearance of the 

 mottled leaves indicates that the mottled Citrus tree is failing to secure 

 something essential in the formation of chlorophyll. The association 

 between mottling and low humus suggests that the missing substance 

 may be some' organic compound normally formed during the decom- 

 position of organic matter in the soil or associated with the formation of 

 humus, in which event the "humus" content would be indicative to some 

 extent of the amount of this substance formed. Until further information 

 is available in this connection, practical considerations point to the imme- 

 diate enrichment of the humus content of the soil as the most promising 

 specific for mottle-leaf. 



RELATION OF MINERAL CARBONATES TO MOTTLING OP ORANGE TREES 



The mineral carbonates in the soils of the area studied consist for the 

 most part of calcium carbonate (limestone). The percentage is usually 

 low (see Table I), although large deposits of limestone are found in some 

 of the hills rising from the floor of the valley. No significant correlation 

 was found to exist between the percentage of mineral carbonates and the 

 percentage of mottled leaves (correlation coefficient = 0.07 ±0.06). In 

 other words, there is no evidence that the amount of mineral carbonates 

 within the limits found in these soils bears any relation to mottling. 

 Most of the groves in the areas studied have not been limed, and where 

 lime has been used, the amount applied has with few exceptions been 

 so small as to be negligible in the determinations. For example, an 

 application of a ton of limestone per acre would mean an increase of only 

 two one-hundredths per cent when calculated on the weight of the soil 

 to a depth of 3 feet. The effect of heavy applications of lime on mot- 

 tling has not yet been definitely settled by properly controlled field 

 experiments. This matter should furthermore not be confused with 

 the evident beneficial effect of lime in improving the physical condition 

 of some of the soils in the area studied. 



RATIO OF HUMUS TO MINERAL CARBONATES AS AFFECTING MOTTLING OF ORANGE 



TREES 



The ratio of humus to mineral carbonates in orange groves is plotted 

 in figure 2 against the percentage of mottling. While the relationship 

 is not marked, the mottling tends to diminish as the humus-lime ratio 

 increases. The correlation between the reciprocal of this ratio and the 

 mottling was computed and found to be 0.17 ±0.06. Since no relation- 

 ship was observed between the lime content of the soil and the percentage 

 of mottling, it seems probable that the correlation observed in the case 

 of the humus-lime ratio is dependent wholly on the humus correlation. 

 The result indicates that the humus content of the soil should be taken 



