Aug. 7, 1916 Mottle-Leaf of Citrus Trees 739 



areas studied were badly mottled in all cases, so far as observed in these 

 studies. This was also the case where sodium nitrate was used alone or 

 as the principal fertilizer for some years. 



The results of the soil analyses show in the case of oranges a marked 

 inverse correlation between the humus content of the soil and the per- 

 centage of mottling, the latter tending to diminish as the humus content 

 increases. An impartial statistical study of the data from the individual 

 orange groves shows that approximately one-half the mottling can be 

 accounted for by the low humus content of the soil. 



The humus content of the lemon soils studied is much below that of 

 most of the orange soils, averaging less than 0.1 per cent. This amount 

 of humus is apparently too low to produce a normal foliage growth, all 

 of the lemon groves being badly mottled. 



No correlation was found between the mineral carbonates of the 

 soil and the mottling of orange trees. In lemons the mottling 

 decreased slightly as the mineral carbonates increased, but the corre- 

 lation is low. The lime content of nearly all the Citrus soils studied is 

 low, and the effect of heavy applications of lime can only be determined 

 by suitably controlled field experiments. The present study indicates 

 that the application of lime would be more likely to benefit lemon trees 

 than orange trees. 



The percentage of mottled leaves on orange trees is definitely corre- 

 lated with the increase of the ratio of organic carbon to humus, indicat- 

 ing the importance of the organic matter in the soil being well decomposed. 



No relation was found between the percentage of leaves mottled and 

 the total nitrogen content in the soil in either the orange groves or the 

 lemon groves studied. 



The principal conclusion of this investigation is that the mottling of 

 orange trees in the areas studied is definitely correlated with the low 

 humus content of the soil, the mottling diminishing as the humus con- 

 tent increases. A study of the data by statistical methods shows that 

 approximately one-half of the mottling can be accounted for on this 

 basis. The incorporation of organic matter with the soil in such a man- 

 ner as to be accessible to the roots during its decomposition is indicated 

 as a promising treatment for mottle-leaf. 



