Aug. 7, 1916 



Mottle-Leaf of Citrus Trees 



737 



RELATION OP HUMUS IN THE SOIL TO MOTTLING OF THE LEMON LEAVES 



It was found that the humus content of the soil in the orange groves 

 varied inversely with the leaf mottling. In the case of the lemon groves 

 no definite relation appears at first sight to exist between these two fac- 

 tors. However, a comparison of Table II with Table IV shows that 

 the humus content of most of the orange groves was much higher than 

 that of the lemon groves. It will be noted from Table IV that the 

 humus content of the lemon-grove groups was in every case less than 

 one-tenth of 1 per cent, an extremely low value. Orange groves in 

 which the humus content approximates 0.1 per cent (Table II) show as 

 high a percentage of mottling as the lemon groves. It would therefore 

 appear that the humus content in the lemon groves is less than is 

 necessary for the growth 

 of a leaf comparatively 

 free from mottling, as- 

 suming that lemon 

 leaves would mottle to 

 the same extent as 

 orange leaves under the 

 same conditions. 



RELATION OF MINERAL CAR- 

 BONATES IN THE SOIL TO 

 LEAF MOTTLING 



An indication of a 

 slight relationship be- 

 tween the mineral car- 

 bonate content of the 

 soil and the percentage 

 of mottled leaves was 

 obtained in the case of the lemon groves, as shown in figure 4. The 

 correlation coefficient is —0.31 +0.09, the minus sign indicating that the 

 mottling decreases as the mineral carbonates (chiefly lime) increase. 

 This correlation coefficient would indicate an association between mot- 

 tling and lime content of about 10 per cent. The probable error is 

 relatively so large that the result can be considered to be little more 

 than indicative of the inverse character of the relationship. The per- 

 centage of mottling is very high even in the case of the highest lime 

 content. The average amount of lime carbonate in the lemon-grove 

 soils was about the same as in the orange-grove soils. 



It is recognized that the lime-carbonate content is very low in all the 

 soils represented in these areas and possibly a higher range of this con- 

 stituent would bring out more definite results. The data presented are 

 not sufficient to justify recommending the application of lime to lemon 

 52169°— 16 2 



0.0? 0.05 0.0s 0.07 O.Off 0.09 o./o 



P£V? C£~ATr /*7/A/£r/?s4/. 01/?&0/V^7-£& //V SO//. 



Fig. 4. — Graphical presentation of the relationship between the 

 mineral carbonates in the soil and the percentage of mottled 

 lemon leaves (from data in Table IV). 



