734 



Journal of Agricultural Research 



Vol. VI, No. 19 



into consideration in applying lime and that such treatment would be 

 more likely to be beneficial in the case of soils with a high humus content. 



RELATION OF ORGANIC CARBON TO MOTTLING OP ORANGE LEAVES 



The correlation between the total organic carbon in the soil and the 

 leaf mottling is very low ( — o. 1 o ± o . 06) . Organic matter is not effective 

 in nutrition until decomposition has set in, and the results indicate that 

 the amount of those decomposition products effective in the control of 

 mottle-leaf and available in the soil at a given time is not necessarily 

 proportional to the total organic carbon present. The negative sign 



of the correlation co- 

 efficient shows that 

 the mottling tends to 

 decrease as the organic 

 matter increases. 



/oo 



/ 



2 



Fig. 2. — Graphical presentation of the relationship between the ratio of 

 organic carbon to humus in the soil and the percentage of mottled 

 orange leaves (from data in Table II). 



RATIO OF ORGANIC CAREON 

 TO HUMUS IN RELATION 

 TO MOTTLING OF ORANGE 



LEAVES 



e- 



The data presented 

 in Table II for the 

 six groups of orange 

 groves show that an 

 increase in the ratio of 

 organic carbon to hu- 

 mus is accompanied 

 by an increase in mot- 

 tling (fig. 3). This re- 

 lationship may be 

 parti}' due to the fact 

 that the mean organic 



content of the soils of the several groups is nearly the same throughout, 

 although the reduction in mottling is accompanied by a slight increase in 

 organic carbon. 



A correlation of 0.43 ±0.06 was found between the organic carbon- 

 humus ratio and the percentage of mottling. While this correlation 

 may be partly associative, as in the case of the lime-humus ratio, the 

 results indicate that it is important in the nutrition of the orange tree 

 that the organic matter be decomposed, so far as possible, into humus, 

 since the greater the proportion of humified organic matter, the smaller 

 the percentage of mottling. This, of course, does not necessarily in- 

 dicate that what we term "humus" is the most effective form of or- 

 ganic matter for promoting a healthy growth of orange leaves; but if a 



