Aug. 7, 1916 



Mottle-Leaf of Citrus Trees 



73 1 



RELATION OF HUMUS IN SOIL TO LEAP MOTTLING OP ORANGES 



/OO 



The relation of the percentage of leaf mottling to the percentage of 

 humus in the soil is shown in figure 1, the humus being plotted as the 

 abscissas and the mottling as ordinates. While the points by no means 

 form a smooth curve, there is a very evident inverse relation, showing 

 that a high humus content is correlated with a low percentage of 

 mottling. 



As already mentioned, some time is required for an orange or lemon 

 tree to respond to an application of manure. Consequently, in cases 

 where manure has been 

 recently added to a 

 grove a measurable 

 increase in humus may 

 result without suffi- 

 cient time having 

 elapsed for a leaf re- 

 sponse. Furthermore, 

 when a leaf is well ad- 

 vanced in mottling, it 

 does not recuperate 

 (except by special leaf 

 treatment), but re- 

 mains mottled until it 

 drops. Hence, a new 

 set of leaves must be 

 grown before the mot- 

 tling will disappear 

 from the tree, although 

 the first stage of mot- 

 tling, especially in a 

 young leaf, may disap- 

 pear as the leaf grows, 

 if conditions become 

 favorable. With these 

 facts in mind, one 

 would expect that the humus graph presented in figure 1 would show 

 some inconsistencies, especially since other soil factors besides humus 

 undoubtedly influence the nutrition of the tree. 



The relationship between humus and mottling has been examined in 

 more detail by the use of statistical methods. The form of the graph in 

 figure 1 suggests an approximate hyperbolic relationship between humus 

 content and mottling. In order to reduce the data to a suitable linear 

 form for calculating the coefficient of correlation, the reciprocal of the 

 humus content of each soil was calculated — that is, the number of grams 



°o./o 



0.24 



0./2 0./4 o./e o./& 0.20 0.22 



Fig. 1. — Graphical presentation of the relationship between humus 

 content of soil and percentage of mottled orange leaves (from data 

 in Table II). 



