156 PLANT DISEASE 



in lime, while those immune to it contain a 

 higher percentage of this mineral. 



One can understand the lime disintegrating 

 the soil so that the roots can avail themselves 

 of every particle of iron, irrespective of a 

 better oxidation of the said iron, which I 

 believe leads to an increased nitrification, as 

 explained under Nitrogen ; but beyond that it 

 looks as if the presence of lime had something 

 to do with this nitrification, which, as far as I 

 know at present, is neither recognized nor 

 understood. But the fact remains that liming 

 the soil round trees suffering from aphis pro- 

 duces a healthier growth, and under favour- 

 able circumstances, drives off the aphis, which 

 I have not the least doubt would be destroyed 

 if the soil were also manured with iron. 



Professor E. W. Hilgard, Director of the 

 College of Agriculture, California, says 



" The attack of phylloxera on the vines 

 produces a diminution of potash and albumen 

 normally contained in the juice. The indica- 

 tion that potash with nitrogenous manures 

 would mitigate the effect on the vines produced 

 by phylloxera has been verified by experience. 



