INSECT PESTS AND DISEASES. 175 



of which a sufficient quantity was not previously contained 

 in the soil." 



It remains to say that the manures necessary to restore 

 a tree to health vary as the soils, although the ashes of 

 the wood of the apple tree contain seventy-one per cent, 

 of lime an exceptionally large quantity it would not 

 be necessary to supply this element on a lime formation ; 

 nor would soda be required in a soil near the sea, although 

 on other geological formations or situations a deficiency 

 of one or both may be the cause of canker. 



Ville lays down the rule that soils generally contain 

 sufficient of all the mineral elements except potash, lime, 

 and phosphorus, and the gaseous element nitrogen, and 

 says it is only necessary to supply to the soil manures 

 which contain these four. This may be sufficient for the 

 general purposes of cultivation, but more recent experi- 

 ments have conclusively proved that the addition of a 

 small quantity of iron largely increases the development 

 of foliage, and consequently of the plant. In dealing 

 with a mysterious disease, such as canker, I should not 

 leave out either iron or magnesia. 



" The following formula, which may be varied as cir- 

 cumstances require, is suitable for the apple tree: 



Superphosphate of Lime ... ... 12 parts. 



Nitrate of Potash 10 



Chloride of Soda ... ... ... 4 ,, 



Sulphate of Magnesia ... ... 2 



,, Iron ... ... ... 1 part. 



,, Lime ... ... ... 8 parts. 



" This may be used at the rate of a quarter of a pound 

 to the square yard over the whole extent of soil within 

 reach of the roots. It need not be dug in. One effect of 



