46 COMMON WEEDS 



the latter. In some cases the leaf surfaces of plants 

 are covered with a greyish waxy excretion or " bloom," 

 which prevents the solution from adhering to or wetting 

 the plants ; the solution collects in drops, which roll 

 off easily, or are shaken off by the wind. 



Copper sulphate is more poisonous to plants, and 

 can therefore be used in weaker solutions, than iron 

 sulphate. A 2 or 3 per cent solution of copper 

 sulphate is about as effective as a 15 per cent solution 

 of the iron salt. Which of them is the more economical 

 depends of course upon the relative price at which the 

 two substances can be purchased. The iron salt is 

 always much cheaper pound for pound than the copper 

 compound ; but the market price fluctuates consider- 

 ably, and which to use must be determined after 

 obtaining quotations for both. 



The amounts generally used are 40 to 50 gallons 

 per acre of a 2, 3, or 4 per cent solution of copper sulphate 

 (say on an average 1 2 Ib. in 40 gallons of water per 

 acre), or 40 to 50 gallons of a 15 per cent solution 

 of iron sulphate (say 60 Ib. in 40 gallons of water per 

 acre). Stronger solutions than these are liable to do 

 permanent damage to crops among which the weeds 

 are growing. 



Hundreds of trials have been made in various parts 

 of the world, to determine the influence of these solu- 

 tions upon many kinds of weeds. They have been 

 found to be most active upon Charlock (Sinapis arvensis) 

 and Wild Radish (Raphanus Raphanistrum), and it may 

 be said that it is for these two weeds alone that spray- 

 ing is generally practised. 



On the other hand, it is true that many other weeds 

 are much damaged by the solutions of copper and iron 

 sulphates. It was found, for example, in 1899, at the 

 Holmes Chapel Agricultural and Horticultural College, 



