CHAKLOCK SPRAYING. 367 



mainly the calcium carbonate) upon the copper sulphate. In 

 America the grain is soaked in a solution of lib. copper sul- 

 phate in 24 gallons of water for 12 hours and then for 

 5 minutes in lime-water." Dressings of copper sulphate are 

 also now recommended for barley and oats, for preventing 

 smut. 



Copper sulphate is also employed in solution for spraying 

 plants, with the object of preventing fungoid diseases. For 

 this purpose a solution containing about O5% of the salt is 

 usually employed. Stronger solutions would be apt to injure 

 the foliage of certain plants. 



Another use of copper sulphate is in the destruction of 

 cruciferous weeds in cereal crops, e.g., charlock in barley or 

 oats. This is effected by spraying the field when the char- 

 lock plants are still small, best when 2 or 3 inches high, and 

 before the stem and flower are formed with a 2 or 3 % solu- 

 tion of the salt, at the rate of about 40 gallons per acre. To 

 be successful the operation should be performed in dry, sunny, 

 calm weather. It is then found that the charlock leaves blacken 

 and the plants die, while the barley and clover not only are 

 not injured, but appear, in many cases, to be benefited by the 

 process. This plan of dealing with charlock was apparently 

 first tried in 1897 by Girard in France. He used a 5% 

 solution and ascribes the destruction of the charlock to the 

 poisonous effect of the solution, which would be retained on 

 the rough and more or less horizontal leaves of the charlock, 

 while it would quickly run off the smooth and erect leaves of 

 the cereals. It is doubtful whether this opinion is entitled to 

 much weight, as clover, which also has horizontal leaves, 

 suffers little or no damage. It is possible that the action is in 

 some way dependent upon the presence in the charlock (as in 

 other ' cruciferae) of organic sulphides or sulphocyanides, and 

 that some reaction of the copper upon these compounds is the 

 cause of the injury. 



Another possible explanation of the toxic action of copper 

 and iron sulphate solutions is that in contact with the cells of 

 plants, osmotic pressure is set up owing to the liquid outside 

 being more concentrated than that in the protoplasm of the 



* F.u-mevs' Bulletin, No. 75, U.S. Dept. of Agrk-. 



