INSECTICIDES, FUNGICIDES, ETC. 189 



iron from it, the one metal probably displacing the other. How 

 long the copper may remain in the leaf under ordinary conditions, 

 and how far it may preserve the leaf from fungus attacks while 

 there, requires further investigation. What is known at present 

 is that the protective effect of spraying does not generally last 

 for more than a few weeks, and that even this effect is not due 

 solely to the absorption of copper by the leaf, but depends also 

 on the deposit of fungicide left on the surface. 



Clark's value for the smallest amount of copper which affected 

 a fungus in those cases which he investigated was 0-00004 per 

 cent, of the metal, and copper, even in some of its most insoluble 

 compounds, will dissolve almost to this extent : ignited copper 

 oxide in aerated water gave a solution containing 0-00003 per 

 cent, of the metal (XI, 109), and the action of the oxide in causing 

 leaf -scorching was not only perceptible, but even energetic (XI, 

 150) . The toxic effect of copper on plant-growth has been traced 

 in water-culture experiments down to strengths of 0-00002 per 

 cent., 1 and even metallic copper itself is sufficiently attacked by 

 water to prevent the development of plant roots in its vicinity : 

 such roots will not grow through a sheet of clean copper gauze ; 

 but if the gauze be coated with a protective covering of paraffin 

 wax, they will do so. 



In view of these facts it is somewhat remarkable that no 

 deleterious effect on the fertility of the soil has been noticed as 

 resulting from spraying with copper. The application of only 6 or 

 7 oz. of copper sulphate to an acre would convert the water in 

 the top 6 inches of soil into a solution containing copper sufficient 

 to affect plant-growth, whilst the amount of copper applied 

 may, in the case of Burgundy mixture (according to the recom- 

 mendations for 1917), amount to an quivalent of 25 Ibs. of 

 sulphate to the acre at each spraying. That even several such 

 sprayings may not affect the fertility of the soil was shown at 

 Woburn, where various plots of grass and of potatoes were 

 sprayed in 1917 with Burgundy mixture from one to seven times : 

 in the following season the crop of grass from the sprayed plots, 

 as compared with that from the unsprayed ones, showed an 

 excess of 3 per cent., and a crop of onions, grown on the tilled 

 portions of the plots, showed a deficit of 0*2 per cent., such 

 differences being quite insignificant. Yet the soil contained 

 much copper : when sampled, fourteen months after the spraying, 

 the top 6 inches were found to contain 0*0026 grams of metallic 

 copper per kilogram of dried soil where there had been one 



1 Brenchley, Annals of Botany, XXIV, 571. 



