112 Journal of Agnculture, Victoria. [10 Feb., 1919. 



chemical aspect of the question. The explanation is to be found in the 

 following extract from an article by Professor Fonzes-Diacon : — * 



This salt (sodium carbonate), being very soluble in water, as opposed to lime 

 which is only very slightly so, its reaction on the copper sulphate is immediate; for 

 these reasons it would appear that one might employ the theoretical quantity 

 of dry soda carbonate giving the maximum of tetracupric sulphate, that is, 680 

 grms. for 2 kilogs. of copper sulphate (.3.2 lbs. for 10 lbs. of copper sulphate; but 

 it is well to slightly force the theoretical dose, in order to compensate certain 

 secondarj'^ reactions, due to the liberation of carbonic acid gas, and it is for this 

 reason tlaat I have indicated the doses of 750 and 800 grms. of solvay carbonate 

 of soda (equivalent to 3.33 lbs. and 3.6 lbs. pure soda carbonate for 10 lbs. blue- 

 stone) as giving spray mixtures rich in tetracupric sulphate, but more or less 

 slightly acid. . . . These acid boniUies hourguignonnes, of which the fungicide 

 action is, according to M. Ravaz, as energetic as that of Bordeaux mixture, are 

 thus of more constant composition, and their preparation presents somewhat less 

 difficulty. 



Summary. 



From the foregoing it will be seen that copper soda, as usually pre- 

 pared by our orchardists and j)otato-growers, presents several serious 

 defects. Unless the washing soda used be very impure, the precipitate 

 will inevitably contain its copper in the carbonate form. This, it is true, 

 possesses considerable fungicide power, as is evidenced by the protection 

 hitherto afforded by the copper soda sprays used in the orchard and field. 

 Nevertheless, there are sound reasons for prefei-ring a copper soda spray 

 containing its copper mainly in the form of basic sulphate; the advan- 

 tages being greater stability, less scalding of foliage, and higher fungi- 

 cide power. Such a spray is obtained by following the practical direc- 

 tions given above, which entail the use of considerably less soda than is 

 usually recommended. 



It may further be explained that if too little soda carbonate be 

 used the mixture will be acid, and will scald the foliage by the excess 

 of copper sulphate it contains. Since soda carbonate does not burn the 

 foliage, it would at first sight appear that an excess of soda would be 

 of no consequence. As a matter of fact, copper soda is more severe on 

 the foliage if alkaline than if it be acid. The explanation of this seem- 

 ing contradiction is as follows. In the case of alkaline copper soda, 

 leaf injury is not due to excessive soda, but to the presence of copper 

 carbonate. The class of mixture which causes least leaf damage is, 

 therefore, that which contains the maximum of tetracupric sulphate. 

 Another serious drawback to the use of too much soda is the liability 

 to malachite deterioration, as has been explained. 



It is thus evident that copper soda differs radically from Bordeaux 

 mixture, in connexion with which an excess of lime does not present any 

 undesirable features. 



The substitution of soda ash for washing soda constitutes a consider- 

 able improvement; being of constant composition, the requisite quantity 

 can be accurately determined by weighing. This is not so with washing 

 soda, owing to the variability of its pure soda content. 



• ProgreM Agrieole, 2nd May, 1915, dealing with copper soda. 



