New York Agricultural Experiment Station. 385 



The ainonnt of lime necessary to form the Bordeaux mixture was 

 formerly determined by weighing, using two-thirds as much lime as 

 copper sulphate, but by means of the color tests as explained below 

 the necessity of weighing the lime is now obviated. 



Excess of Lime. — It is important that enough \hv.e be added, 

 otherwise the mixture may injure the foliage, while au excess of 

 lime will not harm the foliage. 



Color tests. — Yarious color tests may be used for determining 

 whether or not sufficient lime has been added to the copper sulphate 

 solution to form the Bordeaux mixture, as explained in Bulletin 84 

 of this Station. The one most commonly known is the potassium 

 ferrocyanide test, which is used as follows : 



Pour the lime into the copper sulphate solution, stir the mixture 

 thoroughly and then add a drop of the potassium ferrocyanide. If 

 enough lime has been added the drop will not change color when it 

 strikes the mixture, otherwise it will immediately change to a dark 

 reddish brown color. More lime must then be added till the potas- 

 sium ferrocyanide does not change color when dropped into the 

 mixture. It sometimes happens if the mixture has not been thor- 

 oughly stirred, that some of the copper sulphate in the bottom of 

 the barrel has not yet been precipitated, while at the surface the 

 mixture shows no color when the test is applied, so that after the 

 mixture has been standing a few minutes the potassium ferrocyanide 

 will again give the dark color, showing that not enough lime had 

 been used. On this account it is best to add more lime after the test 

 shows no change of color, thus insuring an excess of lime, which does 

 no harm. A mixture with not enough lime in it will hurt the foliage. 



The potassium ferrocyanide, also known as the yellow prussiate 

 of potash, is a poisonous substance. It is a yellow salt which readily 

 dissolves in water, and a solution may conveniently be kept on hand 

 in a small bottle. The commercial form of the potassium ferrocy- 

 anide may be used. A few cents should purchase enough to last 

 through the season. 



Copper Sulphate Solution. — As explained before, lime is added 

 to the copper sulphate solution in making Bordeaux mixture, to pre- 

 vent the solution from injuring the foliage or fruit, but in some 

 cases the copper sulphate solution is used without the lime in 

 making applications in the spring before the leaves put forth. It is 

 made by simply dissolving the copper sulphate in water and diluting 

 to the required strength. In treating raspberry canes for anthracnose 

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