Formula 3 is the formula for general use on apples, pears, asparagus, grapes, tomatoes, melons, strawberries, etc.
Formula 4 is the strongest formula that is often used. It is considered best for potatoes and cranberries. It may be used on grapes, on apples and pears before blossoming and sometimes on other crops. It was once more commonly used, but, except as here quoted, it is generally being displaced by Formula 3.
Normal or 1.6 per cent. Bordeaux mixture:
| Copper-sulfate (Blue Vitriol) | 6 pounds |
| Quick-lime (Good stone lime) | 4 pounds |
| Water | 50 gallons |
Six pounds of sulfate of copper dissolved in fifty gallons of water, when applied at the proper time, will prevent the growth of fungi. However, if applied in this form, the solution will burn the foliage. Four pounds of quick-lime to six pounds of copper will neutralize the caustic action. When sulfate of copper and lime are added in this proportion, the compound is Bordeaux mixture.
Weighing of copper and lime at time of mixing is very inconvenient. Bordeaux mixture is best when used within a few hours after being mixed. Therefore a stock mixture of Bordeaux is impracticable. It is, however, practicable to have stock preparation of sulfate of copper and of lime ready for mixing when required.
The lime should be fresh quick-lime and when slaked must always be covered with water to exclude the air. In this manner a "stock" mixture of lime can be kept all summer unimpaired.
Sulfate of copper can be dissolved in water and held in solution until needed. One gallon of water will hold in solution two pounds of copper sulfate. To accomplish this the sulfate should be suspended at the surface of the water in a bag. The water most loaded with copper will sink to the bottom and the water least loaded will rise to the surface. If fifty pounds of sulfate are suspended in twenty-five gallons of water on an evening, each gallon of water will, when stirred the next morning, hold two pounds of sulfate. This will form the stock solution of copper sulfate.
If three gallons of this solution are put in the spray barrel, it is equivalent to six pounds of copper. Now fill the spray barrel half full of water before adding any lime. This is important for if the lime is added to so strong a solution of sulfate of copper, a curdling process will follow. Stir the water in the lime barrel so as to make a dilute milk of lime, but never allow it to be dense enough to be of a creamy thickness. If of the latter condition, lumps of lime will clog the spray nozzle. Continue to add to the mixture this milk of lime so long as drops of ferrocyanide of potassium (yellow prussiate of potash) applied to the Bordeaux mixture continue to change from yellow to brown color. When no change of color is shown, add another pail of milk of lime to make the necessary amount of lime a sure thing. A considerable excess of lime does no harm. The barrel can now be filled with water and the Bordeaux mixture is ready for use.