9 BULLETIN 785, U. S. DEPARTMENT OF AGRICULTURE. 
tion than the standard spray calendar provides. It is difficult to 
estimate the effects of the complicated set of local factors that deter- 
mine the effective period of any spray application. A suitable 
chemical test of the spray coating would seem to be a desirable aid 
in forming judgment as to the proper time for renewal. Such test 
should be simple, rapid, and reasonably accurate. The more refined 
methods of chemical analysis, while very exact, would not well serve 
the practical purpose, because of the special requirements in profes- 
sional training, apparatus, and time. 
A PRACTICAL METHOD OF FIELD TESTING. 
_ The following method has been used during two seasons and has 
been found to be reasonably satisfactory in field practice. This is 
indicated by the consistent results obtained with varying strengths 
of copper sprays, many of which have been checked by exact chem- 
ical analyses of duplicate samples. 
A 200-gram fresh weight sample of leaves is washed at least three 
minutes, with occasional stirring, in 1,000 c. c. of a 0.2 per cent 
solution of chemically pure nitric acid in water, either distilled or 
sufficiently pure to give correct results. A convenient amount of 
this acidulated wash water is treated with a few drops of 2 per cent 
solution of potassitum-ferrocyanid solution, sufficient to precipitate 
the copper. A color comparison is then made with a series of known 
dilutions of a standard copper solution, similarly treated with the 
potassium-ferrocyanid solution. From this comparison the copper 
content of the wash water is determined. For more exact readings 
a colorimeter may be used. 
The standard copper solution is made by dissolving 3.928 grams of 
pure crystals of copper sulphate in water to make 1,000 c.c. Each 
cubic centimeter of such standard solution would then contain 1 
milligram of copper, and by proper dilution with water stock solutions 
may be readily prepared for the comparison series containing one-half 
to 10 mg. of copper per 100 c. c. of water, in one-half or 1 mg. grades. 
If the wash water gives readings above 8 or 10 mg. of copper per 100 
c.c.itshould be diluted for accurate color comparison and account of 
this taken in figuring results. On the basis indicated above, any 
direct reading multiplied by 5 will express the amount of copper in 
the spray residue in milligrams per 100 grams fresh weight of leaves, 
a convenient unit for tabulation purposes. The solutions of potassium 
ferrocyanid and of nitric acid may be varied in strength for con- 
siderable differences in the copper content of the spray coating, so as 
to avoid using unnecessarily strong solutions, with consequent re- 
duction in the delicacy of the test. There must always be a sufficient 
amount of nitric acid to dissolve completely the copper compounds 
and of potassium ferrocyanid to precipitate them completely. 
