Florida Agricultural Experiment Stations


have found that liming of acid soils containing large accumula-
tions of copper reduced copper toxicity symptoms.
 The fact that many citrus groves already contain sufficient
amounts of copper to meet the tree needs and that many old
groves and vegetable fields contain excess amounts of copper
indicates a need for a convenient rapid method of determining
the copper content of the soil. Such a rapid test, accurate to
within 50 pounds copper per acre, is described here. The test
is based on the brown precipitate formed when sodium diethyl-
dithiocarbamate is added to a copper solution. This reagent
is made specific for copper according to the method of Cheng
and Bray (3) by adding disodium ethylenediamine tetraacetate
(EDTA) to the carbamate reagent to eliminate interference
of iron and other heavy metals. The technique of running the
test is similar to that described by Cheng and Bray (2) for
magnesium in soil. The copper is leached from a small cone
of soil on filter paper by the addition of several drops of 1 N HCI.
The filter paper is then turned over and the color developed by
the addition of a strongly buffered solution of the carbamate
and EDTA reagent. The intensity of brown color is compared
with a standard color chart.

 REAGENTS AND MATERIALS

 1. Leaching Solution.-Approximately 1 N HC1. Dilute 8 ml.
concentrated HCI to 100 ml. Add 5 drops of a wetting agent
such as Ultra Wet 60 L. The wetting agent is not necessary on
most soils but does give better leaching action on soils which
are hard to wet.
 2. Carbamate Reagent.-Dissolve 16 grams of disodium ethyl-
enediamine tetraacetate (diNaEDTA)2 and 0.250 grams sodium
diethyldithiocarbamate in 80 ml. of an ammonium citrate solu-
tion containing 22 grams ammonium citrate and 130 ml. con-
centrated NH4OH per liter. Dilute to 100 ml. This solution
is stable for several months if kept in well stoppered dark glass
bottles or if kept in the dark in clear glass bottles. The usual
interfering elements-iron, cobalt, chromium and bismuth-do
not give interfering colors with this reagent in the presence of
the above concentrations of diNaEDTA and NH40H.

 STrade names for diNaEDTA are SEQUESTRENE NA, which is manu-
factured by Alrose Chemical Co., Providence, R. I., and Disodium Versenate
which is manufactured by Bersworth Chemical Co., Framingham, Mass.