Joint International Conference on Long-term Experiments ...

COMPARISON OF SOIL-TEST EXTRACTANTS FOR ZINK AND COPPER
Rita Kremper - Sándor Berényi - Jakab Loch – Andrea Balla Kovács
Debrecen University, Centre for Agricultural Sciences
Department of Agrochemistry and Soil Science
ABSTRACT
KCl-EDTA, Lakanen Erviö and CaCl2- DTPA solutions were compared as soil Cu and
Zn extractant. The former two methods are the Hungarian official methods, while
CaCl2- DTPA is a frequently studied method word-wide. 27 agricultural soil samples
were collected for the study from the Soil Information and Monitoring System (SIMS)
database. LE and KCl-EDTA extracted similar amounts of Zn and Cu. CaCl2-DTPA
extracted much smaller amounts both for Zn and Cu (approximately half-third as much)
than the other two extractant. It can be attributed to the pH of the extractants. We
suppose that KCl-EDTA and LE solved Zn and Cu oxides-hydrates, Zn and Cu
carbonates at pH 4.36 and pH 4.6 respectively which can not be solved by CaCl2-
DTPA at pH 7.2. We found good correlation between the three methods despite of the
small sample number.
Keywords: extractrant, DTPA, KCl-EDTA, Lakanen-Erviö, microelements
INTRODUCTION
The three sequential steps generally followed to develop micronutrient cation soil tests
are extractant selection, greenhouse evaluation and field calibration (Martens and
Lindsay, 1990). The first step is to select an extractant that will solubilize a
proportionate part of labile forms of micronutrient cations from different soils. The
second step is to evaluate if amounts of cations extracted are related to quantities
absorbed by plants from different soils. This step commonly is completed int he
greenhouse rather than in the field. The third step field calibration of a soil extractant is
conducted if a suitable relationship is established during the greenhouse evaluation.
During field test scientist separate soils into responsive and non-responsive categories,
or determine the insufficient, sufficient and transitional zone of the available
micronutrient cations in the soil.
Most of the Zn and Cu in soils is unavailable to plants and may be divided by chemical
extraction into five chemical pools (McLaren and Crawford, 1973)
1. as free and complexed ions in soil solution
2. as non-specifically and specifically adsorbed cations
3. as ions occluded mainly in soil carbonates and hydrous oxides
4. in biological residues and living organisms
5. in the lattice structure of primarily and secondary minerals.
In soils Zn and Cu cations would be specifically adsorbed by carbonates, hydrous
oxides of Al, Fe and Mn, soil organic matter and phyllosilicates (Udo et al., 1970)
Soil extractants for plant available Cu and Zn forms can be divided into water and
neutral salts, weak and strong acids and chelating agents. Neutral salts by itself (as
CaCl2) proved to extract too small amounts for Zn and Cu which are not measurable
accurately. Dilute acids as (0.025-0.1M HCl) has been used on acidic soils as
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