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DIVISION S-2—SOIL CHEMISTRY

Soil Chemical Properties Controlling Zinc²⁺ Activity in 18 Colorado Soils

^a Neptune and Company, 1505B 15th Street, Los Alamos, NM 87544
^b Dep. of Soil & Crop Sciences, Colorado State Univ., Fort Collins, CO 80523
^c 205 Woodside Drive, Provo, UT 89604
^d MS J495, Los Alamos National Lab., Los Alamos, NM 87545

* Corresponding author (kcatlett{at}neptuneandco.com)

Zinc is a heavy metal of much interest since it is a plant micronutrient as well as a potential contaminant in soils. In soil solution, the speciation of Zn, and thus the free Zn activity, determines the plant availability of Zn as a micronutrient and its characteristics as a heavy metal contaminant. A better understanding of the mechanisms that control free Zn activity could improve soil treatments of Zn deficiency or toxicity. Possible controlling mechanisms for Zn activity include adsorption or precipitation. In our study, Zn²⁺ activity was measured by chelation and was related to soil properties for 18 alkaline soils from three farms in eastern Colorado. Soil organic C (OC) and pH were statistically significant parameters in a multiple regression with log Zn²⁺ activity. The significance of OC may suggest that adsorption onto organic matter controls Zn solubility in some of our soils. Log Zn²⁺ activities plotted with pH fell near the soil-Zn solubility line. However, the slope of the regression line was -1 rather than an expected -2, which indicates that another mechanism besides precipitation and dissolution of soil Zn may occur. Another possibility is that there are two different regions of solubility, one below pH 8.4 and one above pH 8.4. It is suggested that free Zn ions may adsorb on organic matter in a region of low pH and may precipitate as franklinite or other minerals, such as a Zn-containing kerolite, at high pH.

Abbreviations: CEC, cation-exchange capacity • EC, electrical conductivity • EDTA, ethylenediaminetetraacetic acid • IC, inorganic C • ICP-AES, inductively-coupled plasma-atomic emission spectroscopy • IS, ionic strength • ISE, ion-selective electrode • MF, mole fraction • OC, organic C • XAFS, X-ray absorption fine structure spectroscopy • XRD, X-ray diffraction

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