Copper in cultivated soils of Finland

Markku Yli-Halla

doi:10.23986/afsci.72721

Abstract

Soil samples from the plough layers of 105 fields in different parts of Finland were analyzed for Cu fractions. Vertical distribution of Cu was also studied in a smaller material. Total Cu (Cutot , HNO3-HCIO4-HF-H2SO4 digestion) in the surface soil ranged 6.9-97.4 mg kg-1 (mean 37.1 mg kg-1) and was highest in clay soils (mean 59.0 mg kg-1) and lowest in fine sand and moraine soils (mean 18.3 mg kg-1 ). Copper in the water-soluble, exchangeable and mainly organically bound fraction was extracted with 0.1 M K4P2O7 (Cupy), and Cu bound by poorly crystalline Fe, A and Mn oxides (Cuox) was dissolved subsequently with 0.05 M oxalate (pH 2.9). The average percentages of Cupy and Cuox were 18% and 12% of Cutot in mineral soils and 34% and 19% of Cutot in organogenic soils, respectively. Residual Cu (Cures ) incorporated in mineral lattices was calculated to constitute 70% and 47% of Cutot in mineral and organogenic soils, respectively. In two thirds of soils the potentially plant-available reserves of Cu (Cupy + Cuox) were more plentiful than those of Zn (Znpy + Znox). An acetic acid - ammonium acetate - Na2EDTA solution used in routine soil testing extracted 56% and 71% of the sum of Cupy + Cuox in mineral and organogenic soils, respectively. In soil profiles, CuEDTA was higher in the plough layer than in the subsoil but a few soils rich in Cutot had abundant reserves of CuEDTA below the rooting depth of annual field crops.

Highlights

Soil Cu is commonly divided into fractions with different extractants applied sequentially (McLaren and Crawford 1973, Shuman 1979, 1985, Liang et al 1991)
The material consisted of 25 clay soils, 20 silt and very fine sand soils, 26 fine sand and moraine soils, 14 mull soils and 20 peat soils
The vertical distribution of Cu was studied on seven soil profiles of cultivated fields as well as on 15 pairs of samples from the plough layer (A p horizon) and from the respective subsoil (3035 cm)

Summary

Introduction

Soil Cu is commonly divided into fractions with different extractants applied sequentially (McLaren and Crawford 1973, Shuman 1979, 1985, Liang et al 1991). It is assumed that each solution dissolves a specific fraction retained by a given mechanism or soil constituent; Cu in soil solution, exchangeable, adsorbed, complexed by organic matter or by Fe, Al and Mn oxides and residual Cu incorporated mainly in the lattices of primary minerals (Viets 1962). The residual fraction is considered unavailable to plants, while the other ones, collectively called secondary fractions, are, at least to some extent, sources of plant-available Cu (Gallardo-Lara and Torres-Martin 1990, Liang et al 1991). A few sediment samples mainly from polluted industrial areas of Finland have been analyzed for the fractions of Cu (Räisänen and Hämäläinen 1991) but the fractional distribution of Cu in cultivated soils of the country is unknown. Jokinen et al (1993) found that this extractant dis-

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