Copper in cultivated soils of Finland

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.