Availability of Cu, Zn and Mn in soils: I.—Influence of soil pH, organic matter, and extractable phosphorus

Samples from Ap horizons of 36 cultivated Wisconsin fields were tested for concurrent availability of Cu, Zn and Mn. The effects of soil pH, organic matter, and available P were evaluated by using four chemical extract ants. Oats were used as the test crop and were grown using a self‐watering pot‐culture technique in a plant‐growth room. The divergent soils had the following averages: pH, 6–4; organic matter, 2–6%; available P, 37 ppm; total Cu, 20 ppm; total Zn, 35 ppm; and total Mn, 631 ppm. Concentrations of the micronutrient elements in plants and soils were determined by atomic absorption spectrophotometer. NPK fertilisation resulted in greater plant uptake of Cu, Zn and Mn. Significant interactions between the soil properties and the different chemical fractions influenced the plant uptake of each micro‐element; interactions between Cu, Zn and Mn in the same chemical fraction also influenced their individual uptake. Copper uptake was best predicted by inclusion particularly of soil pH, or the same chemical fractions of Zn and Mn in the regression equation; Zn uptake by inclusion of soil organic matter and available P, Mn uptake, or the chemical fractions of Cu and Mn; and Mn uptake by inclusion of available P, Cu chemical fraction, or Zn uptake in the equation. The extractants N ammonium acetate (pH 7) 10 · 01 M EDTA and 0·1 N ‐HCI show promise in soil tests for the simultaneous availability of Cu, Zn and Mn.