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A Simple Microbial Bioassay for Plant Available Manganese¹

ABSTRACT

A bioassay using the soil bacteria Bacillus brevis or Ensifer adhaerens for determining plant-available Mn in organic soils was developed. For this assay, a 1:10 soil-agar plug was prepared in a nutrient agar plate and then overlaid with semisolid nutrient agar inoculated with either test organism. Manganese-induced zones of growth stimulation were measured after 24-h incubation at 27°C. There was a linear relationship between the logarithm of standard concentrations of Mn and the zone of growth stimulation. The enhanced growth was a specific response to Mn; Ca, Co, Cu, Fe, Mg, Mo, Ni, Sr, Zn, NO^-₃, PO^3-₄, and SO^2-₄ did not stimulate growth of either organism. Addition of glucose as a C and energy source or yeast extract as a source of vitamins and growth factors had no effect on growth. Six organic, six Mn-amended organic, and 17 mineral soils were tested with this assay. This new assay clearly identified five nonamended organic soils that were Mn-deficient as determined by the growth of spring wheat (Triticum aestivum) in a growth chamber study. There was a statistically significant correlation between the zone of growth stimulation on bioassay plates and plant relative yields, Mn concentration, and Mn uptake. The DTPA extracts of these soils did not predict this Mn deficiency. Separation of responding from nonresponding soils was achieved using the E. adhaerens bioassay. This assay allows rapid and inexpensive identification of Mn-deficient soils.

¹ Contribution no. R423, Saskatchewan Institute of Pedology.

² Assistant Professor, Assistant Professor (NSERC), and Professor, Dep. of Soil Science, Univ. of Saskatchewan, Saskatoon, Saskatchewan, Canada, S7N 0W0, respectively.

Received for publication January 23, 1985. Accepted for publication July 11, 1985.