Laboratory studies were conducted to determine fractions of native soil and applied manganese 54 that contribute to the manganese extracted by DTPA (0.005 M diethylenetriamine pentaacetic acid, 0.005 M CaCl2, 0.1 M triethanolamine, pH 7.3). Since DTPA-extractable manganese is closely related to manganese uptake by plants, such studies should provide information about fractions of soil manganese that contribute most to its being taken up by plants. Eight soil types, varying in pH from 4.8 to 7.0, were treated with 448 kilograms nitrogen per hectare as NH4NO3 and labeled with 1 microcurie of manganese 54 (as 54MnCl2) per 100 grams of oven-dry soil. Samples of each soil were fractionated in sequence into water-soluble, exchangeable, organically bound, reducible, iron-bound, and residual manganese fractions before and after extraction with DTPA.
The reducible fraction accounted for an average of nearly 45 percent of the total manganese before DTPA extraction. The exchangeable, organic, and iron-bound fractions each contained intermediate amounts (13 to 19 percent), and the water soluble had less than 2 percent of the total. The distribution varied widely among soils, however, and generally was related to initial soil pH. With some exceptions, soils of lowest pH had the highest amounts of water-soluble, exchangeable, and organic manganese, whereas soils of highest pH contained large amounts of reducible manganese. The distribution of added manganese 54 in soil, both before and after DTPA extraction, generally paralleled that for native soil manganese.
The extraction of native manganese or added manganese 54 by DTPA varied among soils and generally was related to soil pH. In soils of lowest pH, the exchangeable, organic, and reducible fractions contributed most to the manganese extracted by DTPA. In soils of highest pH, the reducible fraction contributed most to extractable manganese.