Organic matter can accumulate in surface soil when reduced tillage methods are used. This in turn can influence the plant-availability of micronutrients. This paper reports an incubation study carried out to determine the effects of organic matter and micronutrient addition (Mn, Cu, and Zn) on the distribution of Mn, Cu, Fe, and Zn among soil fractions. Eight topsoils were treated with 0, 20, and 50 g/kg of ground straw and none or one level of the above combined metals. After 5 mo, soils were extracted sequentially to determine metals in the exchangeable, organic, Mn oxide, amorphous Fe oxide, crystalline Fe oxide, and residual fractions. Metals were also extracted using Mehlich 1, Mehlich 3, and DTPA.
Increasing organic matter caused Mn and Fe to move from the less soluble forms to more plant-available forms (exchangeable and organic). Oxidation-reduced effects are the probable mechanism of this movement. Copper was not affected significantly. Increasing organic matter caused Zn to increase in the Mn and Fe oxide fractions at the expense of the Zn in the other fractions. Released Zn may have been occluded in these fractions or been strongly adsorbed. Added metals were evident in the exchangeable and organic fractions. Metals extracted with Mehlich 3 increased with increases in organic matter, but the other extractants did not consistently show this pattern. Correlations between metals in the fractions and that extracted by the extractants showed that the soil test extractants were solubilizing the metals from the exchangeable and organic fractions. These results demonstrate that accumulated organic matter near the soil surface can increase plant availability of Mn and Fe and possibly decrease the availability of Zn by causing a redistribution of elements between soil fractions.
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