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Soils that were fertilized with 0 and 20 ppm of applied Mn (as MnSO4) and previously cropped with soybeans were sown to sudangrass to evaluate the residual effect of Mn fertilization. Prior to planting, soil samples were taken from each pot, and carrier-free 54Mn was mixed with the remaining soil. Isotopically exchangeable Mn (L value) was calculated by measuring the 54Mn in the plants at each of three harvests. Soil samples were equilibrated with 0.1 N H3PO4 containing 54Mn to obtain E values. Soil-extractable Mn by six extractants and L and E values were correlated with Mn uptake by the sudangrass.

Three successive harvests of sudangrass differed significantly in their Mn uptake between fertilized and unfertilized pots, showing the residual effect of Mn applied as MnSO4. The L and E values were useful parameters in predicting Mn uptake response due to the residual effect of Mn fertilization. Correlation of soil tests with the average Mn uptake (three harvests) by sudangrass occurred in the order: H3PO4 > steam/ NH4OAc > NH4OAc > E value > L value > DTPA > NH4H2PO4 > HCI. Inclusion of pH and bases ratio (CA + Mg/K) in the prediction equation of the soil tests improved the R values of these equations markedly. The critical levels of Mn in soil for sudangrass grown in a greenhouse, as determined by L value, E value, and 0.1 N H3PO4, were 32, 70, and 14 ppm of Mn, respectively.

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