DETERMINATION OF PLANT-AVAILABLE MANGANESE FROM SOILS BY ACID AMMONIUM ACETATE-EDTA EXTRACTION

Manganese was extracted with a solution containing 0.5 M NH4-acetate, 0.5 M acetic acid and 0.02 M Na2-EDTA at pH 4.65 (AAAc-EDTA) from 86 soil samples collected from plough layers in Finland. The results were compared to the quantities of exchangeable, reducible (three methods) and total Mn of the soil samples as well as to Mn uptake in a pot experiment in which four yields of ryegrass were grown. Mn(AAAc-EDTA) ranged from 1.8 to 158.8 mg/kg, mean 32.2 mg/kg. Mn(AAAc-EDTA) correlated more closely with reducible Mn (r = 0.82*** -0.87***) than with total Mn (r = 0.50***) or exchangeable Mn (r = 0.45***), suggesting a relationship between reducible Mn and Mn(AAAc-EDTA). In order to take into account the effect of pH on plant-availability of Mn(AAAc-EDTA), the Mn(AAAc-EDTA) indices were multiplied by two different pH correction coefficients. The pH correction resulted in a closer correlation between Mn(AAAc-EDTA) and exchangeable Mn, but in a poorer correlation between Mn(AAAc-EDTA) and reducible Mn. The pH-corrected Mn(AAAc-EDTA) indices or exchangeable Mn explained the variation in the Mn content of the first ryegrass yield to a higher degree (R2 = 33-38 %) than did the original Mn(AAAc-EDTA) indices (R2 = 3 %). However, the original Mn(AAAc-EDTA) indices explained 38-55 % of the variation in the Mn content of subsequent ryegrass yields, whereas the pH-corrected indices explained only 16-34 % of the variation. Thus, Mn(AAAc-EDTA) is an indicator of the potentially plant-available reserves of Mn, while the pH-corrected indices reflect the quantity of the readily available Mn in the soil.