Abstract
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. MnAAAc-EDTA ranged from 1.8 to 158.8 mg/kg, mean 32.2 mg/kg. MnAAAc-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 MnAAAc-EDTA. In order to take into account the effect of pH on plant-availability of MnAAAc-EDTA, the MnAAAc-EDTA indices were multiplied by two different pH correction coefficients. The pH correction resulted in a closer correlation between MnAAAc-EDTA and exchangeable Mn, but in a poorer correlation between MnAAAc-EDTA and reducible Mn. The pH-corrected MnAAAc-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 indices (R2 = 3 %). However, the original 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, MnAAAc-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.
Highlights
The supply of soil Mn to plants is of interest both at low levels where deficiency may occur and at excessive levels where toxic reactions may arise
In Denmark, the Mn fertilization recommendations are based on soil pH instead of soil Mn indices (Nilsson 1984)
Reisenauer (1988) classified the extractants used for soil Mn into five groups: (1) water and dilute salt solutions, (2) ammonium acetate, pH 4 or pH 7, with or without reducing agent, (3) dilute acids, (4) chelate solutions (DTPA and EDTA) and (5) total soil Mn fusion analysis
Summary
The supply of soil Mn to plants is of interest both at low levels where deficiency may occur and at excessive levels where toxic reactions may arise. Mn toxicity can be a growth limiting factor in acid soils (Wright et al 1988), in soils high in total Mn or in soils of low oxygen levels caused by poor drainage, compaction or excessive irrigation or rain (Reisenauer 1988). A number of properties control Mn availability and cause plant-available Mn levels in the soil to vary with time. These properties include: soil pH, total Mn content, soil aeration status, microbial activity and organic matter content (Foy 1984). Reisenauer (1988) reviewed, that most workers have had little success in relating soil analysis to plant uptake of Mn. In general, the acid extracts and DTPA have given the closest correlations
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