Growth and manganese uptake by soybean in highly calcareous soils as affected by native and applied manganese and predicted by nine different extractants

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Abstract
Manganese (Mn) becomes a limiting factor of plant growth under some soil conditions. High pH and abundance of free calcium carbonate in calcareous soils are conducive to Mn‐deficient plants. The Mn status of highly calcareous soils in Iran has not been studied in detail. This experiment was conducted to obtain such information. Twenty‐three surface (0–20 cm) soil samples with pHs from 7.7 to 8.2 and calcium carbonate equivalent (CCE) from 19 to 46% were used in a 7‐week greenhouse study with soybean [Glycine max (L.) Merr.]. The experiment was a 23 × 3 factorial with three replicates, i.e., 23 soils and 3 Mn levels (0, 15, and 30 mg/kg soil as manganese sulfate). Extractable Mn of the untreated soils were determined before planting by nine different procedures. Dry matter yield, Mn concentration of soybean tops, and Mn uptake were used as the measure of plant response. Multiple regression equations showed that the most influencial soil properties in extractability of Mn in highly calcareous soils are cation exchange capacity (CEC) and calcium carbonate equivalent (CCE). Application of Mn increased dry matter, Mn concentration and Mn uptake of soybean plants in most soils. The Mn concentration of plants on some of the soils, however, decreased following the application of Mn. This was attributed to dilution effect caused by enhanced growth. The recovery of applied Mn in all soils was low, persumably due to conversion of the applied Mn to unavailable forms. Regression equations were developed to predict dry matter, Mn concentration, and Mn uptake by plants from soil Mn extracted by water, hydrochloric acid, disodium‐EDTA, and EDTA‐ammonium acetate.