An experiment was conducted for two consecutive years (2000-2001) to study the effect of manganese (Mn) fertilizer on soybean yield and to determine critical level of the soil Mn. Twenty soybean farms were selected from a soybean growing area in the eastern part of Mazandaran province. The soils had a wide range of CaCO3 (5–39 %) and DTPA extractable Mn of 1.56 to 9.10 mg/kg. The experiment on each farm was a randomized complete block ,one with three replications and two treatments , 20 soils and two levels of manganese sulfate (0 and 40 kg manganese sulfate per hectare). Each experimental plot was 49 m2 , planted with Pershing variety of soybean. The aerial parts of plants were harvested at the end of growth period. The results indicated that manganese sulfate application significantly increased the seed yield, Mn concentration and Mn uptake by 24, 10.3 and 37 percent, respectively, and also increased thousand kernel weight, number of seed per plant and seed protein content by 6.5, 10 and 5.1 percent, respectively. Based on farm experiments for obtaining 85% relative yield, manganese critical levels in these soils were calculated to be 3.54 and 4.40 mg/kg, while with 90% relative yield they were found to be 5.10 and 6.20 mg/kg, using Mitcherlich-Bray and Cate-Nelson methods, respectively
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