Soil Distribution and Soybean Plant Accumulation of Manganese in Manganese-Deficient and Manganese-Fertilized Field Plots

Manganese (Mn) additions are required on certain southern Coastal Plain soils for maximum yield of soybeans (Glycine max (L.) Merr.). This study was conducted to determine the soil profile distribution of added Mn, the accumulation of Mn in soybean plant tops, and the production of plant dry matter in field plots having five rates of Mn and two soil-pH levels. ‘Ransom’ soybeans were grown on an Olustee-Leefield sand, an Ultic, Haplaquod-Arenic, Plinthaquic Paleudult, adjusted to pH levels of 6.3 and 7.0 and supplied with Mn at 0, 5.6, 11, 22, and 56 kg/ha each year for three years. Near the end of the growing season in the first and third years, dry matter production and plant accumulation of Mn were determined on whole plants (R6 growth stage). At the end of the third year, extractable (DTPA and double-acid) soil Mn was determined in 15-cm increments to a depth of 90 cm. Considering all Mn rates and pH levels, < 0.5% of the added Mn could be accounted for in the plant tops. Soil depth samples showed that most of the added Mn remained in the upper 30 cm of soil, even though at the high Mn rate, a total of 168-kg Mn/ha had been applied. Summation of the Mn contents of all soil depth increments revealed that double acid extracted 32 to 57% and DTPA 2.6 to 7.2% of the total amount of applied Mn. Total dry matter production was reduced at the 0- and 5.6-kg Mn/ha rates at the high pH level compared to the 56-kg Mn/ha rate. Maximum dry matter production was obtained on treatments producing plants containing as little as 59-g Mn/ha. Correlations between plant top Mn content (g/ha) and extractable-soil Mn (kg/ha) in various depth increments gave r-values significant at the 1% level for both extractants. DTPA-extractable Mn accounted for 72%, however, but double acid only 22% of the variation in plant top Mn, considering all Mn rates over both pH levels. Nearly all of the broadcast Mn applied to this very sandy soil remained in the rooting zone. Only a small fraction was accumulated into the tops of soybean plants, despite the fact that, in some treatments, the plants were under Mn-deficiency stress. These results emphasize the inefficient utilization of broadcast Mn by soybean plants and suggest that other methods of application, such as foliar or band, should result in more efficient use of applied Mn.