Richard Taylor, Extension Agronomy Specialist; rtaylor@udel.edu
Driving around the state, I’ve noticed a number of fields with portions of soybean fields showing manganese (Mn) deficiency. Certain areas of the state are very prone to this deficiency, such as western Kent County, portions of western New Castle County (NCC) as well as around Taylors Bridge road in eastern NCC, and any field with relatively high soil pH especially on lighter, sandy soils. Manganese deficiency is characterized by interveinal chlorosis of the younger leaves since Mn is immobile in the plant. Plants showing Mn deficiency can be distinguished from plants impacted by poor nodulation due to inadequate soil aeration from the heavy rainfall we’ve received in many parts of Delaware, since the latter condition will show symptoms similar to nitrogen (N) deficiency with general yellowing of the older, lower leaves.
Foliar Mn is very effective in reducing or eliminating the yield loss associated with Mn deficiency according to data from researchers in Virginia and North Carolina as well as our own research. Although as little as a tenth of a pound of Mn was effective in restoring yield potential, it was necessary, in severe cases, to apply foliar Mn with multiple applications. I generally recommend applying at least a half a pound of chelated Mn or tecmangam (manganese sulfate) as a single application when soybeans are large enough to easily absorb the Mn. To be completely effective at restoring yield, however, Mn should be applied before deficiency interferes with pod set, flower number, and seed fill. Although best applied before flowering, especially with full-season soybeans, Mn can be effective as long as the field is still in the R2 or full bloom stage of development.
For double-crop soybeans, the very short time between the plants becoming visible above the small grain stubble and their reaching full bloom means that scouting fields and responding with foliar Mn quickly is very important for obtaining maximum yield in fields with a history of Mn deficiency or in fields in the critical areas mentioned above.