Richard Taylor, Extension Agronomist; rtaylor@udel.edu
An interesting fact came up recently about the research retired Purdue University plant pathologist Don Huber has done linking glyphosate and reduced uptake of several nutrients in field crops. I found the notation that significantly lower tissue levels of the micronutrients manganese (Mn), zinc (Zn), and iron (Fe) are being taken up by the field crops he studied. In Delaware, we frequently see Mn deficiency symptoms on soybeans, especially on sandy soil or where the soil pH is maintained near neutral or above.
Just yesterday driving back from the University of Delaware Research and Education Center, I noticed Mn deficiency symptoms showing up in several fields. Manganese deficiency is characterized by dark green veins and light green (mild deficiency) to yellow (moderately severe deficiency) to white (severe deficiency) interveinal leaf tissue. The symptoms often are most severe on the most recently emerged leaves. Manganese deficiency symptoms are similar to the deficiency and toxicity symptoms of some of the other micronutrients.
Photo 1. Moderately severe interveinal chlorosis on no-till single-crop or full-season soybean. Note dark green veins with tissue between veins yellow. Younger leaves are most affected since Mn is not mobile in the plant.
Yield reductions can be avoided to a large degree by early diagnosis and treatment with foliar application of Mn. Multiple applications of foliar Mn may be needed especially when Mn deficiency is severe. If enough leaf area is present to absorb adequate Mn, a single application higher rate (1 to 2 lb Mn/acre) was shown to be effective by Virginia and North Carolina researchers. Ignoring or not catching the problem until later in the season can not only reduce yield potential but make a foliar application more difficult and possibly more expensive since driving over the soybeans may cause damage on drilled beans. You may need to treat early season symptoms several times since the leaf area available to absorb Mn is limited so always rescout treated fields to be sure Mn deficiency does not reappear after treatment.
Photo 2. Moderately severe interveinal chlorosis on no-till single-crop or full-season soybean. Note dark green veins with tissue between veins yellow.
Where the symptoms are widespread and moderate to severe, foliar Mn applied at 1 to 2 lbs Mn per acre can boost yields significantly. Since the crop is still in the vegetative stage, mild to moderate symptoms can be alleviated with a 0.5 lb Mn per acre foliar spray. Researchers in Delaware, Virginia, and North Carolina have shown that soybeans are very responsive to foliar Mn especially when applied well before soybeans begin to bloom.
Even if you do not apply foliar Mn, you should be making note of which fields and where in the field symptoms occur so you can monitor these areas in the future. If wheat or barley are to be planted this fall, careful early monitoring will allow you to apply Mn to the small grains before they are severely injured by Mn deficiency. You should also note the areas so you can do soil testing to determine the underlying problem. Check to see if the native Mn concentration in the soil is too low or whether the soil pH is too high since the higher the pH the lower the availability of Mn in the soil. Also, any factor restricting root growth (compaction, drought, etc.) can aggravate Mn deficiency symptoms and should be corrected.
Dr. Joseph Heckman at Rutgers University is writing a series of articles on Mn deficiency in Rutgers Plant and Pest Advisory publication. These publications are available on the web through the Rutgers New Jersey Agricultural Experiment Station. A recent article (Vol. 16, No. 7, page 3) showed research Dr. Heckman conducted comparing manganese sulfate and chelated manganese and this article can be found at the following web address: http://njaes.rutgers.edu/pubs/plantandpestadvisory/2010/vc051210.pdf