Yellow soybean fields are a common occurrence in 2007 throughout the Southeast. Though the reason may simply be lack of water, much of the yellowing is more likely a deficiency of one micronutrient or another.
Drought stress will lead to a lower rate of water uptake by the soybean roots. An imbalance between water uptake by roots and water loss by transpiration causes the plant to wilt.
Under drought conditions soybean plants defend themselves from water loss by closing the stomata (the pore openings at its leaf surface). Closing the pores helps the plant to avoid fast dehydration, but closed pores also keep the plant from exchanging carbon dioxide and oxygen between its internal tissue and outside atmospheric air.
Closing the pores also stops or drastically slows the flow of water through the plant, reducing the uptake of nutrients. Once nutrient intake is shut down by the plant, deficiencies occur. Although some are more affected by drought, including manganese, all are affected by the soybean plant's drought protection mechanism.
In the upper Southeast manganese deficiency has been the primary culprit and a season long problem in areas most severely affected by drought. Manganese activates important enzymes in chloroplast formation which drives photosynthesis. Manganese also is used to activate other enzymes, which are important in nitrogen metabolism and synthesis.
In Virginia, Virginia Tech Soybean Specialist David Holshouser says manganese deficiencies aren't anything new. However, in 2007, he notes manganese deficiencies occurred earlier than normal and advanced rapidly — seemingly overnight in some fields.
Holshouser notes that it was unusual that in some fields yellow soybeans were bordered on both sides by green soybeans. Lab analysis showed both yellow and green soybeans were significantly low in manganese. The difference was the level of soil compaction.
In the tighter soils, which the grower later confirmed to be compacted by tillage equipment, soybeans were green. In the looser soils, soybeans were yellow.
Virginia Tech Soil Scientist Mark Alley explains, “Manganese becomes more available to the soybean plant when the soil has less oxygen. In the looser soils, manganese stayed in its oxide form, which is not available to plants.” Though all the plants had manganese deficiency, the plants on the looser soils were just slower to exhibit symptoms.
To correct manganese deficiencies, Holshouser recommends application of three fourths pound of chelated manganese. If chelated is not available, one pound of inorganic manganese can be used. Holshouser points out lower rates of manganese may not be adequate to correct true deficiencies. He also stresses that manganese should be applied before soybean bloom.
Word of warning
Though growers routinely tank-mix manganese supplements with other applications, they should be careful to avoid tank-mixes with herbicides containing glyphosate. Most forms of manganese are antagonistic to glyphosate.
The EDTA formulation is not antagonistic and can be tank-mixed with glyphosate. Other formulations of manganese should be applied separately from glyphosate.
Though manganese deficiency has been the most frequent nutrient deficiency tied to drought conditions in 2007, it is not the only cause for yellowing or discoloration of soybeans. Among the seven micronutrients considered essential for soybean growth all are affected by drought or other stress conditions.
Boron, chloride, copper, iron, manganese, molybdenum, and zinc are the seven nutrients known to be needed for crop production. Some suggest that sodium and cobalt are also essential, but evidence that the latter two are essential is limited.
Like manganese, zinc deficiencies usually occur on soils with pH greater than 6.5. Deficiencies are also related to sandy soils, high phosphorus levels and low organic matter. Soybeans are much less sensitive to zinc deficiencies than corn, small grains, or sorghum.
Zinc deficient soybean plants are stunted, leaves are yellow or light green, and lower leaves may turn brown and drop. Flowers are scarce and pods that do set are abnormal and slow-maturing. Corrective measures are generally via preventative soil application, but a foliar application of one half pound per acre of elemental zinc or one pound per acre of zinc sulfate or zinc oxide can be made.
Molybdenum is needed by the soybean plant itself and also by the nitrogen-fixing rhizobia bacteria in the soil. Unlike other micronutrients, molybdenum availability increases with soil pH. Molybdenum deficiencies seldom occur with soil pH above 6.0.
Since the element is critical for nitrogen fixation, the pale green or yellow plants are identical to a nitrogen deficiency. In this case, leaves generally begin to yellow first on the lower leaves. Molybdenum deficiencies usually do not occur on soils high enough in nitrogen to make up for lack of nodule fixation.
Molybdenum may be applied as a foliar spray at the rate of one half pound of elemental per acre or as a seed treatment at the rate of one ounce per acre.
Boron deficiencies are rare in soybeans. However, Georgia researchers have indicated that yields can be increased by one half to three bushels per acre with a single application of 0.25 pound of soluble boron to soybeans at full bloom to beginning pod elongation (R2/R3 growth stage).
With the prolonged drought conditions in Georgia and Alabama, it is expected that some of the yellowing of soybeans may be a result of boron deficiencies. Supplemental boron applications should only be applied after soil analysis proves a true deficiency.