Manganese Deficiency in Winter Wheat

Pale green or yellow patches are showing up in many more wheat fields than normal this year. Some of this is probably due to a deficiency of manganese, brought on by the variable weather this spring. The recent cool conditions, in particular, will be slowing the release of manganese from soil organic matter. Mn deficiency symptoms are not as striking in wheat as in soybeans, showing up as yellowing of the upper leaves, or a faint striping on the leaves. Some plants will also show greyish spots between the veins, that may coalesce into larger spots.

Figure 1 - Manganese deficiency on winter wheat
(pale-yellow interveinal strips on the leaf) occurs most frequently
on high pH, sandy soils or on organic soils.
Photo courtesy of University of Georgia (Athens).

The patterns of these yellow patches in the field are often puzzling, and point out the unique nature of manganese behaviour in soils. These patterns are often more diagnostic of manganese deficiency than the symptoms on individual plants.

Manganese behaves differently from most other nutrients, in that it exists in different forms in the soil that vary greatly in availability. Soil pH, organic matter and moisture content all have large, and often conflicting, effects on manganese availability. Understanding some of these factors can help us to see where to expect manganese problems, and to comprehend some of the striking patterns that we see in soybean and cereal fields.

The immediately available form of manganese in the soil is the Mn2+cation, but this is present in tiny quantities. A much larger pool of manganese is in insoluble forms, or in the soil organic matter. This organic manganese becomes available during the natural process of organic matter breakdown. Sandy areas with low organic matter will have the lowest manganese content, simply because there is very little to hold it.

Solubility of manganese declines rapidly as the soil pH increases. This tends to make manganese deficiencies worse in eroded areas, where the pH is higher.

Some forms of organic matter can also tie up manganese so tightly that it is not available to plants. This will commonly occur in muck soils, or in low spots in a field where the organic matter content is higher. However, the breakdown of organic matter is also a major source of plant available manganese. Whether the presence of large quantities of organic matter is a benefit or a hazard depends on the conditions for the breakdown of organic matter. This is favoured by warm weather with moderate soil moisture, so in cool conditions with either wet or dry soils, the tie-up of manganese by organic matter can be greater than the release.

Anaerobic soils will have an increased manganese availability, because much of the manganous oxide will be converted back to the Mn2+ ion when the soil bacteria use the oxygen for their own purposes. These conditions can occur in compacted soils. We can often see the tire tracks from the sprayer or the fertilizer spreader showing up as green strips through the yellow patches. The compaction of the tires has created conditions where just enough manganese is released to keep the crop from showing the deficiency.

Figure 2 - Manganese deficiency in wheat showing classic effects of wheel tracks.
Photo courtesy of University of Georgia.

An over-riding factor in all of this is that the roots have to reach the manganese to absorb it from the soil. Wet soils, cool weather, soil compaction, root diseases, nitrogen deficiency and herbicide damage can all limit root growth. In this situation, the plant may not be able to absorb enough manganese to fill all of its needs even though there is adequate manganese in the soil. These plants will usually grow out of the deficiency once the weather warms up, and the roots get going again.

Foliar applications of manganese are preferred to correct a deficiency, both because it avoids tie up in the soil, and because very small amounts of the nutrient are needed. The product of choice for correcting manganese deficiency is manganese sulphate, applied to provide 2 lb/ac of actual Mn (8 lb/ac of product). This should be applied in high rates of water (40 gal/ac), with a spreader-sticker to help carry the manganese through the cuticle of the leaf. Chelated manganese is much easier to use, and is more compatible with herbicides if they are mixed, but much more expensive. Scientific opinions on the effectiveness of chelates range from about 2X as effective as the sulfate (U. of G.) to equally effective (Michigan State), but the cost is roughly 10X higher for the chelate per pound of manganese. Very low rates of chelated product are not effective.

Figure 3 - Foliar application of Mn in wheat

Soil applied manganese is not effective because the mineral gets tied up in unavailable forms. Adding Mn to the soil simply increases the amount of unavailable Mn. There is some suggestion that on some of the acid sandy soils, which have very low manganese content, that adding slag containing Mn can help overcome a Mn deficiency. This makes sense, for that particular situation, but there have been no trials to confirm this.

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