Wheat Nutrition – Secondary Macronutrients and Micronutrients

Richard Taylor, Extension Agronomist; rtaylor@udel.edu

Sulfur (S)
In past years, quite a few small grain fields located on sandy soils in both Delaware and Maryland have shown large areas of yellowed, stunted plants for both wheat and barley. Usually symptoms are noticed or appear shortly after spring fertilization with nitrogen (N). In many cases, deficiency of either S or Mg has been confirmed with both tissue and soil samples. Examples of fields showing classic deficiency symptoms for S are shown in photos 1 to 4. The symptoms include stunting of plants, general yellowing or chlorosis especially of the new growth (remember that S is immobile in the plant causing symptoms first to occur on new growth but when deficiencies are severe symptoms can involve the whole plant), and poor root development. In the barley shown in Photos 1 through 4, the root systems of the affected plants were limited to the upper 2 to 4 inches and the soil type was loamy sand.

S deficient barley field

 

 

Photo 1. Field view of barley showing sulfur deficiency (tissue test indicated 0.11% S and >5% N) on very sandy soil that had been fertilized with 20+ pounds per acre of S that likely was leached by heavy spring rains.

stunted, S deficient barley plants

Photo 2. Barley showing sulfur (S) deficiency symptoms of stunted plants with generally yellowing but beginning with newly emerged leaves ((tissue test indicated 0.11% S and >5% N) on very sandy soil that had been fertilized with 20+ pounds per acre of S that likely was leached by heavy spring rains.

stunted, S deficient barley field

Photo 3. Barley showing sulfur (S) deficiency symptom of severe stunting (tissue test indicated 0.11% S and >5% N) on very sandy soil that had been fertilized with 20+ pounds per acre of S that likely was leached by heavy spring rains.

chlorotic, S deficient barley field

Photo 4. Barley showing sulfur (S) deficiency symptom of stunting and general chlorosis or yellowing (tissue test indicated 0.11% S and >5% N) on very sandy soil that had been fertilized with 20+ pounds per acre of S that likely was leached by heavy spring rains.

With S deficiency, the symptoms often can be attributed to a high tissue N concentration. With inadequate S uptake, plants are unable to synthesize S-containing amino acids, limiting the ability of the plant to make the proteins and enzymes that do much of the work producing yield. In essence, a high tissue N content can worsen S deficiency.

Why does S deficiency sometimes occur? A severe winter or very cold spring can limit root development. If excessive rainfall occurs on sandy soils, S can leach below the crop rooting zone. Compaction issues that limit rooting depth also impact S availability. Many of the fields that have shown S deficiency in the past were low in soil organic matter (SOM), which supplies some S. Cold spring conditions can inhibit SOM mineralization reducing S availability. In the above case (Photos 1 to 4), S was applied along with N but heavy spring rainfall likely leached much of the S below the shallow rooting depth of the crop.

To alleviate the deficiency, sulfur should be applied as either a foliar spray for quick relief of the symptoms or as a soil application. Several products have been used including Epsom salts (magnesium sulfate), Sul-PoMag or K-Mag [Sulfate of potash-magnesia (K2SO4-2MgSO4), containing about 22% K2O (potash), 11% magnesium (Mg) and 22% S], and ammonium sulfate. Epsom salts does have limited solubility in water and ammonium sulfate can cause some plant burn if applied at high rates. All of the above products can be used to provide the needed nutrients. Each product should be evaluated on a cost per pound of nutrient basis. Don’t forget that the potash (K) applied with the Sul-PoMag or K-Mag can be used by the double-crop soybean planting.

Magnesium (Mg)
Other very sandy fields have shown both S and Mg deficiency symptoms in both barley (Photos 5 and 6) and wheat (Photos 7 and 8). In some cases, the soil pH was about 6.0. Typically, Mg deficiency is found on soils with a pH of less than about 5.2 in our area. A pH of 6.0 would indicate that both calcium (Ca) and Mg soil test levels were adequate. Why was Mg deficiency showing up where you would expect adequate soil test level of Mg? Grasses, which both wheat and barley are, often are unable to absorb adequate Mg in cool, wet springs. This situation most often is seen in forage grasses. It leads to a condition in grazing cattle called “grass tetany” characterized by low blood serum Mg levels due to poor absorption of Mg by the grass under certain environmental conditions. It is best to confirm suspected Mg deficiency with both a soil test and tissue test.

S and MG defiecient barley leaf

Photo 5. Barley showing sulfur (S) and magnesium (Mg) deficiency symptom of interveinal chlorosis or yellowing (tissue test confirmed S and Mg deficiencies) on a loamy sand soil near Laurel, DE

S and Mg deficient barley field

Photo 6. Field view of barley showing sulfur (S) and magnesium (Mg) deficiency (tissue test confirmed S and Mg deficiencies) on a loamy sand soil near Laurel, DE

S and Mg deficient wheat leaf

Photo 7. Interveinal chlorosis of wheat showing sulfur (S) and magnesium (Mg) deficiency (tissue test confirmed S and Mg deficiencies) on a loamy sand soil near Laurel, DE

S and Mg deficient wheat field

Photo 8. Field view of wheat showing sulfur (S) and magnesium (Mg) deficiency (tissue test confirmed S and Mg deficiencies) on a loamy sand soil near Laurel, DE

Can anything be done about the problem or should anything be done? An S, Mg, and potassium (K) containing fertilizer (KMag or Sul-PoMag-potassium magnesium sulfate) can be applied at a very low additional cost. This fertilizer will provide K, Mg, and S to the small grain crop and more importantly to the following soybean crop. Another fertilizer option that contains S and Mg is Epson salts or magnesium sulfate. The choice between the two depends on the costs of each compound and whether soil test K levels are high or not. High levels of soil test K can inhibit the uptake of Mg by grass crops. Even with this addition of S and Mg, the upcoming soybean or other crop should be observed carefully for deficiency symptoms so early intervention can solve developing deficiencies before yield is lost. In the current crop if the crop has headed out, there may be only a minimal potential for yield increase and much of the benefit will accrue from the double-crop portion of the rotation. If applied prior to heading, the crop usually responds enough to cover the cost of both the application and the fertilizer.

The value of tissue testing has been confirmed in past years. When evaluating tissue test results, pay attention not only to the absolute concentration of each nutrient but also the relative ratios of certain nutrients. In the above cases, the relative high N content in relation to the S content confirmed a S deficiency problem. A high K concentration with respect to the Mg concentration would indicate a problem with Mg even if the absolute Mg concentration were not below the critical level.

In the above cited instances of S and Mg deficiency, growers reported exceptional responses to the nutrients applied. Deficiency symptoms did not subsequently appear on a double-crop soybean planting.

Manganese (Mn)
Although we sometimes find Mn deficiency in small grains in the fall, the most common time that I have observed Mn deficiency is in the spring shortly after a spring green-up N application. Wheat and particularly barley are susceptible to Mn deficiency problems especially on soils where soil pH increases with depth. This problem has decreased in the past decade since most of the lime we apply today is very finely ground limestone with low amounts of the coarser limestone fractions. Years ago in situations where the coarser fraction predominated agricultural limestone and lime was applied on a regular (every two or three year) basis, the deeper layers of soil (8-16 inches) were often much higher in pH than the surface soil.

In these instances, the deficiency showed up following spring N application since N and warming soils stimulate crop root growth into deeper soil layers where the high pH soil leads to Mn deficiency. The availability of Mn and other micronutrients not only depends on the amount of nutrient in the soil but also on the soil pH. As soil pH rises, the availability of many micronutrients and especially Mn decreases. The only micronutrient this does not hold true for is molybdenum which increases in availability as the pH approaches neutral.

Mn deficient wheat plants

Photo 9. Interveinal chlorosis of wheat showing manganese (Mn) deficiency on a high pH loamy sand soil.

Mn deficient wheat field

Photo 10. Pattern of interveinal chlorosis of wheat showing manganese (Mn) deficiency on a high pH loamy sand soil

Early scouting of fields is essential in preventing stand loss. The usual interveinal symptoms of Mn deficiency such as seen on soybeans are not as striking in wheat. The deficiency shows up as yellowing of the upper leaves (it’s not a mobile nutrient in the plant) or as a faint striping on the leaves. Some plants show grayish spots between the veins that can coalesce into larger spots. Symptoms are sometimes hard to see on young small grain seedlings although poor growth and a gray steel coloration of leaves are often seen. The patterns of yellowing or stunting in the field often are more diagnostic of Mn deficiency than individual plant symptoms.

To confirm the problem, the best choice is to obtain a tissue sample for testing and to pull a soil sample from the affected area and a nearby area showing normal growth. For a tissue sample, take whole plant samples from a number of affected plants across the field but avoid contamination with soil. Again, take a reference sample from an area of normal growth. You should take soil samples in four inch increments and go at least a foot deep to accurately assess the impact of soil acidity on micronutrient availability. Be sure to make note on the soil test information sheet of the depth of soil sampled. Foliar Mn application can help the crop recover but more than one application may be needed if the seedlings are very small and the leaf area available for foliar absorption is limited. We typically apply between 1 and 2 lbs of actual Mn per acre. Plant coverage with the spray is very critical for helping the crop recover. Chelated Mn can be used as well as MnSO4 (techmangam). Both fertilizers are effective so the decision is usually made based on product cost. Chelated Mn is often much more expensive than techmangam but do not try using very low rates (0.25 to 0.5 lb Mn/acre) of chelated Mn to save on cost. Low chelated Mn rates have been shown to be less effective than techmangam. Fast action and complete plant coverage are critical factors to consider.

Copper (Cu)
Copper deficiency was last positively identified on wheat in Delaware back in the 1950s according to conversations I had years ago with Dr. Leo Cotnoir. The deficiency symptom appears as leaf tip die-back followed by a twisting or wrapping of the leaves. Copper deficiency can lead to delayed maturity and stunted, misshapen heads. Traditionally, this deficiency is associated with muck or organic soils but Dr. Cotnoir did observe it in northern Delaware on highly limed soils before the widespread use of superphosphate. Superphosphate (0-20-0) contained enough copper to eliminate the symptoms. Today, most producers apply triple-superphosphate that does not contain Cu. Copper deficiency is unlikely on soils that have received poultry manure and some biosolids since these products can contain enough Cu to supply crop needs. In soils where the more refined triple-superphosphate has been the sole phosphorus source for many decades or where soil pH is kept near neutral or the soil is relatively high in soil organic matter or has received applications of organic materials low in Cu, this deficiency might occur again. Copper deficiency should be confirmed with a tissue test before applying copper sulfate which is very effective in alleviating symptoms. Copper-containing fungicides also can be effective. A foliar rate of Cu is 0.1 to 0.25 lb actual Cu/acre. Document any applications of Cu since this element can easily build up in the soil to harmful levels and in certain situations Cu application is limited by law.

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