Do Small Grains Need Micronutrients?

By Dr. Jochum Wiersma, University of MN small Grains specialist
wiers002@tc.umn.edu

Mineral nutrients, along with carbon dioxide, provide the raw building materials ultimately transformed and accumulated in the plant as dry matter. Of the 16 elements known to be essential for plant growth, seven are needed and used in very small amounts—thus, that’s why they’re classified as micronutrients. These are zinc (Zn), iron (Fe), manganese (Mn), copper (Cu), boron (B), molybdenum (Mo) and chlorine (Cl). 

Micronutrients participate in many ways in biochemical reactions in plant cells. Some are structural cofactors in proteins and other large molecules critical to the function of the cells. Plants deficient in iron, manganese, or copper tend to show the symptoms in the new growth first.  However, zinc deficiency commonly shows up in old leaves first, suggesting that possibly the plant redirects this nutrient to new growth at the expense of old growth. Plants deficient in any one of the micronutrients cease to make new growth and therefore are usually stunted and tiller poorly.

Two Sources of Micronutrients
Micronutrients are supplied to plants from two sources:
1) soil minerals and
2) organic matter.
They are released as the soil minerals break down over a period of time by weathering. The major portion of the micronutrients made available to plants, however, probably comes from the breakdown (mineralization) of the organic matter.

Generally, micronutrients need not be applied as part of a fertilizer program in the Northern Plains to achieve attainable yield, as wheat is not very sensitive to low soil levels of these nutrients in most soils in this region. Yield-limiting micronutrient deficiencies can occasionally occur, however. Deficiencies of zinc, manganese, copper, and iron are problems mainly in alkaline soils.

Conversely, it is possible to apply excessive amounts of the micronutrients, and this can result in a subsequent reduction in yield.  Special attention should be given to the application of boron. Excessive rates can produce barren stalks in corn. Since the expected response to applying boron is very rare, it’s best not to include this nutrient in the fertilizer.

Copper Deficiency May Be a Problem in the Red River Valley
Of the seven primary micronutrients, only copper (Cu) may be a problem in the Red River Valley.  Most soils in this area contain adequate amounts of this nutrient for optimum crop yields. Organic or peat soils are exceptions, and copper might be needed in a fertilizer program when small grains are grown on these soils. Availability of Cu is related to soil pH, according to George Rehm, University of Minnesota extension soils specialist. As soil pH increases, the availability of this nutrient decreases. Copper is not mobile in soils.

According to North Dakota State University, copper deficiency can be observed as browning of wheat leaf tips, higher incidence of ergot and false black chaff symptoms.

NDSU recommendations suggest treating Cu deficiency with a preplant application of 3 lb/acre copper sulfate applied to the soil and incorporated. Copper sprays are not effective after the symptoms are seen. Canadian recommendations call for supplemental copper when copper soil test levels fall below 0.6 ppm using the DTPA soil extractant.

Wheat is the most sensitive to copper deficiency. Although barley and oat crops are less sensitive, copper use will increase their yield when grown on organic soils.

You will find more information on predicting the need for copper in U of M Extension Bulletin FS-6790-GO “Copper for Crop Production,” authored by Rehm and Michael Schmitt. This can be found online at www.extension.umn.edu/distribution/cropsystems/DC6790.html

Other Elements That Affect Fertility
Sulfur (S) and chloride (Cl) are not considered primary micronutrients, but are other elements which can affect soil fertility.

Sulfur fertilization can increase wheat yields when the crop is grown on sandy soils with low organic matter. U of M research trials have shown that there is no need to add sulfur to a fertilizer program when wheat is grown on fine-textured soils. Levels below 16 lb/acre in the top 2 feet are generally low in S and are likely to respond to S fertilizer. Plant tissue testing is also very helpful in diagnosing S deficiency. The addition of 10 lb/acre S at planting is usually adequate to correct S deficiency in wheat.

Wheat is sensitive to low chloride levels. Adequate chloride reduces disease incidence and severity (although there is no definitive research which proves Cl will suppress scab development), and Cl is also necessary for photosynthesis.  It is not a nutrient whose level can effectively be built up in the soil.  South Dakota State University research indicated that chloride application is most justified when levels are under 30 lb/acre. If soils are very low in chloride, even a modest application of 10 lb chloride/acre (20-25 lb 0-0-60/acre) can be very helpful.

Soil testing is the best way to determine if your soil is lacking in micronutrients and the major nutrients, N, P, and K. For more soil fertility information, see the following online links:

•  Fertilizing Hard Red Spring Wheat, Durum, Winter Wheat and Rye, NDSU -- www.ext.nodak.edu/extpubs/plantsci/soilfert/sf712w.htm

•  ProCrop 2001 Fertilizer Menu -- www.ag.ndsu.nodak.edu/aginfo/procrop/fer/

•  Fertilizer Recommendations for Agronomic Crops in Minnesota -- www.extension.umn.edu/distribution/cropsystems/DC6240.html