Yellow wheat may or may not be N deficient
In a wet spring, wheat plants may become yellow. This is usually due to a nitrogen deficiency. Usually ... but not always.
click image to zoom Other nutrient deficiencies producing yellow wheat plants in North Dakota include sulfur and iron deficiency. If the plants are examined when the yellowing begins to develop, it is often possible to determine if the deficiency is due to a shortage of nitrogen, sulfur, or iron by the appearance and location of the symptoms on the plant.
Nitrogen (N) is mobile within the plant. A nitrogen-deficient wheat plant will mine its older tissues to keep the younger tissues alive. As a result, a nitrogen deficiency in its earliest stages will appear as a general yellowing of the lower, older leaves. Figure 1 shows a nitrogen deficiency in a wheat plant. The first main stem leaf is labeled as MS-1. Notice that it is quite yellow. Subsequent leaves on the main stem (MS-2, MS-3, MS-4) are not as yellow as MS-1. The degree of nitrogen deficiency in this photograph is quite severe, as the plant has failed to form tillers. There is only a main stem.
click image to zoom Sulfur (S) is less mobile in the plant than nitrogen. In the earliest stages of a sulfur deficiency, the lower leaves will be greener than the younger leaves. Figure 2 shows sulfur deficiency in two wheat plants, at about the same stage of growth as the plant in Figure 1. Notice how the oldest parts of the plants, main stem leaf 1 (MS-1), main stem leaf 2 (MS-2), and main stem leaf 3 (MS-3) have a normal green color. However, younger plant parts, such the fourth main stem leaf (MS-4) and the T1 and T2 tillers, are yellow. This is an indication that the plant is suffering from a shortage of a nutrient that is relatively immobile in the plant, in this case sulfur.
However, as plants age and yellowing affects the whole plant, it becomes more difficult to differentiate between a nitrogen and sulfur deficiency based on visual symptoms. In such a case, one needs to use a tissue test to differentiate between a nitrogen and sulfur deficiency. Fortunately, an index called the "N:S ratio" is helpful in differentiating between a nitrogen and sulfur deficiency. A wheat tissue sample needs to be analyzed for % total N and % total S. Dividing the % total N in the plant by the % total S will give what is called the N:S ratio in the plant. If sulfur is adequate, this ratio will be less than 15 or so. For example, if the % N in a young wheat plant is 3.0%, and the % S in the same plant is 0.1%, the N:S ratio would be 3.0 divided by 0.1, giving a N:S ratio of 30. That would indicate a severe S deficiency.
click image to zoom Farmers normally associate iron (Fe) deficiency with soybeans, but wheat can occasionally develop iron deficiency as well. Level soils, lower on the landscape, are often poorly drained, with a high water table in the spring. Such soils will commonly have lime in the topsoil, and the topsoil will "fizz" when treated with acid. It would be rare for a sulfur deficiency to occur on such soils, but iron deficiency is possible. Iron deficiency in wheat is not as common as iron deficiency in soybeans, but I have observed it in North Dakota. Elevated salinity can also aggravate the problem. Iron is even less mobile in the plant than sulfur, and the deficiency symptoms appear first on the younger leaves. Figure 3 shows an iron-deficient wheat leaf from a field near Leonard, ND. Notice the stripes, as the veins remain green, and the tissue between the veins turns yellow. If this happens on the youngest leaves of wheat, grown on a poorly-drained soil with lime in the topsoil, it is possibly an iron deficiency.
Tissue analysis is usually not reliable for diagnosis of an iron deficiency in wheat. Plant tissues can contain inactive forms of iron, and dust contamination can raise apparent iron levels dramatically.
Differentiating the visual symptoms of nitrogen, sulfur, and iron is best done when the deficiency begins to develop, when the yellowing begins. A rescue treatment is usually indicated when nutrient deficiencies are properly identified in a young wheat crop. Farmers should consult with their local agronomist or to select proper fertilizer rates, sources, and methods of application.
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