Late Season Soybean Nutrient Deficiency Identification

Essential Plant Nutrients for Soybean Production

A plant nutrient is considered “essential” if it meets all of the following criteria:

1. The plant cannot complete its life cycle without the nutrient.
2. The function of the nutrient within the plant cannot be replaced by another.
3. The nutrient is directly involved in the plant’s growth and reproduction.

There are 17 essential plant nutrients, with 14 supplied by the soil and three (carbon, hydrogen, and oxygen) supplied by air and water. They are usually subdivided into primary and secondary macronutrients, which are needed in larger quantities, and micronutrients of which smaller amounts are required—though they are still essential for plant growth.

Approximate Quantity of Soybean Plant Nutrients Required per Bushel

• In many corn/soybean rotations, the fertility needs of soybean—except for nitrogen (N)—are provided as a residual from corn fertilization programs. The increase in soybean yield potential has led to more of an interest by farmers in providing nutrients to the soybean crop directly instead of relying on residual nutrients from the corn production year.
• Nitrogen, phosphorus (P), and potassium (K) are considered the primary macronutrients as they are used in the largest amounts of all the essential plant nutrients. For each bushel of soybean (60 lb) produced, an estimated 4.20 lb, 0.40 lb, and 1.25 lb of N, P, and K, respectively, is required.¹
• Calcium (Ca), magnesium (Mg), and sulfur (S) are the secondary macronutrients for soybean since the overall crop requirements are lower compared to the primary macronutrients. Soybean use of the secondary macronutrients is estimated at 0.20 lb, 0.23 lb, and 0.20 lb per bushel for Ca, Mg, and S, respectively.¹
• Micronutrients—including boron (B), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), and zinc (Zn)—are nutrients that are needed in relatively small amounts. Of the micronutrients, a deficiency of iron is probably the most common, usually a result of high soil pH conditions.
• Maintaining soil pH is critical for maximizing yield potential. Soil pH for a soybean crop should be maintained between 5.5 and 7.0 to help ensure the availability of fertilizer nutrients. In acidic soils with a pH lower than 5.5, growers should consider treating with lime to achieve the desired pH for soybean production while basic soils with a pH above 7 may be treated with elemental sulfur. If soil pH is below 5.5, the soybean’s ability to fix N can be reduced.
• In general, noticeable fertility deficiencies in soybean are rare—though nutrient deficiency symptoms can appear when vascular and root diseases interfere with the movement of water and nutrients from the soil into the plant. For example, aboveground symptoms of soybean cyst are often associated with nutrient deficiencies.
• Awareness of nutrient deficiency symptoms can help in distinguishing them from symptoms of foliar diseases, herbicide injury, sun scald, and other stresses.

Symptoms of Primary Macronutrient Deficiency in Soybean

If a plant nutrient deficiency is suspected in the later soybean growth stages, tissue sampling can be a useful tool to help confirm a diagnosis that was originally based on visual symptoms. The recommended growth stages for sampling soybean plants for tissue analysis later in the season is at R1 through R2 (beginning flower to full bloom). Collect the top two or three fully developed trifoliate leaves from the top of the soybean plant, sampling 20 to 30 random plants. As soybean plants begin to develop pods, sampling is no longer recommended.

Nitrogen Deficiency in Soybean

Soybean plants that are nitrogen-deficient appear pale green to yellow (Figure 1). Late season deficiencies can be caused by several factors that impact the ability of the plant to fix the necessary amounts of N, such as:

• Wet and compacted soils
• Saline or calcareous soils
• Drought
• Soils with less than 1.5% organic matter
• Reduced numbers of nitrogen-fixing Rhizobia bacteria caused by low soil pH

Nitrogen uptake is greatest from the onset of flowering to pod fill. Should a deficiency exist and if irrigation is possible, an application of 20 to 40 lb/acre of N at the R3 growth stage may be considered.² Another study found that while soybean did respond to additional N applications, such applications may only be cost effective in high-yield environments.³ Soil tests are not useful for indicating if supplying additional N will result in a positive yield response. Applied N is not likely to increase yield unless a field has a pH of less than 5.5, organic matter is less than 1.5%, soybean have not been grown on the field in recent years, or there is an absence of active nodules on the roots.²

Phosphorus Deficiency in Soybean

Phosphorus-deficient plants may appear stunted, have small leaves, and may have interveinal reddening on lower leaves (Figure 2). Because P is mobile within the plant, symptoms are generally more severe on lower leaves. If a leaf tissue analysis of the newest leaves at the R2 growth stage reveals a P concentration of less than 0.25%, the plants may be phosphorus deficient. Phosphorus concentrations of 0.25 to 0.30% should be considered low.⁴ There is limited information to support the use of additional P during pod set to offset deficiencies.

Potassium Deficiency in Soybean

Potassium deficiency appears first on the oldest leaves as a yellowing of outer leaf margins (Figure 3) which can turn brown and necrotic later. Since K is mobile within the plant, symptoms can appear throughout the plant. Potassium moves to the roots by diffusion from the soil solution. As such, drought can predispose soybean to potassium deficiency. A leaf tissue test finding a K concentration of less than 1.5% at the R1 to R2 growth stages indicates K deficiency, 1.5 to 1.8% should be considered as low, and greater than 1.8% is sufficient to help maximize yield.⁴ If K deficiencies are noted before the R5 growth stage, K fertilizer may be applied and watered in with irrigation or rainfall.⁴ Foliar applications may be justified under rainfall production systems when nutrient-deficiency symptoms are obvious, with confirmed K-deficient soil tests, or when soil or climatic factors (other than drought) limit nutrient uptake in late spring and early summer.

Symptoms of Secondary Macronutrient Deficiency in Soybean

Calcium Deficiency in Soybean

Calcium is important for cell wall development, the transportation of other nutrients, plant strength, and for potentially counteracting the effects of alkali salts and organic acids in the plant. A balance of Ca, Mg, and K needs to be maintained within the plant, or the plant may become deficient in one or two of these nutrients. The newest soybean leaves appear necrotic if a Ca deficiency occurs (Figure 4).

Late Season Magnesium Deficiency

Magnesium is a much-needed nutrient in soybean as it plays an important role in photosynthesis, starch production, leaf and root growth, carbon fixation, maturation uniformity, and the uptake of P. Magnesium deficiency is mostly observed on very acidic soils (below pH 5.5) which usually have textures of sandy loam, loamy sand, or sand. High levels of K, ammonium (NH4), or Ca can cause Mg deficiency. Deficiency symptoms appear first on older leaves and include pale-green leaves with interveinal yellowing (Figure 5).⁵

Sulfur Deficiency in Soybean

Sulfur is an essential element for the formation of proteins. When S is deficient, soybean growth is reduced, maturity is delayed, protein formation is reduced, and foliage becomes pale green to yellow with non-prominent veins (Figure 6). Sustained deficiencies can result in leaves becoming pale brown to bronze. Sulfur deficiencies are favored by cold wet soil, low soil organic matter, and low soil pH.

Symptoms of Soybean Micronutrient Deficiencies

When diagnosing plant nutrient deficiencies, it is helpful to know which ones are mobile within the plant and which are immobile. Deficiency symptoms of immobile nutrients will likely appear in newer plant tissue since, by definition, the plant cannot move immobile nutrients from older to developing tissue. All of the micronutrients discussed below, except for molybdenum (Mo), are immobile within the plant.

Boron (B) Deficiency in Soybean

Boron is necessary for nodulation, as it accelerates atmospheric N fixation. Deficiency appears as yellowed leaves with curled leaf tips, interveinal chlorosis, tip dieback, and stunted roots (Figure 7). Flowering can stop under severe deficiency conditions.

Copper (Cu) Deficiency in Soybean

Copper is necessary for plant enzymatic activities, chlorophyll production, and seed development. Most Midwest soils supply adequate amounts of Cu for crop production. However, deficiencies can occur on organic soils, sandy-textured soils, soils with increased amounts of oxides and carbonates, and soils with a pH of 7.5 or greater. A deficiency of Cu can lead to increased susceptibility to diseases. Deficiency symptoms include reduced nodulation and N fixation, delayed flowering and maturation, pollen sterility, necrosis of leaf tips and stems, and yellowing of leaves.

Iron (Fe) Deficiency Chlorosis

Iron is needed for chlorophyll synthesis in all crops, and for nodule formation in soybean. High levels of manganese can induce Fe deficiencies and vice versa. Iron deficiency is common in calcareous soils which are characterized by a high concentration of calcium carbonate (lime), along with other minerals such as magnesium carbonate and silicates. This high calcium content can result in soil pH values ranging from 7.3 to more than 9.0. The soil may appear light-colored, especially when dry, due to the reflective properties of calcium carbonate. Iron becomes less soluble as soil pH values and the amount of calcium carbonate increase.

Deficiency symptoms are generally referred to as iron deficiency chlorosis (IDC) with symptoms appearing as interveinal chlorosis of the newest leaves, reduced nodulation, and reduced nitrogen fixation (Figure 8). Tissue analysis of chlorotic leaves, however, may often reveal adequate amounts of iron in the plant, but this iron is not in a soluble form that the plant can utilize.⁶

Manganese (Mn) Deficiency in Soybean

Manganese plays a vital role in photosynthesis through chloroplast formation, chlorophyll development, and the development of carbohydrates. Manganese also influences enzyme activity. Deficiency symptoms include interveinal yellowing with veins remaining dark, similar to Fe deficiency. However, Mn deficiency can appear across the entire plant (Figure 9). Manganese deficiency is common in soils with pH levels above 6.8, low organic matter, or droughty coarse-textured soil. The most economical and effective method for correcting manganese deficiency is foliar applications of manganese sulfate at 1 to 2 lb/acre of actual manganese. Apply one lb/acre of actual manganese when the first deficiency symptoms appear, when plants reach six inches tall, and apply an additional one pound per acre in 10 days if deficiency symptoms reappear.

Molybdenum (Mo) Deficiency in Soybean

Molybdenum is vital for two important processes, nitrogen fixation and nitrate reduction. Molybdenum deficiencies in soybean are very rare. If a deficiency does occur, it will most likely be in an acidic soil and plants will have the same light-green appearance associated with N deficiency, due to the lack of N-producing nodules.⁴

Zinc (Zn) Deficiency in Soybean

Soils lacking zinc can reduce plant growth and yield potential as this nutrient is necessary for the development of carbohydrates, proteins, and chlorophyll. Symptoms include interveinal mottling or chlorosis (Figure 10). These symptoms are more apparent when soil temperatures are cool and in soils that are fine-textured, sandy, of low organic matter content, eroded, or under fallow-syndrome situations. The symptoms of Zn deficiency can be confused with symptoms of iron deficiency. Soils with high phosphate concentrations may also result in soybean plants that exhibit Zn deficiencies.

Sources

¹Nutrient requirements – soybean. Iowa State University, Integrated Crop Management. https://crops.extension.iastate.edu/encyclopedia/nutrient-requirements-soybean
²Wortmann, C.S., Krienke, B.T., Ferguson, R.B., and Maharjan, B. 2018. Fertilizer recommendations for soybean. Nebraska Extension, NebGuide. G859. https://extensionpublications.unl.edu/assets/html/g859/build/g859.htm
³La Menza, N., Grassini, P., Specht, J., Brhel, J., Timmerman, A., Whitney, T., and Glewen, K. 2018. Is soybean yield limited by nitrogen supply? University of Nebraska–Lincoln, CropWatch. https://cropwatch.unl.edu/2018/soybean-yield-limited-nitrogen-supply#:~:text=Soybean%20has%20a%20large%20nitrogen,240%20lbs%20N%20per%20acre
⁴Slaton, N., Roberts, T., and Ross, J. 2013. Chapter 5: Fertilization and liming practices. In Arkansas soybean production handbook. University of Arkansas. https://www.uaex.uada.edu/publications/pdf/mp197/chapter5.pdf
⁵Smith, D. 2023. Magnesium magic. AGWEB. https://www.agweb.com/news/crops/crop-production/magnesium-magic
⁶2023. Managing iron deficiency chlorosis in soybean. University of Minnesota Extension. https://extension.umn.edu/crop-specific-needs/managing-iron-deficiency-chlorosis-soybean
Web sources verified 07/24/26. 1313_142344