For optimal yields on mineral soils with subsoil pH greater than 6.0 (generally western Ohio), the pH range should be maintained between 6.0 and 6.8. On mineral soils with subsoil pH less than 6.0 (generally eastern Ohio), the range should be higher (6.5 to 6.8). Lime should be added to soybean fields when pH levels drop below the optimal range. A soil test will be necessary to calculate lime requirements based on buffer pH or lime index (buffer pH multiplied by 10). Lime may be applied anytime for recommendations of 2 tons per acre or less. Fall applications will allow time for lime to raise the soil pH before spring planting. Split applications or application over multiple years will be required for recommendations larger than 4 tons per acre. Regardless of the recommendation, no more than 8 tons per acre of lime should be applied in one season. Lime application rates for no-till fields should be half of recommendations given for a tilled field sampled to an 8-inch depth.
Nitrogen (N)
Soybeans, like other legumes, have the ability to form a symbiotic relationship with nitrogen-fixing bacteria called Bradyrhizobium japonicum. Bacterial infection occurs soon after emergence, and nitrogen fixation begins as early as growth stage V2 (second trifoliolate leaf). In Ohio, even under high-yielding conditions (>70 bushels per acre), farmers seldom see a positive economic return by adding nitrogen to well-nodulated soybeans. Soybeans do not respond to starter nitrogen; most soils have the ability to provide adequate nitrogen until the Bradyrhizobium bacteria infect roots and form nodules. Soybeans adjust to early-applied nitrogen by fixing less nitrogen from the atmosphere. Applications after flowering have not shown a consistent or predictable yield advantage.
Yield-limiting deficiencies of nitrogen are uncommon in soybeans. Deficiencies may occur temporarily during extended cool and/or wet soil conditions after planting. These short-term situations should not lower yields and nitrogen fixation will quickly resume with warmer temperatures and drier soils. Deficiencies seldom occur later in the growing season. However, diseases, soybean cyst nematode, or extended hot and dry weather may limit the ability of plants to absorb nutrients and produce symptoms that resemble nitrogen deficiency.
Nitrogen fertilizer may be necessary the first time soybeans are planted in a field, even when seed inoculation is used. To ensure a reliable source of inoculation in new fields, soybeans should be grown for two years and the seed inoculated each year.
Phosphorus (P)
Soybeans require relatively large amounts of phosphorus. It is not unusual for a 60-bushel-per-acre crop to contain 48 pounds of P2O5 in the grain. Although phosphorus is taken up throughout the growing season, the period of greatest demand occurs during pod development and early seed fill (growth stages R3—R5). Deficient plants seldom exhibit specific leaf symptoms. Generally, phosphorus-deficient plants will be stunted, a symptom easily confused with disease and environmental stress symptoms. Plant and soil tests are the most reliable methods to ensure against phosphorus deficiency. Soiltest phosphorus levels should be maintained between 20 and 40 parts per million based on a Mehlich-3 extraction. Phosphorus recommendations are based on the yield potential of the field and the corresponding phosphorus levels from a recent soil test (Table 5.4). If soil-test phosphorus is above the critical level of 20 ppm, no yield response is expected with additional fertilizer application. In an Ohio study conducted in 2014 and 2015 at four locations, there was no yield benefit when 100 pounds of P2O5 per acre was applied to soybean when soil-test phosphorus levels were within the recommended critical level (Figure 5.4).
Potassium (K)
Soybeans require large amounts of potassium. It is essential for vigorous growth, yet never becomes a part of protein molecules and other organic compounds. Potassium is not involved extensively in biological activities in the soil. Most of the total plant potassium will be in the seed at maturity (1.15 K2O pounds per bushel).
For soils low in potassium, recommendations are designed to provide more potash than crop removal so that soils will build up above the critical level in four years. Potash should be applied annually until soil-test potassium is above the critical level. Once above the critical level, recommendations are made to replace soil potassium removed by the crop. These recommendations are slightly above the critical level to account for soil sampling or analytical variation. In soils with a cation exchange capacity (CEC) greater than 5 milliequivalents per 100 grams, the range to maintain soil-test potassium levels optimum for soybean production is between 120 and 170 ppm (Mehlich-3 K). Potash recommendations are provided in Table 5.5. These recommendations are dependent upon a field’s yield potential, CEC, and soiltest level. If soil-test potassium is above the critical level, no yield response is expected with additional fertilizer application. In an Ohio study conducted in 2014 and 2015 at four locations, there was no yield benefit when 100 pounds potash was applied to soybean when soiltest potassium levels were above the recommended critical level (Figure 5.4).
Calcium (Ca) and Magnesium (Mg)
In loam and clay soils, soybeans require a minimum exchangeable soil-test level of 200 and 50 ppm (Mehlich-3) of calcium and magnesium, respectively In most cases, these requirements are automatically met when soils are maintained at the proper soil pH. Soybeans will grow well over a wide range of calcium-tomagnesium ratios and should not need additional calcium as long as the proper pH is maintained and soil calcium levels are higher than magnesium. Soils naturally low in magnesium (eastern, extreme southern, and sandy soils of northwestern Ohio) should be limed with dolomitic limestone. Dolomitic lime is an economical source of magnesium and still contains generous amounts of calcium.
Sulfur (S)
Soybeans use large amounts of sulfur. A crop yielding 60 bushels per acre contains about 25 pounds of sulfur, 11 pounds of which is in the grain. Soils with more than 1% organic matter usually supply adequate sulfur for high yields. Deficiencies generally occur during cool, wet weather on sandy soils and/or soils low in organic matter. Soil tests are not reliable in predicting crop response to sulfur. A continuing plant analysis program is the best guide to confirm the need for additional sulfur. If a need for sulfur is identified, several suitable materials, such as gypsum ammonium sulfate, or thiosulfate, will correct the deficiency.
Manganese (Mn)
Even though manganese deficiency in soybeans is not a widespread problem, its occurrence is more common than the other micronutrient deficiencies. Deficiencies are most likely to occur in glacial lakebed, glacial outwash, and peat and muck soils. Soil pH is the most important factor affecting manganese availability (becomes less available at higher pH levels), but other such factors as organic matter, soil type, and weather may magnify dificiencies. On silt loams and clayey soils, deficiencies seldom occur below a pH of 6.8. Deficiencies may occur on sandy soils that are high in organic matter with a pH as low as 6.2. Muck and peat soils occasionally are deficient at a pH as low as 5.8. Pale yellow to nearly white leaves with distinct green veins (interveinal chlorosis) is the most visual symptom of manganese deficiency. Deficiency symptoms will first appear on younger leaves. In severe cases, the plants will become stunted.
Manganese may be banded for wide-row soybeans, but narrow rows require foliar applications. Generally, when the plants have two or three trifoliolate leaves (growth stages V2 or V3), a foliar application of 4 to 8 pounds of manganese sulfate will usually correct minor deficiencies. Multiple applications may be needed when both the surface and subsoil have high pH values.
Manganese fertilizers should probably not be mixed with such herbicides as glyphosate to prevent the loss of weed control. Producers should examine the herbicide label to confirm that the product selected will not interfere with the activity of the herbicide. Spraying at the optimal time for weed control and using the manganese chelate product EDTA may lower the potential for antagonism between fertilizer and herbicide.