Soybean Soil Fertility (SF1164 Revised February 2018)

All of NDSU soil fertility recommendations now have no yield-based formulas. The soybean fertility recommendations were modified to be in line with these new guidelines.

D.W. Franzen, Extension Soil Science Specialist

Photo by Dave Franzen

D.W. Franzen photo

Soybeans need 14 mineral nutrients: nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), magnesium (Mg), copper (Cu), iron (Fe), manganese (Mn), zinc (Zn), boron (B), chloride (Cl), molybdenum (Mo) and nickel (Ni).

Of these, North Dakota soils provide adequate amounts for soybean production except for N, P, K, S and Fe.



Although the atmosphere is 78 percent nitrogen gas, plants cannot use it directly. Plants can use only ammonium-N or nitrate-N. Soybean is a legume and normally should provide itself N through a symbiotic relationship with N-fixing bacteria of the species Bradyrhizobium japonicum. In this symbiotic relationship, the plant supplies carbohydrates and minerals to the bacteria, and the bacteria transform nitrogen gas from the atmosphere into ammonium-N for the plant to use.

Soybean infection by N-fixing bacteria and symbiotic N fixation is a complex process between the bacteria and the plant. The right species of N-fixing bacteria must be present in the soil, either through inoculation of the seed or the seed zone at planting.

N-fixing bacteria are attracted to soybean roots by chemical signals from the soybean root in the form of flavenoid compounds (1). Once in contact with the root hairs, a root compound binds the bacteria to the root hair cell wall. The bacteria release a chemical that causes curling and cracking of the root hair, allowing the bacteria to invade the interior of the cells and begin to change the plant cell structure to form nodules (2)(3)(4) (Figure 1).

The bacteria live in compartments, up to 10,000 in each nodule, called bacteroids (Figure 1). Each bacteroid is bathed in nutrients from the host plant, and the bacteroid takes nitrogen gas from the soil air and converts it to ammonium-N using the enzyme nitrogenase, which consists of one Fe-Mo (iron-molybdenum)-based protein and two Fe (iron)-based proteins. In this region, iron deficiency chlorosis (IDC) may result in poor nodulation and may contribute to N deficiency as well as iron deficiency.

Photo by RJ Goos, NDSU

Figure 1. Nodules formed on soybean roots through infection by Bradyrhizobim japonicum inoculation in soils with no previous soybean inoculation history.  (R.J. Goos, NDSU)

Photo by Louisa Howard, Dartmouth College

Figure 2. Soybean nodule cross-section micrograph showing individual bacteroids. (Photo courtesy of Louisa Howard, Dartmouth College)


If soybean will be planted in a field for the first time, the seed will need to be inoculated with Bradyrhizobium japonicum (soybean inoculum). Several inoculums types can be used: peat-based, liquid-based or granular. Of the three, granular appears to be the most fool-proof for a first inoculation. The other two also can be used, but the frequency of mistakes is much higher. No formulation is free of error.

For the peat- and liquid-based treatments, all seed should have inoculum attached to it when it enters the soil. Peat-based inoculants can vibrate off the seed if they are not applied with an adequate sticking agent. Liquids could be calibrated incorrectly and may not hit all the seed if the application is not made carefully. Even granular inoculum will have problems in performing if the seeder is calibrated improperly.

With proper care in handling and application, the success rate of all inoculation is very high. In the rare event that nodulation does not take place, supplemental N will have to be applied to reach yield potential. In-season foliar N application is not recommended, and slow-release liquid N sources have no higher foliar N efficiency, compared with UAN (urea-ammonium nitrate solutions).

If a field has been seeded to soybeans previously and nodulation was effective, chances are small that inoculating again will be economically effective. Studies in North Dakota have compared many inoculum brands in fields where soybeans previously were grown and successfully inoculated. Only very small yield benefits have been seen; most often, inoculation results in no yield benefit (Table 1).

In the region, soybeans grown in soils with conditions that did not support iron deficiency chlorosis (IDC) sometimes responded to higher soil nitrate levels (4). However, higher soil nitrate levels increase the severity of IDC in soils where IDC-supporting conditions prevail (5).

Soybean does not require nitrogen if adequate inoculation is present. In North Dakota experiments, the only responses to supplemental N have been to first-year soybeans, where initial inoculation resulted in poor nodulation (6).

Table 1


North Dakota soils typically are low in phosphorus (P). The soil test supported by NDSU recommendations is the Olsen sodium bicarbonate extraction method. The Olsen test best predicts crop response in the soils within state boundaries that range from below pH 5 to more than pH 8. More than one-half of soils in North Dakota have a pH higher than 7.

Soybeans have a great demand for P if soil tests are medium or below. Most fields are seeded with grain drills and air seeders for solid-seeding soybeans, and many growers would like to apply their P with the seed. Although up to 100 pounds/acre of 11-52-0 or equivalent P fertilizer can be applied with the seed row spacing through 12 inches in width, higher yields are achieved when the P is broadcast-applied (Table 2).

Table 2

Experiments in Nebraska with broadcast compared with band resulted in almost a 5 bushel per acre advantage to broadcast. Similar experiments in Minnesota resulted in almost a 3 bushel advantage to broadcast versus band.

In North Dakota, while seed-placed or near-seed-placed P application is usually profitable in small grains, canola, corn and sugar beet, profit is not assured in soybeans (Table 3). No fertilizer P should be applied with the seed in 15-inch rows or wider because stand reduction will overwhelm any yield benefit of P application in wider rows (Table 4).

Table 3

Table 4

In the central U.S. Corn Belt, P fertilizer commonly is applied to corn only in a corn-soybean rotation. This practice tends to provide good benefits to soybeans and corn because the soil P levels are most often in the high range.

In North Dakota, the P test is much lower, so P should be applied to soybean in a separate application. Fertilizing each crop is important until the field P test reaches the high availability range (Table 5).

Table 5

Phosphate inoculants sometimes are promoted to North Dakota soybean growers. The predominant inoculants are formulations of Penicillium bilajii, a fungus developed for commercial use in Canada about 20 years ago. According to the developer, the fungus excretes acid from its hyphae.

The acid can release trapped P from “occluded” P in the soil; P that is separated from the soil by a carbonate coating that can be present if the soils are high in free calcium carbonate. If this P is present in soil, up to 10 pounds P2O5 per acre may be made available during the course of the season.

The application of a P inoculant cannot be expected to act as a starter. The P inoculants will not release P in soil with a pH below 7 (7). Six site-years of work at Carrington in soybean found no significant yield increase from the use of P inoculants (Endres, unpublished 2012). Recommendations for rates of P2O5 based on soil test can be found in Table 7.

Table 7


Potassium requirements for soybean are lower than they are for corn. However, in a corn and soybean rotation, fertilizing for higher nutrient levels is important.

Although the critical soil level for K using the 1-N ammonium acetate extraction method is only 100 parts per million (ppm) for soybean, the critical level for K in corn is closer to 200 ppm. Therefore, adding K to replace what the soybean crop may remove will make fertilizing corn less economically and logistically painful the next year corn is intended for the field.

In dry seasons, soybean has shown K deficiency symptoms even when soil test K is higher than 100 ppm. Recommendations for K fertilization of soybean based on soil test can be found in Table 7.

In-season Application of Foliar Sprays of N, P and K

Due to soybean plant physiologists’ observations that the host plant’s soybean nodule sustenance is reduced soon after pod initiation, foliar fertilizer trials have been conducted in soybean-growing areas for the past 40 years.

Single nutrients or combinations of nutrients have not resulted in consistent yield responses to these applications (8) (9). Occasionally a yield response is recorded, but the great majority of experiments and on-farm trials have resulted in no yield increases and even yield decreases.

Some of the uncommon yield increases to foliar NPK applications have been related to low availability of soil nutrients. The rest of the yield increases are unexplained. The frequency of yield increases from all studies is so low and so unpredictable that foliar application of NPK to soybean is not recommended.


Sulfur is as important to soybean as it is to most other crops. The sulfur soil test poorly predicts the chance for S deficiency. Knowing the soils and considering the past precipitation is a better way to determine whether S is needed each season.

If the soils are loam or coarser in texture, and the fall was wet, snowfall was normal to above normal and/or spring was wet prior to planting, the application of 10 pounds of S as a sulfate form (ammonium sulfate or gypsum) would be recommended. Elemental sulfur of all formulations is not a recommended S source in North Dakota due to poor oxidation potential to sulfate.


North Dakota soils contain about 5 percent iron (Fe) by weight. However, only a tiny fraction ever is available to plants.

Iron in well water is reduced iron (Fe++ or ferrous iron). Ferrous iron is very soluble in water. A No. 2 carpenter nail can be dissolved in water if it was ferrous iron.

Unfortunately, as soon as ferrous ion is exposed to oxygen, it oxidizes to oxidized Fe (Fe+++ or ferric iron). Ferric iron is a trillion times less soluble than ferrous iron. Plants, except for aquatic plants such as rice and pondweed, implement Fe uptake strategies to improve Fe nutrition and avoid deficiency.

In soybean, Fe is mobile from germination through the first mono-foliate leaf. As the first trifoliate leaf emerges, Fe becomes immobile in the plant and must be taken up continually through the season to avoid deficiency.

The soybean strategy for Fe uptake begins by soybean roots acidifying the soil environment directly around the soybean root. The acid soil environment is necessary for the activity of an Fe-reducing protein that the soybean root secrets (10). If the root remains acidic, the Fe-reducing protein contacts oxidized iron and reduces it to soluble ferrous iron, making it available to the plant.

In soils that are susceptible to iron deficiency chlorosis (IDC), the causal soil condition is the presence of carbonates (CO3--) (11). As the soil becomes wetter, the solubility of carbonates increases, producing bicarbonate (HCO3-) (12). Bicarbonate neutralizes the acidity around plant roots and makes the Fe-reducing protein secreted by the roots ineffective (13).

Iron foliar sprays generally are not effective in correcting a deficiency. The best application to reduce IDC is ortho-ortho-EDDHA Fe chelate applied with water in-furrow at seeding. The ortho-ortho-EDDHA not only succeeds in delivering Fe to the plant root early in the season, but after conveying its original Fe, it has the ability to go back into the soil solution, grab additional Fe and deliver it to the plant root with the soil water stream (Goos and Lovas, unpublished data, 2012).

The amount of ortho-ortho EDDHA (Figure 3) in relation to ortho-para EDDA (Figure 3) is very important. Recent research at NDSU has shown that the response of soybeans to EDDHA fertilizer is directly proportional to the percentage of ortho-ortho EDDHA Fe (Figure 4).

Figure 3 Left

Fiugre 3 Right

Figure 3. Ortho-ortho EDDHA (top) ortho-para EDDHA (bottom).

Photo by RJ Goos  and S Lovas NDSU

Photos by RJ Goos and S Lovas, NDSU

Figure 4. Effect of a 1.5 percent Fe as ortho-ortho EDDHA added to soil at different rates (top), compared with a 5.5 percent Fe as ortho-ortho EDDHA applied at the same rates (bottom). (R. J. Goos and S. Lovas, NDSU)

An effective IDC-prevention strategy should not rely on the application of ortho-ortho-EDDHA alone, but on a comprehensive approach to the condition. An IDC-tolerant variety should be selected.

A recent four-state study that NDSU led found that the highest yield for a soybean field having soils with and without susceptibility to soybean IDC would be best managed by seeding a high-yielding IDC-intolerant cultivar in non-IDC soils and an IDC-tolerant cultivar in the IDC-susceptible soils (14).

To reduce IDC pressure, the soybean should be seeded in wider than 15-inch rows. Although the exact mechanism for denser stands is not known, many growers have seen this effect when the planter stops within the field and leaves a high strip of seed behind when it resumes planting. Soybeans in densely seeded areas are taller and have less IDC symptoms, compared with the normally seeded fields.

Similar reduction in IDC symptoms are seen as soybeans are seeded closer to each other in wider row spacings or higher seeding rates (15). The causes of the denser-seeding IDC reduction could be related to reduced soil moisture under the row, higher root-zone acidity that would favor activity of the Fe-reducing substance secreted by the soybean root, or other unidentified mechanisms.

A three-state study several years ago found that seeding a cover crop of 1 bushel per acre of oats or other easily killed small-grain cover crop about the day of soybean planting can reduce excess water and take up some excess soil N (Figure 5). Depending on soil moisture, the oats may be killed with herbicide early if conditions are dry, or up to the five-leaf stage of oats if the season is wet.

Photo courtesy of J Lamb, Univ of MN

Photo by J Lamb, Univ of MN

Figure 5a. One hundred pounds of N/acre with no oat cover crop (top), compared with 100 pounds of N/acre with oat cover crop (bottom). (Photos courtesy of J. Lamb, University of Minnesota)

Photo courtesy of J Lamb, Univ of MN

Photo courtesy of J Lamb, Univ of MN

Figure 5b. No N applied and no oat cover crop (top). No N applied with oat cover crop (bottom). (Photos courtesy of J. Lamb, University of Minnesota)

The use of an oat cover crop resulted in as high as 40 bushels per acre more soybean where oats were used, compared with where they were not at a Minnesota site in a wet season (5) (Table 6).

Table 6

Because soil salinity aggravates and increases the severity of IDC, a comprehensive, rotation-based strategy should be imposed to reduce soil salinity as much as possible. The strategy should include selection of better salinity-tolerant crops; use of alfalfa strips to reduce roadside salinity; use of alfalfa above saline seeps to reduce the severity of the seep; use of cover crops when possible before, during or after cropping to reduce field water table; and possibly tile drainage if possible, practical and socially and/or regulation permissible.

Other Nutrients

Deficiencies of zinc, manganese, boron, molybdenum, nickel, chloride or copper have not been recorded in North Dakota. Field experiments have not revealed any supplemental requirement for these nutrients above what our soils currently provide.


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15. Goo, R.J., and B. Johnson. 2001. Seed treatment, seeding rate and cultivar effects on iron deficiency chlorosis of soybean. Journal of Plant Nutrition 24:1255-1268.

February 2018
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