Fertilization Recommendations

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 test index (buffer pH multiplied by 10). Lime may be applied anytime for recommendations of 2 tons or less. Fall applications will allow time for lime to raise the soil pH before spring planting. Split applications will be required for recommendations larger than 4 tons per acre: half before plowing and half after plowing. Regardless of the recommendation, no more than 8 tons of lime should be applied in one season. Lime application rates for no-till fields should be one 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. In Ohio, even under high-yielding conditions (>70 bushels per acre), farmers seldom see a positive economic return and little benefit in yield have been obtained by adding nitrogen to well nodulated soybeans. 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. 

Soybeans also do not respond to starter nitrogen (most soils have the ability to provide adequate nitrogen until the Bradyrhizobia bacteria infects roots and forms nodules). Bacterial infection occurs soon after emergence and nitrogen fixation begins as early as growth stage V2 (second trifoliolate leaf). 

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, disease―such as 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. If the crop does not have a dark green color by early July, 75 pounds of nitrogen per acre should be applied as urea. 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 phosphate (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 insure against phosphorus deficiency. Soil-test phosphorus levels should be maintained between 15 and 30 parts per million (based on a Bray P extraction) or 21 and 43 parts per million (based on a Mehlich 3 extraction). Phosphate recommendations are based on the yield potential of the field and the corresponding phosphorus levels from a recent soil test (Table 5-5). If soil-test phosphorus is above the critical level of 15 ppm Bray P (21 ppm Mehlich P), 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 phosphate per acre was applied to soybean when soil-test phosphorus levels were within the recommended critical level (Figure 5-4). 

 

Table 5-5: Phosphorus (P2O5) Recommendations for Soybeans. 

 

Yield Potential (bu/ac) 

       

Soil test (Bray P) 

30

40

50

60

70

ppm (lb/ac) 

lb P2O5/acre

       

5 (10)* 

75

80

90

100

105

10 (20) 

50

55

65

75

80

15-30 (30-60)** 

25

30

40

50

55

35 (70) 

10

15

25

25

30

40 (80) 

0

0

0

0

0

*Values in parentheses are pounds per acre.
**Maintenance recommendations given for this soil test range. 

Figure 5-4: Effect of additional phosphorus (P) and potassium (K) fertilizer when soil-test phosphorus and potassium are adequate according to state guidelines.

 

---PICTURE---

 

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 (1.4 pounds per bushel). Deficiencies are not common but are easy to recognize by yellow leaf margins. 

Soil Cation Exchange Capacity (CEC) affects potassium availability so the critical level increases as the CEC increases. The critical level for soybeans (ppm) is 75 + (2.5 x CEC). 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. Depending on the CEC, the range to maintain soil-test potassium levels for optimum soy- bean production is between 100 and 180 ppm. Potash recommendations are given in Table 5-6. These recommendations are dependent upon a field’s yield potential, CEC, and soil-test 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 soil-test potassium levels were above the recommended critical level (Figure 5-4). 

Table 5-6: Potash (K2O) Recommendations for Soybeans at Various Yield Potentials, Cation Exchange Capacities (CECs) and Soil-Test Levels. ​​​​​​

 

Yield Potential 

bu/ac

30

40

50

60

70

Soil-Test K 

 

---lb K2O per acre--- 

       

ppm (lb/ac) 

CEC 

---5 meq/100g---

       

25 (50)1 

 

140

155

170

180

195

50 (100) 

 

110

125

135

150

165

75 (150) 

 

80

90

105

120

135

88-118 (175- 235)2 

 

60

75

90

105

120

130 (260)

 

25

30

35

40

45

140 (280)

 

0

0

0

0

0

 

CEC

---10 meq/100g---

       

25 (50) 

 

175

190

205

215

230

50 (100) 

 

135

150

165

180

195

75 (150) 

 

100

115

130

140

155

100-130 (200-260)2 

 

60

75

90

105

120

140 (280) 

 

30

40

45

50

60

150 (300) 

 

0

0

0

0

0

 

CEC

---20 meq/100g---

       

50 (100) 

 

210

225

240

255

270

75 (150) 

 

160

175

190

205

220

100 (200) 

 

110

125

140

155

170

125-155 (250-310)2 

 

60

75

90

105

120

165 (330) 

 

30

40

45

50

60

175 (350) 

 

0

0

0

0

0

 

CEC 

---30 meq/100g---

       

75 (150) 

 

250

265

280

290

300

100 (200) 

 

185

200

215

230

245

125 (250) 

 

125

140

155

165

180

150-180 (300-360)2 

 

60

75

90

105

120

190 (380) 

 

30

40

45

50

60

200 (400) 

 

0

0

0

0

0

1 Values in parentheses are pounds per acre.
2 Maintenance recommendations given for this soil test range.

Calcium (Ca) and Magnesium (Mg) 

Soybeans require a minimum exchangeable soil test level of 200 and 50 ppm (400 and 100 pounds per acre) 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 to magnesium 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, 15 pounds of which is in the grain. Soils with more than 1 percent 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, potassium sulfate or potassium sulfate magnesia 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, peat and muck soils. Soil pH is the most important factor affecting manganese availability (becomes less soluble at higher pH levels), but other factors such organic matter, soil type, and weather may magnify the problem. On silt loams and clayey soils, it seldom occurs below pH 6.8. It 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 sur- face and subsoil have high pH values. 

Manganese fertilizers should probably not be mixed with herbicides such 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.