Soybeans: Fertility Management

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Pub 811: Agronomy Guide > Soybeans > Fertility Management

Excerpt from Agronomy Guide for Field Crops
Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Nitrogen

Nitrogen fertilizers are not usually required for soybeans.(see Inoculation). Research into nitrogen fertilizer applied at planting has shown that nitrogen, even in small amounts, can delay nodule fixation.

If nodulation does not occur, and the soybeans are pale green and N-deficient, the recommended remedial measure is to apply 50 kg/ha of N at first flower, as urea or calcium ammonium nitrate, when the foliage is dry.

Phosphate and Potash

Phosphate and potash recommendations for soybeans are given in Table 2-16, Phosphate and Potash Recommendations for Soybeans Based on OMAFRA-Accredited Soil Tests. Occasionally, potassium deficiency will appear in soybeans as yellowing or browning of margins in older leaves (see Plate 14).

For additional information about soil testing or soil test recommendations, see Fertilizer Recommendations.

Plate 14. Potassium (K) deficiency appears as yellowing or browning of leaf margins on older leaves.

Methods of Application

Do not place fertilizer in contact with soybean seeds, due to the sensitivity to fertilizer salts. Unlike corn, there is no yield advantage to this practice. The fertilizer may be broadcast and plowed down or worked into the soil either in the fall or spring. A planter with a separate attachment for fertilizer placement may also be used to place the fertilizer 5 cm (2 in.) to the side and 5 cm (2 in.) below the seed. For further information, see Table 9-21, Maximum Safe Rates of Nutrients.

¹ HR, MR, LR, RR, and NR denote, respectively, high, medium, low, rare and no probabilities of profitable crop response to applied nutrient. Profitable response to applied nutrients occurs when the increase in crop value, from increased yield or quality, is greater than the cost of the applied nutrient.
² Where manure is applied, reduce fertilizer applications according to the amount and quality of manure (see Manure). Example of fertilizer application: If a soybean crop is not manured, and the soil tests are 9 for phosphorus and 85 for potassium, the phosphate requirement is 40 kg/ha and the potash requirement 40 kg/ha. These nutrients can be supplied by broadcasting or banding 200 kg/ha 0-20-20 fertilizer.
³ For a nutrient that has an "NR" rating by soil analysis, the application of this nutrient in fertilizer or manure may cause problems due to reduced crop yield or quality. Phosphate additions may jeopardize water quality. Potash additions may induce magnesium deficiency on soils low in magnesium.

Plant Analysis

For soybeans, sampling the top fully developed leaf (three leaflets plus stem) at first flowering is recommended. See Table 2-17, Interpretation of Plant Analysis for Soybeans. For sampling at times other than first flower, take samples from both deficient and healthy areas of the field for comparative purposes.
Take a soil sample from the same area and at the same time as a plant sample.

Micronutrients

Manganese

Manganese is the only micronutrient deficiency diagnosed in soybeans in Ontario, although zinc deficiency may show up in the future where the surface soil has been lost by erosion.

The symptoms of manganese deficiency are upper leaves ranging from pale-green (slight deficiency) to almost white (severe deficiency) with green veins (Plate 15). Soil tests and plant analyses are useful in predicting where manganese deficiencies are likely to occur. Both are available at the OMAFRA-accredited laboratories listed in Appendix C, Accredited Soil-Testing Laboratories in Ontario.

¹ Yield loss due to nutrient deficiency is expected with nutrient concentrations at or below the "critical" concentration.
² Maximum normal concentrations are more than adequate but do not necessarily cause toxicities

Plate 15. Manganese (Mn) deficiency. Upper leaves appear pale green to almost white with green veins.

To correct a manganese deficiency, spray the foliage with 2 kg/ha of actual manganese (8 kg/ha of manganese sulphate) in 200 L of water. A "spreader-sticker" in the spray is recommended. If the deficiency is severe, a second application may be beneficial.

Soil application is not a recommended method of applying manganese, regardless of the source, due to the large amounts required. Application of manganese chelates to the soil has resulted in yield reductions.

In general, beans will give a profitable response to manganese in the parts of the field where manganese deficiency is evident. There is no benefit to applying manganese to beans without deficiency symptoms.