Fertility Requirements

AMOUNT REQUIRED

• Understanding the fertilizer requirements of soybean is critical to reaching optimum yields. The crop will respond to added fertilizer nutrients when soil test levels are low to medium. It is always best to soil test as an aid in developing a sound fertilizer management program.

Nitrogen (N)

• Proper inoculation will eliminate the need for N fertilizer.  Soybeans can be grown on fields with high N levels, but it generally reduces nodulation, contributes to iron deficiency chlorosis and can delay maturity.
• The optimum soil test range is low, <50 lbs/ac.  Removal is 3.8 lbs/bu or 152 lbs/ac based on a 40 bu/ac soybean crop.
• Mid-season N applications are normally not recommended. An exception would be under conditions of failed inoculation and obvious N deficiency.

Phosphorus (P2O5)

• Phosphorous is important for soybean nodule formation, flowering, seed formation, and to help speed up maturity.
• Soybeans can be grown on fields with various P levels. They are very efficient at extracting soil P and have shown to be non-responsive to P fertilizer.  However, a crop rotation strategy that ensures P removal rates of soybeans is balanced with P inputs is encouraged. This may include fertilization of soybeans.
• The maximum safe rate of seed-placed P is 10 lbs/ac for wide rows or up to 20 lbs/ac for narrow rows with good soil moisture.
• Optimum soil test range is medium to high, 10 – 20 ppm.
• Removal is 0.85 lbs/bu or 34 lbs/ac based on a 40 bu/ac soybean crop.
• Even though seed yield may not be increased every year in response to phosphorus fertilizer, the crop may still benefit from earlier maturity.
• Phosphorus moves poorly in soil, so it should be placed near the seed. On the other hand, germination and emergence can be reduced if too much phosphate is placed with the seed.
• Research has also shown that although phosphorus is a limiting factor in many Alberta soils, build-up of soil phosphorus tends to raise available soil phosphorus levels and phosphorus fertilizer responses are often not dramatic.
• Rates of seed-placed phosphate should be reduced if the seedbed has less than ideal moisture conditions. Higher rates of phosphate fertilizer placed in the seed row with narrow openers like discs or knives can damage the emerging seedling and reduce the stand.
• If higher phosphate rates are required, banding the fertilizer away from the seed (sideband or to the side and below) should be considered. If sidebanding, sideband all the phosphate fertilizer, especially when using narrow openers.

Potassium

(K2O)

• Soybeans take up and remove more potassium than other annual crops. Soil potassium should be monitored where crop rotation includes frequent soybeans or forages and on coarse-textured soils. If below critical levels, potash should be applied away from the seed.
• Deficiency of potassium appears as yellowing of leaf margins on older leaves.
• The optimum soil test ranges from medium to high, ≥ 100 ppm or 200 lbs/ac.  Removal is 1.4 lbs/bu, or 56 lbs/ac based on 40 bu/ac soybean crop.
• Many Alberta soils are medium to high in exchangeable potassium, often ranging from 400 to 1,000 lb. of potassium/acre in the 0 inch to 6 inch depth of soil.
• Potassium deficiencies are most likely to occur on sandy soils that are intensively cropped or on Grey-Black transition soils and Grey Wooded soils.

Sulphur (S)

• Soils that receive sulphur fertilizer from our crops in rotation (i.e. corn, canola) generally provide sufficient amounts for soybeans. If grown on coarse-textured soils with low organic matter and no recent fertilization, soybeans may benefit from sulphur application.
• Sulphur is required for optimum yield, but is normally not limited in most irrigated soil as irrigation water contains substantial amounts of sulfate-sulphur (amounts in the water vary over time) – approximately 30 lb./ac. of sulfate/sulphur is added to the soil with 12 inches of irrigation water.
• Some soils are deficient in plant-available sulphur in the topsoil but have enough sulphur in the subsoil to meet crop requirements.
• In wetter, cooler conditions, plants may suffer from a lack of sulphur before plant roots grow down into the subsoil containing sulphur.
• Sulphur deficiencies are frequently a problem in the Black and Grey Wooded soil areas of Alberta and occasionally a problem in the Brown and Dark Brown soil areas.
• Test soil to a depth of 24 inches to determine if sulphur fertilizer is required – if analysis levels are less than 20 lb./ac., follow recommendations of the analysis report.

IRON

• Soybean requires adequate amounts of iron (Fe). Some environmental conditions can reduce the availability and uptake of Fe by the soybean plant, leading to the condition known as Iron Deficiency Chlorosis (IDC).
• Conditions that can lead to IDC include excess moisture, salinity, carbonates and/or high nitrate levels in the soil.
• Symptoms of IDC include yellowing of new soybean leaves between the veins (interveinal chlorosis), and an overall yellowing of soybean fields, particularly during the early vegetative stages in June.
• It is often a temporary condition that resolves itself when soil dries up. Plants that recover by the V5 to V6 stage should experience minimal yield loss. However, if symptoms persist for > 1 week, yield loss can occur.
• There is no effective in-season management option, but it is important to accurately diagnose the problem and adjust management strategies for future years. Visual diagnosis, tissue testing and knowledge of soil characteristics can help you diagnose IDC. (Source:  Manitoba Pulse and Soybean Growers)

Micronutrients

• Micronutrient deficiencies for soybean production, other than iron, have not been identified as a widespread problem through growing areas of Western Canada.
• If a micronutrient deficiency is suspected, it is advisable to analyze soil and plant samples within the suspect area and compare the analysis to soil and plant samples collected from a non-affected area of the same field.
• If the analysis confirms a micronutrient deficiency at a relatively early growth stage, a foliar application of the appropriate micronutrient fertilizer may correct the problem.

IMPACT OF MICRONUTRIENTS ON OTHER CROPS 

• Note that if soybean is not grown continuously on the same land – other rotation crops such as flax, wheat, canola and barley may respond optimally to the topped-up levels of these minerals:
  • The three remaining micro-nutrients – iron, manganese and molybdenum – have a much more critical effect.
  • Most Alberta soils are adequate for iron, but high pH soils or alkaline soils may lock up manganese availability so that a foliar application of this micronutrient may be necessary.
  • Molybdenum becomes much less available in acidic soils (below pH 6.5) especially at pH 5.5 or less.
  • Molybdenum is absolutely essential in the nitrogen fixation process in legumes – without it, no nitrogen can be fixed (in Europe, producers may apply 200 to 300 grams of actual molybdenum to the seed crop or soil every few years or lime the soil to bring up the pH and release more molybdenum).
• Based on soil test results, micro-nutrient fertilizer should be applied in test strips the first year. Sandy, low organic matter may show best response
• For more information, recommended soil micronutrient levels are tabulated in the fact sheet: Minerals for Plants, Animals and Man, Agdex 531-3 and Micronutrient Requirements of Crops, Agdex 531-1.