Fertility Requirements

AMOUNT REQUIRED

• Understanding the fertilizer requirements of faba bean is critical to reaching optimum yields.

Nitrogen (N)

• Among prairie pulse crops, faba beans are a particularly prolific nitrogen-fixing crop, deriving up to 80% of its nitrogen requirements from nitrogen fixation. Application of nitrogen fertilizer is not recommended.
• Nitrogen fertilization application of less then 15 lbs per acre can be put down at the time of seeding.
• Faba bean has the ability to fix nitrogen from the air. The process works like this:
  • The faba bean forms a symbiotic relationship with specific bacteria, which live in association with plant roots.
  • The bacteria infect the plant roots and form nodules.
  • The bacteria use nutrients from the plant and provide nitrogen to the plant in return.
• When the seed is inoculated with the correct rhizobium bacteria, most of the total nitrogen in the plant will come from N fixation.
• In a three-year study at Barrhead, Alberta, faba bean nitrogen fixation ranged from 70 to 223 kilograms of nitrogen per hectare (kg N/ha) or 62 to 200 pounds of nitrogen per acre (lb N/ac) depending on growing conditions. This accounted for 80% or more of its nitrogen requirements. The remaining nitrogen comes from what is available in the soil at seeding, and nitrogen that is released (mineralized) from the soil during the growing season.
• Nodule formation and subsequent nitrogen fixation are very sensitive to external nitrogen sources, including fertilizer and available soil nitrogen. As the supply of external nitrogen increases, the amount of nitrogen fixation decreases. When external nitrogen levels are between 28 to 40 kg N/ha (25 to 35 lb N/ac), addition of nitrogen fertilizer will delay and reduce nodulation. External nitrogen levels greater than 55 kg N/ha (50 lb N/ac) can prevent nodulation and nitrogen fixation.
• It can take up to four weeks after planting for full nitrogen fixation to occur. During this time, growth may be inhibited and plants may appear yellow if soil nitrogen levels are less than 11 kg N/ha (10 lb N/ac) in the top 30 centimetres (cm) or 12 inches (in).
• This early season nitrogen deficiency can be corrected by adding low levels of starter-nitrogen at seeding. Typically, mono-ammonium phosphate (i.e. 12-51-0) provides the necessary amount of nitrogen for early plant growth. Although higher levels of starter nitrogen may help the crop overcome an early season nitrogen deficiency, final seed yields may not increase.
• Use of additional nitrogen, other than that which comes with other fertilizer elements such as 11-51-0, is not recommended.
• Ensure the field is low in available N (less than 20 lb. N/ac., 0 to 24 inch depth) or the crop may not mature properly.
• To allow for best N fixation, avoid fields manured in the past two years.
• Excess N fertilizer will reduce the amount of N fixed by a faba bean crop, delay crop maturity, increase disease levels and reduce standability.
• Mid-season N applications are normally not recommended. An exception would be under conditions of failed inoculation and obvious N deficiency.

Phosphorus (P2O5)

• Faba beans are a relatively high user of phosphorus.
• A soil test is recommended to determine optimum phosphate fertilizer rates.
• Adequate levels of phosphorus are critical for optimum yield and early maturity. Phosphorus deficiency restricts top and root growth, resulting in spindly stems with fewer branches.
• Phosphorus promotes the development of extensive root systems and vigorous seedlings. Encouraging vigorous root growth is an important step in promoting good nodule development, nitrogen fixation, and early more uniform maturity.
• The maximum safe rate of actual phosphate applied with the seed is 45 kg P2O5/ha (40 lb P2O5/ac) based on 10% to 15% seedbed utilization (SBU) under good to excellent moisture conditions.
• Calculate SBU by dividing seed spread behind opener by row spacing. For example, a 2.5 cm (1 in) spread with a 22.5 cm (9 in) row spacing gives 11% SBU. Rates of seed-placed phosphate fertilizer should be reduced if the seedbed has less than ideal moisture conditions.
• Higher rates of phosphate fertilizer placed in the seed row can harm the emerging seedlings. If higher phosphate rates are required, band the fertilizer away from the seed (side-band, mid-row, or to the side and below). If side-banding is available, side-band all phosphate fertilizer, especially when using narrow openers.
• When using narrow openers, seed-placed phosphate (P2O5) should not exceed 35 lb. P2O5/ac. (higher rates should be banded before planting or side banded at time of planting).
• 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.
• Even if seed yield increases are not achieved every year, a faba bean crop may benefit from improved stress tolerance as a result of phosphorus application.
• 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)

• Potassium levels in soil are usually adequate in most faba bean growing areas, so potassium is not usually required.
• Faba bean is a high user of potassium, if soil test levels are less than 300 lb./ac., follow the soil test recommendations.
• When potassium fertilizer is placed with the seed, use the following guidelines: the total application of phosphate (P2O5) plus potassium (K2O) must not exceed the maximum safe rate of seed-placed phosphate, which is 40 lb/ac under good-to-excellent moisture conditions.
• 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.
• Banding or seed-placing potassium are the most efficient methods of application.

Sulphur (S)

• Sulphur is required for optimum yields and should be included in the fertilizer blend based on soil test recommendations.
• Sulphate-sulphur, the plant-available form of sulphur, may be used to correct S deficiencies. When ammonium-sulphate fertilizer is placed with the seed, total pounds of nitrogen from ammonium sulphate and other nitrogen-contributing fertilizers should not exceed the maximum safe rate of seed-placed urea-nitrogen.
• Much of the topsoil sulphur is contained in soil organic matter. This is slowly released as sulphate-sulphur (SO4-S), the form of sulphur that plants require. Sulphate-sulphur is similar to nitrate-nitrogen in that both are mobile in soil.
• 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 is required for optimum yields, but is normally not limited on most Brown, Dark Brown or Thin Black soils.
• Lower soil sulphur levels occur more frequently on Black and Grey wooded soil types.
• Soil test to a depth of 24 inches to determine if a sulphur fertilizer is required. For testing purposes, soil samples should be taken from the 0–6 inch, 6–12 inch and 12–24 inch depths to determine the amounts of sulphur at each depth.
• If soil sulphur levels are low, add replacement amounts of sulphur each year – when annual additions are made, the sulphur can be in the elemental form (very little elemental sulphur is available to plants in the year of application).
• If soil test sulphur levels are less than 20 lb./ac., follow recommendations from the soil test report
• Soils testing low in available sulphur should have this deficiency corrected by side-banding, mid-row banding, or broadcasting ammonium sulphate, which contains sulphur in a plant-available form.
• If sulphur is required, apply a sulphate containing fertilizer such as ammonium sulphate (21-0-0-24).
• Elemental sulphur fertilizer won’t be available to the plant in the year it is applied – elemental sulphur is best used in a longer term program to build soil sulphur levels.

Micronutrients

• Faba bean crops require a supply of all the essential micronutrients: boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo) and zinc (Zn)
• 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 a micronutrient deficiency is suspected, a soil test should be done, and if levels are low, a test strip should be tried and evaluated before treating the whole field.
• There is a lack of information on micronutrient deficiencies in faba bean.
• 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 FABA BEAN ON OTHER CROPS

• Faba bean is an annual legume that fits well into cereal crop rotations as it fixes a large amount of nitrogen that is released to succeeding crops.
• Faba bean helps decrease cereal diseases. Faba bean crops that are kept grass and cereal-free break the root disease life cycle for diseases such as take-all (a soil-borne disease of cereal crops).
• Faba bean production allows growers to control competitive grassy weeds that are difficult to control in cereal crops – new grass-selective herbicides can be used on faba bean.
• Faba bean is an alternative crop to pea and lentil (where soils and growing conditions are suitable).
• Note that faba bean 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.