Inoculation and Fertility

Nitrogen: faba bean derives up to 80 per cent of its nitrogen requirements from nitrogen fixation. Application of nitrogen fertilizer is not recommended.

In a three-year study at Barrhead, Alberta, faba bean nitrogen fixation ranged from 70 to 223 kg N/ha (62 to 200 lb N/ac) depending on growing conditions. This accounted for 80 percent 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 three to four weeks after planting before full nitrogen fixation occurs. During this time, plant growth may be poor and plants may appear yellow if soil nitrogen levels are less than 11 kg N/ha (10 lb N/ac) in the top 30 cm (12 inches).  This early season nitrogen deficiency can be corrected by adding low levels of starter-nitrogen at seeding. Typically, monoammonium phosphate (i.e. 12-51-0) provides the necessary amount of nitrogen for early plant growth. Although high levels of starter-nitrogen may help the crop overcome an early season nitrogen deficiency, final seed yields may not increase.

Phosphorus: faba bean is a relatively high user of phosphorus. Phosphorus promotes the development of extensive root systems and vigorous seedlings. Encouraging vigorous root growth is an important step in promoting good nodule development. Phosphorus also plays an important role in nitrogen fixation.

Faba bean planted on soils testing low in available phosphorus, or under cool or wet conditions, may respond to phosphorus fertilizer. Although, yield responses are not always observed, phosphorous fertilizer applications may: hasten maturity, improve resistance to disease, increase nitrogen fixation and improve drought tolerance.

General fertilizer recommendations for phosphate are available from your soil-testing lab. However, it is important to remember, that the maximum safe rate of actual phosphate applied with the seed is 45 kg P2O5/ha (40 lb P2O5/ac) in a 2.5 cm (1 in) spread with a 22.5 cm (9 in) row spacing under good to excellent moisture conditions. Rates of seed-placed phosphate fertilizer must be reduced if the seed bed 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) or use Jumpstart®. If side-banding is available, side-band all phosphate fertilizer, especially when using narrow openers.

Potassium: potassium is usually not required, but deficiencies may exist, especially in sandy Black and Grey soils. Fields low in potassium should be corrected based on 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/acre. This applies under good to excellent moisture conditions.

Sulphur: sulphur (S) is required for optimum yields, especially on Black and Grey Wooded soils. General fertilizer recommendations for sulphur are available from your soil testing lab.

Sulphate-sulphur is the plant-available form of sulphur, and should be used to correct S deficiencies. When ammonium-sulphate fertilizer is placed with the seed, add the pounds of N from the ammonium-sulphate to the pounds of N from other nitrogen fertilizer being placed with the seed. The total N should not exceed the maximum safe rate of seed-placed urea-N. Ammonium-sulphate has a high salt effect.

Micronutrients: micronutrient deficiencies in faba bean production have not been identified as a problem in Western Canada. However, if a micronutrient deficiency is suspected, consult an agronomist to help identify the problem. 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 may correct the problem. The Saskatchewan Ministry of Agriculture fact sheet Micronutrients in Crop Production provides more information about micronutrient fertilization. 

Nodulation and Nitrogen Fixation Field Assessment Guide