Inoculation and Fertility
Nitrogen: Among prairie pulse crops, faba beans are a particularly prolific nitrogen-fixing crop, deriving 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 70-223 kg N/ha (62-200 lbs N/ac) depending on growing conditions. This accounted for 80 per cent 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 28-40 kg N/ha (25-35 lbs N/ac), addition of nitrogen fertilizer will delay and reduce nodulation. External nitrogen levels greater than 55 kg N/ha (50 lbs 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 lbs 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 higher levels of starter-nitrogen may help the crop overcome an early season nitrogen deficiency, final seed yields may not increase.
Phosphorus: Faba beans are a relatively high user of phosphorus (Table 4). Adequate phosphorus fertility promotes the development of extensive root systems. This benefits the plant by encouraging nodule development, thereby increasing nitrogen fixation. It also allows for better access to soil resources, improves disease resistance, and hastens maturity.
Appropriate phosphorus recommendations are best determined by a soil test. However, it is important to remember, that the maximum safe rate of actual phosphate applied with the seed is 45 kg P2O5/ha (40 lbs P2O5/ac) based on 10-15 per cent 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 per cent 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
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 40s lb/ac under good-to-excellent moisture conditions.
Sulphur: Sulphur (S) 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-N.
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.