Pulse crops, including field pea, lentil, chickpea, dry bean and fababean, have the ability to obtain significant amounts of their nitrogen requirement from the air within the soil that surrounds their roots. However, the seed of the pulse crop must be inoculated with specific bacteria called Rhizobium (rhizobia) which converts the nitrogen in soil air to a plant useable form of nitrogen.
“The relationship between rhizobia bacteria and a pulse plant is mutually beneficial with the plant providing energy for the well-being of the rhizobia,” says Neil Whatley, crop specialist with Alberta Agriculture and Rural Development. “Inoculation of pulses is more economical than purchasing synthetic N fertilizer. However, growers must ensure that the inoculant they buy is recommended for the specific pulse crop they plan to grow because only certain rhizobia species will cause nodulation on specific pulse crops. For example, the rhizobia that infects lentil will not infect field pea.”
The appropriate rhizobia/pulse crop compatibility is indicated on the inoculant product label. There are ‘mixed’ inoculant products containing more than one rhizobia species that are applicable to more than one pulse crop and these crops will be indicated on the label. When your cropping plans are complete, order inoculant products from your input dealer ahead of time to prevent running short at seeding time.
Except under drought conditions and unusually low temperatures, properly inoculated pulse crops have the ability to produce 50 to 90 per cent of the crop’s N requirement. Ranking pulse crops according to their estimated ability to fix N is as follows: fababean > pea > chickpea > lentil > dry bean.
“N fixation begins approximately three-to-five weeks after germination and generally there is some residual N in the soil to sustain the seedling until this time,” says Whatley. “However, pulse crops do require phosphorus and a general rule is if the soil contains less than 30 lbs of actual P per acre, top it up with 15-to-30 lbs of P2O5/acre. Due to toxicity to the seed, research suggests the maximum safe amounts of actual P applied with the seed is 0 lbs/ac with dry bean, 15 lbs/ac with chickpea, 20 lbs/ac with lentil, 25 lbs/ac with field pea, and 35 lbs/ac with fababean.
“For example, Monoammonium Phosphate (12-51-0) is generally used, so some N fertilizer is also added. If soil conditions are dry, apply less fertilizer with the seed. Some growers, especially those using narrow seed openers, apply Jumpstart®, which increases the availability of small amounts of P to crops through solubilization of soil phosphate. Some experienced growers add extra P to the previous year’s cereal crop to achieve optimum P amounts in soil destined for pulse crops the following spring.”
Inoculant formulations such as peat-based powder and liquid are applied to the seed while granular inoculant formulations are applied to the soil, either in the seed row or in a side band. Research indicates that when properly applied, all types of inoculant formulations add sufficient rhizobia bacteria to the soil for adequate N fixation, except under dry conditions. In dry years, research shows that yields are higher when granular inoculant is used. However, granular application is more expensive and requires an additional tank and meter on seeding equipment.
“Due to rhizobia being sensitive to some seed-applied fungicides, care should be taken when it is necessary to apply both to the pulse seed,” says Whatley. “It is wise to check inoculant product labels for the various methods that fungicides can be safely applied with the inoculant. One method is the sequential method that suggests applying fungicide to the seed first, allowing it to dry, and then applying inoculant just before seeding.
“Check the inoculant expiry date when you purchase it because over time the number of live rhizobia may reduce, causing inadequate nodulation. Store inoculant in a cool, dry place and away from direct sunlight. For temporary storage, an unheated building is adequate.”
Although some residual rhizobia survive in the soil for several years and can produce N-fixing nodules when its host pulse crop is grown, experienced pulse crop growers inoculate each time they plant a pulse crop. If one chooses not to inoculate and to rely solely on residual rhizobia, rhizobia numbers may be reduced below adequate amounts or they may have become inefficient fixers of N. Applying inoculant when planting pulse crops is a cheap replacement for applying expensive N fertilizer.
For additional information please contact the Ag-Info Centre at 310-FARM (3276) and ask to speak to a Crop Specialist.
Source: Govt. of Alberta Agriculture And Rural Development