Inoculation and Fertility

Pea inoculated with the proper rhizobium (Rhizobium leguminosarum) strain has the potential to fix up to 80 per cent of its nitrogen requirement through nitrogen fixation.

Nitrogen fixation is a symbiotic relationship. Both the rhizobium and the plant benefit from the relationship. Rhizobium enters the root hairs of the plant and induce nodule formation. The plant provides energy for the rhizobium living inside the nodules. The rhizobium, in return, converts atmospheric nitrogen from the soil air surrounding the roots into a form that can be used by the plant. Maximum benefit is derived if the supply of available soil nitrogen is low and the soil moisture and temperature levels are adequate for normal seedling development from the time of seeding until seedlings are well established.

Rhizobium leguminosarum strains will nodulate pea, faba bean, and lentil but some strains may be more effective on certain crops or certain varieties. Manufacturers package the inoculant as either a mixed strain inoculant that contains a mixture of the strain (or strains) or a single-strain inoculant which contains only one rhizobia strain. In either case the best strains are chosen based on their ability to nodulate the crop on the label.

Once the proper inoculant is chosen, steps should be taken to ensure maximum rhizobia survivability. Rhizobium bacteria (either on the seed or in the package) die if they are exposed to stress such as high temperature, drying winds, or direct sunlight. Inoculant must be stored in a cool place prior to use and must be used before the expiry date. Following application of the inoculant, plant the inoculated seed into moist soil as soon as possible.

Inoculants are sensitive to granular fertilizer. Banding fertilizer to the side and/or below the seed is recommended. Never mix inoculant with granular fertilizer. Inoculants are also sensitive to some seed-applied fungicides. Check the label of both the inoculant and seed treatment for compatibility. When using a combination of fungicide and inoculant, apply the fungicide to the seed first, allow it to dry, and apply the inoculant immediately prior to seeding. Granular inoculants are less affected by dry seedbeds and seed-applied fungicides than other forms of inoculants.

High available soil nitrogen levels (over 55 kg N/ha) inhibit nitrogen fixation since the pea plant will preferentially use the soil nitrogen rather than fix nitrogen.

Inoculants are available in different formulations: liquid, powder, and granular.

Liquid-based products offer convenience and better control of application rate, compared to other forms. However, they are also more susceptible to damage from environmental extremes and seed treatment prior to seeding than other inoculant forms. If treated seed is planted immediately into a moist seedbed, liquid formulations perform well.

Powder formulations are more durable and less prone to desiccation and seed treatment damage, compared to liquid formulations. The bacteria can still be killed by desiccation so the same precautions should be taken as with liquid. Some peat-based powder inoculants require the use of a sticker. Adhesion to the seed can be enhanced if the seed is slightly damp during inoculation.

All inoculant formulations will perform equally well if the inoculant is properly applied and if environmental conditions are ideal. Under adverse conditions the best performing formulation should be granular, followed by peat, and then liquid.

Rhizobium bacteria can live in the soil for a number of years. However the most efficient nitrogen-fixing bacteria may not be among those that survive. Generally, native soil strains of Rhizobium leguminosarum are not the optimum strains. This reinforces the recommendation to inoculate each time pea is seeded. Western Canadian research indicated a significant yield response to inoculation of grain legumes in 30 to 50 per cent of the cases. For this reason, most experienced pea producers use an inoculant on their pea crop every year.

Nodulation and Nitrogen Fixation Field Assessment Guide

Checking Nodulation

The effectiveness of inoculation can be checked by examining the pulse crop in early summer at early flowering. It may take three to four weeks after seed germination before nodulation reaches a point where it can be evaluated. Although pea is an excellent nitrogen fixer and the nodules can be easily seen when a plant is pulled from the ground, the best way to check for nodulation is to dig a plant and gently remove the soil from the roots by washing in a bucket of water. Nodules are fragile and readily pull off if the roots are pulled out of the soil.

If the rhizobia are actively fixing nitrogen, the nodules will appear visibly red or pink inside. Nitrogen fixation is synchronized with plant growth, supplying the crop nitrogen during rapid vegetative growth.

Seed applied inoculant should result in nodules forming on the primary root near the crown. If the inoculant was soil applied (granular), nodules should be found on primary and secondary roots. If nitrogen fixation is active, the nodules will be pink or red on the inside. Lack of nodules indicates rhizobia did not infect the pulse plant. Lack of a pink colour (usually green or cream coloured) indicates the rhizobia are not fixing nitrogen. Nitrogen fixation declines once plants begin pod formation and seed development.

Nitrogen: Soil testing is important. If soil nitrogen levels are unusually high, nodulation and nitrogen fixation may be adversely affected. As the supply of nitrogen from soil and fertilizer increases, the amount of nitrogen fixed by the plant decreases. Nitrogen is necessary for high yields, but generally nitrogen fertilizer application is not required for pea. Pea can drive up to 80 per cent of its nitrogen requirements through nitrogen fixation. The remaining nitrogen comes from the soil (available at time of seeding plus mineralized during growing season).

When the combined levels of soil and fertilizer nitrogen reach 28-40 kg/ha (25-35 lb/ac), any additional nitrogen will delay the onset of nodules and reduce nodulation and nitrogen fixation. Combined soil and fertilizer nitrogen levels greater than 55 kg/ha (50 lb/ac) can prevent nodulation and nitrogen fixation.

It can take three to four weeks after planting before nodules are fully functioning. Early plant growth may be poor in soils with nitrogen levels less than 11kg/ha (10 lb/ac), and plants may appear yellow prior to the onset of active nitrogen fixation due to a nitrogen deficiency. This early deficiency can be corrected by adding low levels (10-15 kg/ha) of starter nitrogen at seeding. Although high levels of starter nitrogen may appear to help the crop overcome a nitrogen deficiency during early crop growth stages, final seed yields may not increase. Monoammonium phosphate (ex. 12-51-0) provides the small amount of nitrogen needed for early plant growth and, depending on the soil test, may provide the starter nitrogen required. The Saskatchewan Agriculture publication, Inoculation of Pulse Crops, provides information on what to do if the crop fails to fix nitrogen through inoculation. 

Table 2. Average nutrients taken up by the plant during the growing season and nutrients removed with the grain.

Phosphorus: Pea has a relatively high requirement for phosphorus. A 50 bu/ac pea crop takes up 40 – 54 kg/ha (36 to 48 lb/ac) of phosphate and 35 to 43 kg/ha  (31-38 lb/ac) is removed from the field with the seed. 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 the nitrogen fixing process and in promoting earlier, more uniform maturity.

Pea grown on soils testing low in available phosphorus or under cool wet conditions may respond to phosphate fertilizer. However, dramatic yield responses are not always achieved. Even if seed yield increases are not achieved every year, a pea crop may benefit from improved stress tolerance as a result of phosphorus application.

The maximum safe rate of actual phosphate applied with the seed is 17 kg/ha (15 lb/acre) in a 2.5 cm (1 in) spread and 15-18 cm (6-7 in) row spacing under good to excellent moisture conditions.  Rates of seed-placed phosphate must be reduced if the seed bed 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) or the use of the product Jumpstart® should be considered. If sidebanding, sideband all the phosphate fertilizer, especially when using narrow openers.

Potassium: Pea has a high demand for potassium and is about 135-165 kg/ha (123-150 lb/ac) K2O for a 50 bu/ac crop. This works out to just over 1 kg/ha for every bushel of grain produced. Use soil test to determine whether potassium is needed. Seed-placing potassium may cause seedling damage. As with phosphate, a wider opener may allow for slightly higher safe seed-placed rates.  The sum of seed-placed potassium (K2O) plus phosphate fertilizers must not exceed the recommended safe of phosphate mentioned previously (17 kg/ha or 15 lb/ac). Most of the potassium taken up remains with soil residue and is not removed with the grain. Most soils are sufficient in potassium. However, deficiencies may exist, especially in sandy Black and Grey soils found in northern Saskatchewan.

Sulphur: Sulphur is required in a relatively significant amount. A 40 bu/ac pea crop requires about the same amount of sulphur as a 40 bu/ac wheat crop, approximately 9 to 11 kg/ha (8-10 lb/ac). 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. Most research indicated no yield response from addition of sulphur fertilizer except in fields testing very low in sulphur. 

Micronutrient deficiencies for pea production have not been identified as a problem through pea 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.