Inoculation and Fertility

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

Nitrogen fixation is a symbiotic relationship, in that it benefits both the bacteria and the plant. Rhizobium enters the root hairs of the plant and induce nodule formation. The plant provides energy in the form of carbohydrates for the rhizobium living inside the nodules. The rhizobium, in return, converts atmospheric nitrogen into a form of nitrogen that can be used by the plant. This relationship provides maximum benefit when the supply of available soil nitrogen is low and the soil moisture and temperature levels are adequate for normal seedling development.

Rhizobium leguminosarum strains will nodulate peas, faba beans, and lentils but some strains may be more effective on certain crops or certain varieties. Manufacturers package the inoculant as either a mixed or single strain inoculant.

Once the proper inoculant is chosen, care must be taken to ensure maximum rhizobia survivability. Rhizobium bacteria (either on the seed or in the package) are susceptible to temperature stress, drying out, and damage from direct sunlight. Inoculant must be stored in a cool dark environment prior to use, and expiry dates must be observed. Inoculated seed should be planted as soon as possible.

Inoculants are sensitive to granular fertilizer therefore, banding fertilizer to the side and/or below the seed is recommended. Inoculant should never be tank blended with fertilizer. Inoculants are also sensitive to some seed-applied fungicides. Check the labels 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 then apply the inoculant immediately prior to seeding.

Inoculants are available in different formulations: liquid, peat-based, and granular.

Liquid-based products offer convenience and better control of application rate, compared to other forms. However, the rhizobia in these formulations are more susceptible to damage from environmental extremes and direct contact with seed treatments than other inoculant forms. If treated seed is planted immediately into a moist seedbed, liquid formulations perform well.

Peat-based formulations are more durable and less prone to desiccation and damage from direct contact with seed treatments compared to liquid formulations, although care must still be taken. 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.

Granular formulations offer ease of application and should be applied in the seed row. These formulations remove the risk of incompatibility with seed treatments but care must still be taken to minimize risk of desiccation. Granular inoculants are less affected by environmental stress and seed-applied fungicides than other inoculant forms.

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

Although rhizobium bacteria can live in the soil for a number of years, efficient nitrogen-fixing bacteria may not be among those that survive. This reinforces the recommendation to inoculate each time lentils are 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 lentil producers use an inoculant on their lentil crop every year.

Checking Nodulation

Nodulation efficacy can be verified by examining the pulse crop at early flowering. It may take up to four weeks after seeding for nodulation to reach a point where it can be evaluated. The best way to check for nodulation is to dig up 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. Nitrogen fixation declines once plants begin pod formation and seed development.

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. 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. 

Nodulation and Nitrogen Fixation Field Assessment Guide

Fertility

Nitrogen: Soil testing is important. High soil nitrogen levels will adversely affect nodulation and fixation. 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 lentil. Lentils can derive 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).

Where combined levels of soil and fertilizer nitrogen reach 28-40 kg/ha (25-35 lb/ac development of nodules and nitrogen fixation may be delayed. Combined soil and fertilizer nitrogen levels greater than 55 kg/ha (50 lb/ac) can prevent effective nodulation and nitrogen fixation.

It can take three up to four weeks following planting for nodules to become fully functional. Early plant growth may be poor in soils with nitrogen levels less than 11 kg/ha (10 lb/ac), causing plants to appear yellow prior to the onset of active nitrogen fixation due to 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. Typical applications levels of monoammonium phosphate (ex. 12-51-0) often provide the small amount of nitrogen needed for early plant growth and, depending on the soil test, may provide the starter nitrogen required.

Phosphorus: Lentils have a relatively high requirement for phosphorus (Table 1). 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.

Lentil 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 lentil crop may benefit from improved stress tolerance as a result of phosphorus application.

Table 1. Average Nutrients Taken Up By The Plant During The Growing Season And Nutrients Removed With The Grain

The maximum safe rate of actual phosphate applied with the seed is 22 kg/ha (20 lb/ac) with 10 to15 per cent seed bed utilization (SBU) under good to excellent moisture conditions. Calculate SBU by dividing seed spread by row spacing. For example a 2.5 cm (1 in) spread with 25.4 cm (10 inch) row spacing, equates to 1/10 or 10 per cent SBU. Rates of seed-placed phosphate should be reduced if less than ideal moisture conditions exist. 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, band the fertilizer away from the seed (sideband or to the side and below), or increase phosphate levels in the years prior to growing lentils.

Guidelines for Safe Rates of Fertilizer Applied With The Seed

Potassium: Lentils have a high demand for potassium. Use a soil test to determine whether additional 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 rate of phosphate mentioned previously (22 kg/ha or 20 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 30 bu/ac lentil crop requires about the same amount of sulphur as a 40 bu/ac wheat crop; approximately 9-11 kg/ha (8-10 lb/ac) as lentils remove about 0.2 lbs/bu of sulphur. 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 indicates limited yield response from the addition of sulphur fertilizer except in fields testing very low in sulphur. 

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