Description & Adaptation
Peas are a pulse crop that is part of the Leguminosae family. It is a cool-season annual plant grown adapted to cool temperate zones such as Western Canada.
Pea seed goes through hypogeal germination in which the pea cotyledons and seed coat remain below the soil surface. The first two growing points or nodes do not produce true leaves but are called scale leaves which are relatively small, and the nodes seldom emerge completely from the ground. If the young seedling is injured by adverse conditions such as frost or mechanical damage, re-growth is possible from buds at the base of these scale leaves. Under favourable growing conditions, basal branches will develop from one or both nodes by the sixth leaf stage.
The first true leaf is produced at the third node position. First, true leaves usually consist of one pair of leaflets and a tendril. The second leaf is formed at the fourth node. Growth is usually rapid by this stage and under favourable growing conditions, two nodes can develop in as little as seven days. In semi-leafless peas, tendrils replace the leaflets. Most varieties in Saskatchewan are now semi-leafless except for some forage pea types. The tendrils on the semi-leafless types help the plant intertwine with neighbouring plants which increases standability and improves harvestability.
The varieties grown in Saskatchewan are relatively indeterminate, meaning they will continue to grow and flower over an extended period until some stress factor induces maturity. Depending on the variety, plants usually begin to produce flowers between the 12 and 16 node of development. Typically, one to three flowers are produced at each flowering node. Pea flowers have separate male (stamen) and female (pistil) parts and undergo self-pollination prior to opening. Cross-pollination can occur because of pollinators (insects) but is rare. Most pea varieties have white flowers, but some forage types can have purple flowers.
Pods develop and are fully elongated within seven to 10 days. Over a period of about 24–30 days, the seeds develop and mature until physiological maturity the (dry seed stage) is reached. Physiological maturity occurs when the seeds are fully developed and colour change has occurred. Seeds at physiological maturity are usually around 35% moisture. At this stage, seeds may be detached or easily detach from the pods. Fully formed pods are four to 10 cm long and 1 cm wide and usually contain six to eight seeds. Seed sizes range from 150–280 grams per thousand kernels depending on the variety and growing conditions. Bushel weight for dry peas is 60 pounds.
Peas have a relatively shallow root system. Planting peas on cold, poorly drained soils should be avoided, as it favours the development of seedling diseases and root rots. Peas do not tolerate water-saturated or salt-affected soils. The low potential of hydrogen (pH) can inhibit nodulation, reducing nitrogen fixation and plant growth. Most Saskatchewan soils have a pH range suitable for pea growth. Well-drained, clay loam soils are ideal for pea production. They can tolerate some hot weather or drought stress during flowering, but yields may be reduced. The best growing temperature range is when daytime highs are between 13°C and 23°C. Flower abortion can occur at higher temperatures (over 25°C).
Its relative drought tolerance allows field peas to be grown in the Brown soil zone, however, peas are best suited for growth in the Dark Brown and Black soil zones.
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Varieties
There are many varieties of peas, each with its own characteristics and target markets. Yellow and green cotyledon peas are the most widely grown in Saskatchewan and are suitable for human consumption and livestock feed markets. These varieties have white flowers and are semi-leafless. Historically yellow pea varieties outyield green varieties, but the gap has narrowed with the development of new green pea varieties.
Specialty varieties for use as forage or silage may be leafed or semi-leafless. Marrowfat types are blocky, very large-seeded, green cotyledon peas used in specialty snack food markets in Asia. Purple flowered varieties with coloured seed coats (maple and dun pea) are also produced in Saskatchewan.
Varieties with improved tolerance to Mycosphaerella/Ascochyta blight and Fusarium wilt are being developed, and each variety is rated for its level of disease resistance. All recently released varieties have resistance to powdery mildew. Varieties may also differ in their standability or lodging, height, seed size, level of seed coat breakage, and relative maturity. When choosing a variety consider all the factors. Varieties with good lodging resistance improve harvestability and reduce soil tag. Taller pea varieties do not necessarily have weaker straw strength. Research at the Crop Development Centre (CDC) has shown that taller varieties are more competitive against weeds. Adding the semi-leafless characteristic to new varieties has improved lodging resistance.
Bleaching resistance is an important consideration when choosing green pea varieties. Rain and hot sunny days, especially just prior to harvest, increases bleaching.
Want to see how different varieties compare?
Visit the Interactive SaskSeed Guide for information on all pea varieties available to Saskatchewan producers.
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Seeding
The recommended target plant density for peas is 75–85 plants per square metre (7–9 per square foot). Peas can be seeded early when the minimum average soil temperature at seeding depth is 5°C. In Saskatchewan this means seeding from mid-April to mid-May for most regions. The recommended seeding depth for peas is 3–8 cm or 1.2–3.2 in. Peas can tolerate deep seeding (up to 4 in), but if moisture is near the soil surface, shallower seeding ensures quicker emergence.
Seed Quality
Seed quality is critical for stand establishment, health, and vigour. Seed quality includes genetic and mechanical purity, germination and vigour, and levels of seed-borne disease.
Seed purity is determined by the nature and amount of unwanted contaminants in the pure seed. Impurities include unwanted crop seed, weed seed, and inert material. They can adversely impact crop yield and quality, as well as increase production costs.
Seed germination tests assess the ability of the seed to produce a healthy plant under favourable growing conditions. These tests are generally conducted under controlled conditions that provide ideal moisture, temperature, and light for a prescribed period. Seed lots with low germination often lack the ability to produce strong, healthy seedlings.
Seed vigour tests, undertaken by some seed testing labs, are conducted under more adverse conditions than a germination test. Vigour tests are not standardized, and conditions imposed upon the seed may vary from lab to lab. Vigour tests are an attempt to see how the seed germinates under less-than-ideal conditions and gives an indication of the vigour of the seedling.
Pea seeds are susceptible to mechanical damage during harvest, handling, storage, and seeding. Dry pea seed (14% or less seed moisture) is brittle and difficult to handle without chipping and splitting the seed. All handling should be done as gently as possible. Even nearly invisible seed cracks can result in a reduction in germination. Seed damaged after a sample is submitted for germination and/or vigour testing will not perform as expected based on the results of the test(s). The final cleaned seed lot should be re-tested if handling damage is suspected.
Application of certain herbicides prior to harvest can also affect seed germination and/or vigour. Seed from fields treated with pre-harvest glyphosate should be avoided.
Contamination from seed-borne diseases should be as low as possible. Table 1 summarizes guidelines for seed disease levels when considering a lot for seed.
Table 1. Guidelines For Tolerances of Seed-Borne Diseases in Pea Seed Intended For Planting
| Disease (Pathogen) | Tolerance & Factors Affecting the Level |
|---|---|
|
Ascochyta (Mycosphaerella pinodes, Ascochyta pinodella, Ascochyta pisi) |
Up to 10% Ascochyta infection should not significantly affect plant establishment and yield, as long as the seed has good germination, and spring conditions promote quick germination and good seedling vigour. Seed-to-seedling transmission of Ascochyta in pea under field conditions is considered low. In areas where pea production is common, the primary means of infection is air-borne spores from the overwintering stage of Mycosphaerella pinodes on pea residues. |
|
Seed Rots and Damping-off (Pythium sp. & Phytophthora sp.) |
These are soil-borne diseases and are not tested for at seed testing labs. Seed treatment in field peas may be beneficial when planting under cool, moist soil conditions or if using damaged or cracked seed. Seed Rots and Seedling Blights (Botrytis, Sclerotinia, Rhizoctonia, and Fusarium species) Sclerotinia, Rhizoctonia and Fusarium are primarily soil-borne. Botrytis and Fusarium are also often seed-borne and can be tested for at seed testing labs. Up to 10% infection (Sclerotinia + Botrytis) may be tolerable but will result in significant seedling blight if a seed treatment is not used. The importance of seed-borne Fusarium in seed rot and seeding blight in pulses is not known. Some labs will notify growers if greater than 5% Fusarium infection occurs. If present, add the Fusarium value to the Sclerotinia + Botrytis value above (not to exceed 10%). |
These are guidelines only and should be considered along with farming practices and level of disease risk for the situation. The use of certified seed assures high quality seed with respect to purity, germination, and disease level.
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Inoculation & Fertility
Peas inoculated with the proper Rhizobium (Rhizobium leguminosarum) strain have the potential to fix up to 80% of nitrogen required through nitrogen fixation.
Nitrogen fixation is a symbiotic interaction in which both the Rhizobium and the plant benefit from the relationship. Rhizobium enters the root hairs of the plant and induces nodule formation, while the plant provides energy for the Rhizobium. 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 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 strain inoculant that contains a mixture of the 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 dark place prior to use and must be used before the expiry date. Following application of the inoculant, plant 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. High available soil nitrogen levels (over 55 kilograms of nitrogen per hectare, or 49 pounds of nitrogen per acre) 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/peat, and granular. Granular inoculants are less affected by dry seedbeds and seed-applied fungicides than other forms of inoculants.
Liquid-based products offer convenience and better control of application rate, compared to other forms. However, they are also more susceptible to damage prior to seeding from environmental extremes and seed treatments, than other inoculant forms. If treated seed is planted immediately into a moist seedbed, liquid formulations perform well.
Powder/peat 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.
Granular formulations are the least prone to exposure damage. They allow for precise application rates when an additional seed cart compartment is available. 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 several 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 peas are seeded. Western Canadian research indicates a significant yield response to inoculation of grain legumes in 30–50% of the cases. For this reason, most experienced pea producers use an inoculant on their pea crop every year.
Checking Nodulation
The effectiveness of inoculation can be checked by examining the pulse crop at early flowering. It may take three to four weeks after seed germination before nodulation reaches a point where it can be evaluated. Although peas are 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.
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.
Soil testing is important. If soil nitrogen levels are too 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 peas. Peas can drive up to 80 per cent of their nitrogen requirements through nitrogen fixation. The remaining nitrogen comes from the soil using what is available at the time of seeding, plus what is mineralized during the growing season.
When the combined levels of soil and fertilizer nitrogen reach 28–40 kg per hectare (kg/ha) or 25–35 pounds per acre (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 functional. Early plant growth may be poor in soils with nitrogen levels less than 11 kg/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, 9–13 lb/ac) 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) can provide a small amount of starter nitrogen needed for early plant growth.
Peas have a relatively high requirement for phosphorus. A 50 bushel per acre (bu/ac) pea crop takes up 40–54 kg/ha (36–48 lb/ac) of phosphate and 35–43 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, nitrogen fixation, and early more uniform maturity.
Peas 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 because of phosphorus application.
The maximum safe rate of actual phosphate applied with the seed is 17 kg/ha (15 lb/ac) 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 should be reduced if the seedbed 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) should be considered. If sidebanding, sideband all the phosphate fertilizer, especially when using narrow openers.
Peas have a high demand for potassium at 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 (0.9 lb/ac) for every bushel of grain produced. Use a soil test to determine whether additional potassium is required. 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 in plant residue and is not removed with the grain. Most Saskatchewan soils are sufficient in potassium. However, deficiencies may exist based on region and farming practices. Fertility requirements should be determined based on soil test results.
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 9–11 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.
Micronutrient deficiencies for pea production have not been identified as a widespread 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.
