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Soybeans: Planting and Crop Development
Excerpt from Agronomy Guide for Field Crops (Chapter 4)Order OMAFRA Publication 811: Agronomy Guide for Field CropsTable of Contents
| Top of Page | Seed QualityIt is important to know the quality of the seed being planted. Certified seed must meet purity and germination standards. The quality of common seed is not known unless the germination is tested at an accredited seed lab prior to planting (see Appendix H, Ontario Seed Laboratories Accredited by the Canadian Food Inspection Agency.
Figure 4-1. The Relationship Between Seed Vigour,
Viability and Deterioration Germination is the major quality consideration used in grading seedlots. It is the ability of a seedlot to produce normal seedlings under favourable conditions of 95%-100% humidity and 25°C. Stress conditions in the field following planting often reduce field emergence compared to that in the lab. A better measure of the ability of seed to emerge rapidly and uniformly under a wide range of conditions is the vigour rating of the seed, called the vigour test, or more appropriately referred to as a stress test. Certified seed standards require that seed be tested for germination. In addition to germination, many seed distributors routinely test and report seed vigour. Figure 4-1, The Relationship Between Seed Vigour, Viability and Deterioration, above, illustrates the relationship between germination and vigour. As seed deterioration increases, germination drops slowly, whereas vigour drops very rapidly. With Lot A, deterioration is minimal and germination and vigour are similar. On the other hand, Lot B has excellent germination but low vigour. A number of factors can contribute to loss of seed vigour, including genetics, mechanical seed damage, deterioration in storage and weather conditions prior to harvest and disease. The most important factor affecting vigour appears to be environmental. Time-of-harvest studies conducted by the University of Guelph suggest that vigour is lost if there is a delay between physiological maturity and harvest. Timely harvest is important if soybeans are being grown for seed. | Top of Page | InoculationAdvancements in technology have provided inoculant types at a lower cost per acre, with higher rhizobia concentrations and more options for planting systems. When soybeans are grown on land for the first time, inoculation with soybean rhizobia is essential for high yields. Sterile inoculants containing the bacteria strain 532C have been found to give a 7% yield advantage over other strains when soybeans are grown on virgin land. On non-virgin land, this advantage does not occur over other strains of inoculant bacteria. Use two different products or at least two different lots of the same product to improve chances of good inoculation. A number of different types of soybean inoculants are available. Performance trials comparing inoculants are conducted each year by the University of Guelph. Table 4-6, Increase in Yield From Inoculating First-Time Soybean Fields With Different Types of Soybean Inoculants, presents a summary of 1998-2000 trials conducted in first-time soybean fields that revealed that liquid inoculants applied as in-furrow sprays or dribbled into the seed furrow at planting produced the highest yields. This application method involves equipping the planter or drill with nozzles or tips at each shoe plus a pressurized tank for the liquid inoculant. All major inoculant companies now market liquid inoculants. Liquid inoculants applied to the seed prior to planting have also given good performance. Apply the inoculant at the base of a brush auger when loading the planter. Kits that hang on the side of a truck, tote or gravity wagon are available from dealers. Some manufacturers recommend that under very dry soil conditions, it may be good insurance when using a liquid inoculant to add a half-rate powdered peat for rhizobia protection. Occasionally, some growers have experienced bridging in the planter or build-up in augers from over-application of liquid seed treatments or inoculants. One option to reduce bridging is to apply a low rate of peat inoculant at the same time. Minimizing the time between treatment and planting by mixing in the field will also help. Sterile-carrier inoculants use a powdered peat base that is sterilized prior to the addition of the inoculant strain. These inoculants carry much higher numbers of rhizobia than the older, conventional, non-sterile powdered peat. Non-sterile powdered peat often contains microbial contaminants, which may compete with the rhizobia. Granular inoculants are also non-sterile. They outperform non-sterile powdered peat inoculants because of the higher rate of granular inoculant applied per hectare. Seed treatments may negatively impact inoculant performance. When using inoculants and seed treatments together, only treat the amount of seed expected to be planted within hours of treatment. Trials show that only 40% of the bacteria survive after 4 hours. Inoculant is not required where a well-nodulated, dark-green soybean crop has been grown in the past. Exceptions are acid soils (pH below 6.0) and coarse, sandy soils. Under these conditions, inoculation is recommended for each soybean crop. A grower who is not certain that previous soybean crops were well nodulated should inoculate to avoid the possibility of poor nodulation. Long-term trials indicate a 0.05-0.07 t/ha (3/4-1 bu/ac) yield increase by inoculating soybeans planted into fields that have previously grown well-nodulated soybeans. Even in the absence of a soybean crop, soybean rhizobia will survive in most soils for 7-10 years. Recent studies have not shown any success in attempting to replace existing strains of rhizobia with newer, more-effective strains. Manure applied to soybean fields supplies a readily available supply of nitrogen, which soybeans will use over that provided by the rhizobia. In these fields, nodulation may be delayed, but yields will not be reduced. On virgin soybean ground where manure is applied, nodulation may not occur, and unless soil nitrogen is abundant, nitrogen deficiency may be observed. Soybean roots normally become infected with Rhizobium japonicum shortly after emergence. Nodulation of soybeans may be observed 3-4 weeks after planting. Checking fields at this point will allow time for nitrogen application, should an inoculant failure occur. In virgin fields, nodules will be located on the taproot. In previous soybean fields, nodules will also be found along lateral roots. Eight to 14 nodules per plant indicates adequate nodulation. | Top of Page | Source: University of Guelph 1Values are from seven replicated field trials conducted near Guelph during 1996-2000. All sites were selected to be free of soybean rhizobia. | Top of Page | Planting DateThe highest yields of soybeans are obtained from early plantings, generally the first 10 days of May in the southwest part of the province. Later plantings are likely to incur significant reductions in yield (see Table 4-7, Effect of Planting Date on Yield, Plant Height and Maturity. Generally, if soil temperature and moisture conditions are suitable for planting corn, they are also suitable for planting soybeans. Avoid planting soybeans at soil temperatures below 10°C (50°F). Normally there is a 2-hour delay between air temperature and tilled soil temperature at seeding depth. Absorption of cold water by newly planted soybean seed may result in reduced germination and low vigour. A hard spring frost can kill early-planted soybeans, since the growing point of the emerged seedling is above the soil surface. However, soybean plants can withstand temperatures as low as -2.8°C (27°F) for a short period of time, while corn experiences tissue damage at -2°C (28.4°F). Source: Ablett, Ridgetown College, University of Guelph, 1980-81 | Top of Page | Delayed PlantingWhen planting is delayed, fewer days are required for the plant to reach maturity. A one-month delay in planting results in a 9-day delay at maturity. Delayed planting can reduce the vegetative growth period. This results in shorter plants with lower pods. Late planting also reduces the number of pods per plant because of the shorter flowering period. In addition, planting date has some effect on the duration of the pod-filling period. A 3-day delay in planting date generally results in a 1-day delay in maturity. When planting is delayed beyond June 15, reduce the estimate of heat units available for crop growth by 100-150 for each week up to June 30. When planting after July 1, select a full-season, light-hilum variety (an early frost may cause dark hilums to "bleed" into the soybean). Vegetative growth of late plantings can be improved by selecting taller varieties and planting in narrow rows. Seeding rates should be increased by 10%. This will increase the height of the lowest pods as well as the number of pods per acre. | Top of Page | Double Cropping SoybeansOccasionally, a small number of soybean growers in the southernmost regions of Ontario have attempted to grow soybeans immediately following the harvest of their winter cereal or pea crop. Unfortunately, double cropping of soybeans in Ontario is more often a failure than a success. Do not take out a good red clover stand to double crop. The benefits from the clover stand will outweigh the risk involved in a double crop venture. Double cropping should not be attempted if soybean cyst nematode is a problem in the field. The soybean crop will reduce the benefits of the non-host (winter cereal) crop and increase cyst populations. The following management tips will increase the chances of a successful double crop:
| Top of Page | Row WidthSoybeans grow well under a wide range of row widths in the long-season regions of Ontario. The choice of row width depends on factors such as tillage system, equipment suitability, weed problems, experience, soil conditions, white mould pressure and planting date. Wide rows allow for inter-row cultivation and are less affected by soil crusting. Narrow rows allow the crop canopy to fill in more quickly, providing maximum light interception (see Table 4-8, Row Spacing vs. Days to Full Canopy (May Planting) below. Quick canopy development also contributes to weed suppression. | Top of Page | On heavier soil types such as clays, wider row widths increase the number of seeds per foot of row, which can aid in emergence. Improved air movement in wider rows may help reduce the severity of white mould. The increase in yield potential from growing soybeans in narrow rows is greatest in the short-season areas. The yield advantage decreases towards Southwestern Ontario. Row widths of 18 cm (7 in.) are recommended in short-season areas. In Southwestern Ontario, there may be some advantage in reducing row widths to less than 53 cm (21 in.), as noted in Table 4-9, Effect of Row Width on Yield. | Top of Page | 1Values based on research on clay loam soils in the 3,250 CHU area. Greater response can be anticipated in shorter-season regions. The response to row-width reductions is reduced under stressful growing conditions. | Top of Page | Seeding RatesSoybean seeding rate is less critical than with corn. Soybeans can compensate considerably for differences in stands without effecting yield. Too high a seeding rate adds unnecessary seed costs and may increase lodging. Seed-size differences impact seeding rates. Seed size and seed quality of a variety are influenced by growing and harvest weather of the previous year. There can be as much as 20% variation in the seed size of a variety from one year to the next. Seeding rates listed in Table 4-10, Recommended Soybean Seeding Rates will provide satisfactory stands if 90% of the seeds emerge. Seeding rate must be adjusted upward for seed with a lower germination or vigour rating or for soils that tend to crust. Soybean varieties more prone to lodging will benefit from plant populations slightly lower than those given in the table. Planting rates should be increased by 10%-20% with late plantings into mid-June. Varieties respond similarly to changes in seeding rate. The formula for determining seeds needed per foot of row is: Seeds needed per m (ft) of row = | Top of Page | Table 4-10.
1Calculations
for an 18-cm (7-in.) row width are based on a stand of 445,000 plants/ha
(180,000 plants/ac.) | Top of Page | Planting DepthA uniform seeding depth is important. Due to the high water demand for germination, plant 1 cm (approximately 1/2 in.) into moisture but not deeper than 7.5 cm (3 in.). However, this is difficult to achieve with some seed drills, especially in reduced or no-till fields. Adequate down pressure, ballast and the use of a coulter cart can help achieve proper seeding depth. It is important to have good seed-to-soil contact and a closed seed slot. The key is to plant into adequate soil moisture with a properly adjusted planter/drill. If seeding into moisture with a drill cannot be achieved, consideration should be given to seeding with the planter, rather than waiting for rain. Varieties differ in their ability to emerge from planting depths greater than 5 cm (2 in.). Seed companies typically provide an "emergence score" or hypocotyl length rating, which rates the ability of the seedling to emerge from unusually deep planting. | Top of Page | Soil CrustingCrusting of the soil surface following a driving rain or water ponding can inhibit soybean emergence. The crust can break the hypocotyl arch (the portion of the plant that lifts the cotyledons above the soil surface). If soil is prone to crusting, plan to break the crust before the seedlings are attempting to break through. Light tillage with a rotary hoe, harrows, coulter cart or even the planter or seed drill can help break the soil crust and aid bean emergence. Typically these operations can cause a 10% loss of emerged beans. A higher stand loss can occur when the hypocotyl arch is breaking the surface. "Crust-busting" may not be necessary in thin stands (e.g., 60%) where full yield potential already exists. Refer to Table 4-11, Expected Yield of Soybeans in Optimum and Reduced Stands, to determine yield potential. | Top of Page | 1 hectare = 2.47 acres 1Conducted at Huron and Kemptville, Ontario, research stations, University of Guelph. | Top of Page | Plant DevelopmentTable 4-12, Vegetative Growth Stages in Soybean, and Table 4-13, Reproductive Growth Stages in Soybean, show the growth stages of the soybean. The first two leaves of the soybean plant are unifoliates (single leaflets) occurring opposite each other at the first node above the cotyledons. Subsequent leaves are trifoliate (three leaflets) and are on alternate sides along the stem. When the plant has 2-3 trifoliates, the nodules, which are important for the fixation of atmospheric nitrogen, become visible on the roots. When planted at the optimal time, soybeans will develop 5-8 leaves before flowering begins. Flowering is triggered mainly by day length and temperature changes. Very early-maturing soybeans are nearly insensitive to day length. Instead, flowering is controlled mainly by accumulated heat units. Later-maturing varieties are influenced more by day length. Therefore, late-planted, long-season soybeans take fewer days to mature than those planted early. | Top of Page | Germination and EmergenceGermination begins with the seed absorbing soil moisture until it reaches a moisture content of about 50%. The first external sign of germination is the emergence of the radicle (primary root), which grows downward and anchors itself in the soil. Shortly after, the hypocotyl (the section of the stem above the radicle) starts growing upwards, pulling the cotyledons (seed leaves) with it. Plate 50 shows the hypocotyl straightening and the cotyledons unfolding from VE to VC.
Plate 50: VE and VC stages of emergence showing the hypocoytl straightening and the cotyledons unfolding. Once emerged, the hook-shaped hypocotyl straightens out, the cotyledons fold down and the growing point is exposed to sunlight. Emergence normally occurs about 4-14 days after planting, depending on soil moisture, soil temperature and planting depth. | Top of Page | Updates on Soybeans: Planting and Crop DevelopmentNo updates available at this time. | Top of Page | Related links...| Top of Page | For more information:Toll Free: 1-877-424-1300 Local: (519) 826-4047 E-mail: ag.info.omafra@ontario.ca |
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