Soils, Fertility and Nutrients
Soil Improvements With Legumes
Last Update: December 2005
What are Legumes?
Legumes are plants that bear their seeds in pods. They differ markedly from grasses, cereals and other non-legume crops because much of the nitrogen they require is produced through fixation of atmospheric nitrogen by bacteria in nodules on their roots. As a result, legumes are rich in protein. World-wide more than 16,000 species of legumes are known, including herbs, shrubs and trees, but only about 200 are cultivated. In Saskatchewan, only a few legume species are grown commercially (Table 1), and only lentil, field pea, alfalfa and sweetclover are grown on a large number of acres.
Table 1. Legumes Grown in Saskatchewan
Old and New Interest in Legumes
Figure 1. Effect of Cultivation on Loss of Potentially Mineralizable Nitrogen and Organic Nitrogen in Some Saskatchewan Soils (0-6 in.) Source adapted from Campbell, Paul and McGill, p. 7-101 in Proc. Western Canada Nitrogen Symp., Calgary, Alberta (1976)
Forage legumes, such as alfalfa and sweetclover, were grown in rotation with cereals soon after the homesteaders first broke prairie grassland around the turn of the century. In these early years legumes generally failed to improve soil conditions because the recently cultivated soils still had good structure and large reserves of organic matter. Furthermore, levels of available nitrogen in the soil were so high that biological nitrogen fixation in legumes was largely inhibited.
Over time, soil organic reserves declined due to cereal cropping and frequent fallowing. This resulted in an increase in green manuring, thus an increase in the importance of legumes.
After World War II, however, farming was rapidly mechanized and traditional "legume-cereal" rotations were abandoned as relatively cheap nitrogen fertilizers became widely available and they appeared more efficient than legumes in increasing grain yields.
Another reason for the declining use of legumes in rotations was the mounting evidence that deep-rooted forage legumes frequently depressed the yields of subsequent grain crops by depleting soil moisture reserves, particularly in drier years and areas.
In recent years, the wheat-fallow rotation and fallowing, particularly mechanical fallowing, have come under increasing criticism. Fallowing contributes to increased salinity and wastes soil nitrogen and water. Mechanical fallowing is considered to be a major cause of increased soil erosion.
Figure 2. A Model Showing Past, Present and Future Soil Nitrogen Trends Source: C.A. Campbell and W. Souster, Can. J. Soil Sci. Volume 62:651 (1982)
During the first 70 to 80 years of cultivation, wheat-fallow cropping practices on the prairies have generally resulted in a loss of 40 to 50% of the organic matter in the top 6 inches of soil. Some loss has also occurred from the 6-12 inch depth. Futhermore, the ability of the soil to supply nitrogen to the crop has been degraded to an even greater degree.
Figure 2 shows the net nitrogen mineralized from the soil during an average summerfallow period in Saskatchewan. This nitrogen comes from soil humus and recently grown straw, rootlets and soil microbes. The horizontal line represents the amount of nitrogen required by a 25 bu/A (1700 kg/ha) wheat crop. When the prairie sod was broken the amount of nitrogen released during a fallow period was more than enough to grow a crop. The straw could even have been removed for use elsewhere and enough nitrogen would have been produced from just the humus to satisfy an average crop. Over time, however, tillage and erosion reduced the ability of the humus to supply nitrogen. Now, many fallow fields need nitrogen fertilizer.
Growing concerns about declining organic matter, soil fertility and rising energy and nitrogen fertilizer costs have led to renewed interest in legumes. Thus, the role of legumes as a nitrogen supplier in the rotation and as a builder of soil organic matter will likely gain importance in the future.
Biological Nitrogen Fixation
Research has shown that the biological nitrogen fixation process is the most efficient way to supply the large amounts of nitrogen needed by legumes to produce high-yielding crops with a high protein content. For the fixation process to occur, legume plants must enter into a "symbiotic" or mutually beneficial partnership with certain bacteria called rhizobia. Soon after legume seeds germinate, rhizobia present in the soil or added as seed inoculum invade the root hairs and move through an infection thread toward the root.The bacteria multiply rapidly in the root, causing the swelling of root cells to form nodules.
Nitrogen in the air of soil pores around the nodules is "fixed" by binding it to other elements, and thus, changing it into a plant available form. Some of the carbohydrates manufactured by the plant via photosynthesis are transported to the nodules where they are used as a source of energy by the rhizobia. The rhizobia also use some of the carbohydrates as a source of hydrogen in the conversion of atmospheric N (N2) to ammonia (NH3).
The amount of nitrogen fixed varies according to the legume species and variety. Within a species, the amount of nitrogen is directly related to (dry matter) yield. Most grain legumes can obtain between 50 and 80% of their total nitrogen requirements through biological fixation, but some, like fababean will fix up to 90% (Table 2).
Table 2. Nitrogen Fixation in Inoculated Legumes Grown Under Irrigation in Southern Alberta
*Determined by 15N isotope techniques.
Source: adapted from R.J. Rennie, formerly at Agriculture Canada Research Station, Lethbridge, Alberta
The potential for nitrogen fixation is directly related to rhizobia survival, the extent of effective nodulation and plant growth factors. Any adverse soil condition or environmental stress that affects plant growth is likely to slow down the nitrogen fixation process.
Nitrogen fixation is also affected by the level of available N in the soil. High soil N levels reduce N fixation because legumes will preferentially use most of the available soil N before they begin to fix atmospheric N. Nodule formation will be progressively inhibited as soil nitrate-N levels rise above about 35 lb/A and little fixation will occur with soil nitrate-N levels above 55 lb/A.
Conversely, soil N levels that are too low can also reduce plant growth. It takes approximately a month from the time of seedling emergence (or the onset of forage legume regrowth) for the nodules to form on the legume roots and begin fixing nitrogen. During this period the legume requires about 15 lb/A of N, depending on growing conditions, from other sources. Usually, this much residual soil N will be available. If not, addition of a small amount (20 to 30 lb/A) of N fertilizer placed away from the seed, may be effective. Recent research has shown starter N to be ineffective in increasing yield.
Efficiency of Nitrogen Fixation Versus Fertilization
Nitrogen fixation is very efficient in satisfying the high nitrogen requirements of legumes because the conversion of gaseous N2 to NH3takes place inside the plant. All of the fixed nitrogen is readily available and in the form required for combination with carbohydrates to produce the amino acids used for the manufacture of protein. Furthermore, since nitrogen fixation in root nodules is directly dependent on the translocation of carbohydrates from the leaves, the rate of fixation is fully " synchronized" with the rate of plant growth. This fine tuning between nitrogen supply and demand is another reason for the high efficiency of symbiotic nitrogen fixation.
By contrast, fertilization can be a somewhat less efficient way of providing nitrogen to legumes. Some of the fertilizer can be temporarily or permanently lost. To make use of the remainder, the legume plant must expend considerable energy to move the nitrogen through the cell membranes from the soil into the roots. Once the nitrogen is inside the plant, more energy is needed to convert it to a form that can be metabolized by the plant. Depending upon soil and climatic conditions, the fertilizer efficiency for legumes generally ranges form 20 to 50%.
Table 2 shows how much N is fixed symbiotically by inoculated legumes, under optimum conditions, and the equivalent value of this nitrogen as fertilizer. Table 3 shows just how little inoculation costs.
Table 3. Legume Inoculant Costs Relative To Seed and N-Fertilizer Costs in 1994
*Based on current Saskatchewan Agriculture and Food recommendations.
Source: V.O. Biederbeck, Agriculture and Agri-Food Canada Research Station, Swift Current.
Role of Legumes
Effect of Legumes on Soil Quality
Legumes have long been recognized and valued as "soil building" crops. Growing legumes improves soil quality through their beneficial effects on soil biological, chemical and physical conditions. When properly managed, legumes will:
The extent of these soil improvements depends mainly on the type of legume used, the quantity of plant material returned to the soil, and the soil and climate conditions.
Annual grain legumes (pulse crops) generally have smaller and shorter-lived effects on soil quality than perennial forage legumes. The amounts of nitrogen fixed by grain legumes and their influence on soil physical conditions are limited by their typically small and shallow root system and short growth period. As the major portion of plant nitrogen accumulates in the seed at maturity, most of the fixed nitrogen is removed from the soil with the harvest of the grain of the pulse crop. However, during the growth of grain legumes, considerable amounts of nitrogen are leaked from roots into the soil. Also, the residues from these crops have a higher nitrogen content than cereal straw and they break down more readily, releasing nitrogen into the soil. Figure 4 shows that, even in the drought-prone Brown soil zone, the growing of grain lentil in rotation with wheat has resulted in a cumulative enhancement of the soil's N-supplying power. Thus, cereal crops that follow grain legumes require less N fertilizer. Furthermore, recent research in northeastern Saskatchewan has shown that subsequent cereal crops may derive even greater benefit from the non-nitrogen benefits of pulses, such as disease suppression.
Table 4. Yield of Dry Matter and Nitrogen from Tops and Roots of Sweetclover, Alfalfa and Red Clover in the Second Year on a Degraded Black Loam at White Fox, Saskatchewan
Source: K.E. Bowren et al., Can. J. Plant Sci., Volume 49:61-68 (1969)
Legend: F= fallow; W = spring wheat; GM = sweetclover green manure; H = Alfalfa - bromegrass cut for hay. All three rotations were unfertilized. ( ) indicates the rotation-year sampled. Source: A. C.A. Campbell et al., Can. J Soil Sci. Volume 71:43-53 (1991)
Figure 5. Effect of Legume Green Manure or Legume-Grass Hay Crop on (A) Nitrogen Supplying Power and (B) Soil Organic Matter in Topsoil (0-6 in) at Indian Head, Saskatchewan.
Legend: F = Fallow; W = spring wheat; GM = sweetclover green manure; H = alfalfa-bromegrass cut for hay. ( ) indicates the rotation-year sampled. All three rotations were unfertilized. Source: C.A. Campbell et al., Can. J. Soil Sci., Volume 73:579-595 (1993)
Figure 6. Effect of Legume Green Manure and Legume-Grass Hay Crop on Surface Soil Structure as Indicated by Degree of Stable Aggregation at Indian Head, Saskatchewan.
Forage legumes are much more effective in improving soil quality because of their large and deep root system, longer growth period and greater capacity for nitrogen fixation. In the wetter areas of the province, biennial and perennial forage legumes can produce large quantities of organic matter and nitrogen in the second year after underseeding in cereals (Table 4). For maximum soils improvement, forage legumes should be managed as green manure with the entire growth being turned under prior to full bloom.
The data in Table 4 indicate that, when top growth is harvested for hay or silage and only the stubble is turned under, less than one-third of the legume dry matter and nitrogen is retained by the soil. However, even when legumes are used for hay or silage, the beneficial effects on soil quality and following crops may be substantial.
On degraded soils with typically low organic matter contents, regular green manuring with forage legumes increases soil nitrogen and organic matter over extended periods. The growing of legumes on a Gray soil in Northern Alberta increased the yield of 12 successive wheat crops over that of wheat in a non-legume rotation. The increased wheat yield during the later years was mainly due to physical subsoil improvements from the deep-rooted legume.
The main effect of turning under forage legumes as green manure is to add nitrogen-rich, readily decomposable plant material to the small mineralizable portion of soil organic matter. However, turning under fresh legumes also greatly stimulates the activity of soil microbes and, as a result, speeds up the cycling of nutrients.
Furthermore, even though most plant available phosphorus is found in the 0 to 6 inch depth and little is found below the 2-foot depth, Campbell et al. (1993, Can. J. Soil Sci.) found deep-rooted perennial legumes take up phosphorus from the subsoil. Thus, green manuring these legumes should increase the level of phosphorus in the pool of plant available nutrients.
An increase in readily decomposable or "active" soil organic matter and microbial life also improves soil structure by binding more soil particles together into aggregates and forming more pore spaces. As a result, the soil becomes more friable and less erosive (Figure 6), is easier to till and can hold more water.
Studies carried out on several soils in Saskatchewan have shown that power requirements for tillage were significantly lower on soils following a perennial legume crop than after cereal grains (Table 5). In practical terms this would mean lower energy requirements for tillage operations.
Source: P.B. Hoyt, Can. J. Soil Sci., Volume 70:109-113 (1990)
Figure 7. Increase in Yield of Continuous Wheat After Alfalfa Compared to Continous Wheat After Fallow During 15 Years at McLennan, Alberta.
Legend: F= fallow; W = spring wheat; GM = sweet clover green manure. ( ) indicates the rotation-year sampled. Source: C.A. Campbell et al., Can. J. Soil Sci., Volume 71:363-376 (1991)
Figure 8. Effect of Legume Green Manure on (A) Microbial Biomass and (B) Respiration in Topsoil at Indian Head, Saskatchewan.
Table 5. Draft Requirements for Tillage of Soils With and Without Legumes in the Rotation (measured to a depth of 6 inches using a 16 inch shovel at 3.75 mph)
*Difference not significant.
Source: adapted from M. Grevers and E. deJong, Second Annual Western Provincial Conference on Rationalization of Water and Soil Research and Management, Saskatoon (1983).
Soil surface crusting can reduce emergence of crops on certain soils. Soils with low organic matter content can develop strong surface crusts, and emergence of seedlings of crops such as canola is reduced as crust strength increases. This problem is seldom encountered on soils with high levels of organic matter.
Forage legumes can reduce salinity problems. Alfalfa, with its deep roots and high water consumption, can effectively use excess water.
Penetration by roots of perennial legumes will also improve the internal soil drainage. Thus, fields after alfalfa will drain more quickly in spring, allowing field operations to begin earlier.
Effect of Legumes on Subsequent Production
A study at Outlook with irrigated alfalfa plowed-down in late fall or early spring indicated that the following cereal crop required little nitrogen fertilizer, while the second cereal required two-thirds of its usual amount (Table 6). Although not indicated in the table, the third cereal crop would require the full recommended rate of nitrogen.
Table 6. The Effect of Alfalfa on N-Fertilizer Response of Subsequent Cereals Under Irrigation at Outlook, Saskatchewan.
Source: J.L. Henry, University of Saskatchewan, Saskatoon
Biennial legumes, like sweetclover, can also markedly increase grain production. In a long-term experiment on a thin Black soil at Indian Head, wheat yields in a 3-year rotation with sweetclover green manure were consistently higher than in a comparable rotation with fallow and similar to those of a well fertilized rotation. During the first 18 years of this study the unfertilized green manure-wheat-wheat rotation also provided, by far, the highest net income(Table 7).
Table 7. Effect of Sweetclover Green Manuring on Yields and Economics of Wheat Production at Indian Head, Saskatchewan (1960 - 1977)
Source: re Yields: R.P. Zentner et al., Can. J. Plant Sci., Volume 67: 965-982 (1987)
Annual legumes that are capable of fixing large amounts of nitrogen under good moisture conditions, can significantly improve the nitrogen supply for succeeding crops. A recent study comparing pulse-barley-wheat with barley-barley-wheat rotations during several cycles on Black and gray soils in northeastern Saskatchewan found that fababean, field pea and lentil all improved subsequent cereal quality and gave, on average, a 21% higher barley yield in the first year and a 12% higher wheat yield in the second year. shows the yield response of barley to N fertilizer was slightly greater on barley than on pulse residues. However, the yield curves emphasize that fertilizer alone, even at rates up to 180 lb N/A, was unable to bring barley yields on barley residue up to the maximum yield obtained on pulse residues. This confirms that benefits from use crops are not only due to the added nitrogen they provide to succeeding cereal crops but also to positive `rotational effects' due to disease suppression, improved tilth and other enhancements of soil quality.
Source: A.T. Wright, Can. J. Plant Sci., Volume 70:1023-1032 (1990)
Figure 9. Average Yield Response of Barley to N Fertilizer When Grown on Barley, Fababean, Field Pea and Lentil Residues in Northeastern Saskatchewan.
Legumes and Green Manuring
Legume green manures offer several advantages over conventional summerfallow as they tend to improve, enrich and protect the soil. Sweetclover green manure can be ineffective fallow replacement on Dark Brown soils, provided it is turned under early in the second year to reduce moisture depletion. In the more drought-prone Brown soil zone, however, deep-rooted biennial and perennial legumes are not suitable for green manuring, as their excessive soil moisture depletion will depress the yield of subsequent wheat crops for several years.
The recent introduction and evaluation of high nitrogen fixing and water use-efficient annual legumes has made it feasible to develop a legume green manure system that is more compatible with the short cereal rotations commonly used on Brown soils. In a study on a Brown loam, 4 annual legumes were seeded into wheat stubble with tall stubble strips for overwinter snow trapping. At bloom they were either disced under or desiccated. Figure 10 shows some of the fertility benefits only 3 months after incorporation of the green manures.
Table 8. Effect of Moisture Conditions and Management of Annual Legume Green Manures on Subsequent Wheat Yields at Swift Current, Saskatchewan
Source: V.O. Biederbeck, Agriculture and Agri-Food Canada, Research Centre, Swift Current, SK.
Figure 10. Effect of Green Manures on Nitrogen Mineralized in Top 2 Feet of a Brown Loam Between Legume Incorporation and Late Fall at Swift Current, Saskatchewan (Average for 1984-1989).
The yield of wheat after these green manures was greatly affected by legume management but not by the type of annual legume. Average grain production after inoculated and disced-in green manure was 12% greater than on conventional fallow and 17% greater than after chemically desiccated green manure (Table 8). The importance of snow management to cereal production on Brown soils is emphasized by the 20% yield advantage of wheat grown on fallow with trap strips compared to wheat grown on conventional fallow. Wheat yields after legumes grown to maturity and also after uninoculated disced-in green manure were generally as low as those of well-fertilized continuous wheat because of increased moisture depletion and nitrogen deficiency. The results from this and other recent green manure studies indicate that fallow replacement with annual legume green manure is feasible in the Brown and Dark Brown soil zones, but only in combination with some method of snow management and early incorporation of the legume, to enhance soil water recharge.
Annual legumes can also reduce soil erosion by improving aggregate stability. However, do not over-incorporate these crops. Severe instances of erosion have occurred after over-incorporation. Leave at least 30% of the topgrowth on the surface for soil protection.
Legend: F = fallow; W = spring wheat; Len = Lentil. ( ) indicates the rotation-year sampled. Source: C.A. Campbell et al., Can. J. Soil Sci., Volume 73:597-612 (1993)
Figure 11. Effect of Grain Legume on (A) Wind Erodibility and (B) Resistance to Water Erosion in Surface Soil (0-2 in) at Swift Current, SK.
Agronomy and Management
Annual Grain Legumes
Annual grain legumes are normally grown for grain production although some producers do use them as green manure corps. Choosing which grain legume (i.e. pea or lentil) and which variety of the legume to grow usually depends on anticipated market price for the crop, adaptability of the crop to that area (e.g. lentil and chickpea are more drought-tolerant than pea and fababean), agronomic factors such as disease resistance, and the availability of specialized equipment.
Growing annual grain legumes can increase the yield of succeeding crops in the rotation. This benefit, called rotation effect, is due to more than an increase in high-N crop residue. For example, a pea crop that yielded 2000 lb seed per acre would produce about 3000 lb of crop residues containing 1% nitrogen, or about 30 lb N/A, about half of which would be available to the succeeding crop. However, crops grown after annual legumes yield more than can be attributed to an additional 15 lb N/A (Figure 9). Thus, the rotation effect has been attributed to improved physical, chemical and biological characteristics of the soil, after resulting in reduced duration and severity of attacks by diseases and insect pests.
Seed costs of most annual legumes are prohibitively high for their widespread use as a green manuring crop. However, seed costs can be reduced somewhat by using the small-seeded pea cultivar Trapper with a seeding rate of 110 lb/A. The full nitrogen supplying potential of an annual legume can be harnessed by partially incorporating the crop at the first pod stage. At this time the nitrogen concentration is at its highest (nearly 3%) and the top growth is succulent and decomposes rapidly, resulting in a flush of nitrogen mineralization. Partial incorporation helps reduce volatilization losses of nitrogen directly from the decomposing plant material.
Annual grain legumes are fairly tolerant of early spring frosts and thus can be seeded early. A severe frost may destroy the top growth, but regrowth will occur from one of the scale nodes at or below the soil surface. Pea, lentil and fababean should be sown into firm, moist soil, at a depth of 1 to 3 inches to get below dry surface soil.
Good soil fertility is required to achieve high yield and protein content. Placing phosphate fertilizer with or near the seed is particularly important due to the "pop up"" effect which results in a more vigorous seedling better able to compete with the weeds. Only a limited amount of phosphate fertilizer can be placed in a narrow band with the seed. Surpassing safe rates can lead to germination and seedling damage. For more information on seed-placed fertilizer see the FarmFacts bulletin entitled Guidelines for safe rates of fertilizer applied with the seed.
Legumes preferentially use available soil nitrogen rather than fix atmospheric nitrogen. Thus, high levels of available soil nitrogen (about 55 lb/A) will greatly reduce the amount of nitrogen fixed by the legume. However, in low nitrogen soils (less than about 15 lb/A) a low rate of starter nitrogen (20 to 30 lb N/A) placed away from the seed may boost seedling growth of the legume prior to the establishment of fully functioning nodules. Recent research has shown starter N to be ineffective in increasing yield.
Biennial Forage Legumes
The only biennial forage legume grown on the Canadian prairies is sweetclover. Sweetclover is an upright, broadleaved legume with many stems and branches. In the seeding year plants develop to a height of 12-36 inches. In the second year, flowers are produced and the crop grows 4-5 feet tall at maturity. The two common types are yellow-flowered and white-flowered.
The yellow-flowered type is preferred by farmers. It is more drought-tolerant, shorter, and finer stemmed and leaved. These characteristics make it a more palatable livestock feed and easier to incorporate as green manure.
The yellow-flowered type also grows more rapidly early in the spring and can be harvested or incorporated earlier. This allows a longer period for recovery of soil moisture reserves. Where sweetclover is grown for feed, the variety Norgold should be grown. Norgold is a yellow-flowered variety that produces forage with a low coumarin content. Low coumarin sweetclover poses no danger of causing "sweetclover or bleeding disease" of livestock.
Sweetclover and most other forage legumes have small seeds, thus they will only emerge from shallow depth (less than 1 inch). Any preseeding tillage should be done as shallow as possible to conserve moisture near the surface and provide a firm seedbed. Economic conditions often dictate that the forage legume is seeded with a companion crop. The seeding rate of the companion crop should be reduced to 1/2 to 1/3 or less of the normal rate. The companion crop should be sown first and the soil firmed with harrows and/or packers. The forage legume should then be sown (preferably) at right angles to the companion crop rows to reduce competition. The legume should be sown within a few days of the companion crop to improve its ability to compete and survive with the companion crop.
Cereals are the most suitable companion crops. Limited research with canola indicates that poor stands result, probably due to shading of the legume by the large canola leaves. Use of flax as a companion crop usually results in weedy stands, as flax is not very competitive. Barley is very competitive with forage legumes and is not generally recommended as a companion crop. however, it matures earlier than other spring cereals and, in wetter zones, the legume may have an opportunity to become established after harvest. Wheat and oat are less competitive and alter maturing than barley. They are more useful as companion crop in the Brown and Dark Brown soil zones where rainfall is frequently inadequate for significant fall growth. Where the companion crop can be used as green feed, oat is preferred. Early removal of the companion crop as forage is beneficial for establishment of a forage legume, particularly if the crop is weedy or suffers from lack of moisture. Letting the companion crop mature for grain significantly reduces the seedling vigor of the forage for 1 to 3 years after establishment.
Seed sweetclover as early as possible in the spring. Early seeding takes advantage of favorable moisture conditions and allows the sweetclover seedlings to emerge and become established before weed growth begins. Fall seeding of sweetclover is not recommended. Inoculate the seed immediately prior to seeding with the proper inoculant to ensure optimum nitrogen fixation.
Weed Control and Fertilizer
Refer to the latest "Guide to Crop Protection" for herbicies registered for use in sweeclover.. Production of sweetclover should be planned well in advance to minimize weed populations prior to seeding. Several practices which have proven useful in reducing weed populations and competition are as follows.
An alternate method for use on stubble is to spray winter annual and biennial weeds in the fall and then spray a "burn off" herbicide and zero till seed the sweetclover and companion crop directly into the undisturbed stubble in spring.
Wild oat and green foxtail can be controlled with herbicides, however, the herbicide selected must be compatible with both the companion crop and sweetclover. Refer to the latest Guide to Crop Protection Guide.
Fertilizer applications should be based on soil test results. Where required, phosphorus, potassium and sulphur should be applied. Proper use of fertilizer aids in establishment of vigorous competitive stands of sweetclover, and may contribute to increased yields of the succeeding crop(s). Sulphur deficiency can severely depress sweetclover growth. Because the crop is a biennial, some consideration should be given to supplying enough of these nutrients to also meet the second year's needs. Sweetclover obtains its nitrogen requirements by nitrogen fixation in the root nodules. Where the crop is grown on stubble with a companion crop, it may be desirable to use some nitrogen fertilizer for the companion crop. However, high rates of nitrogen should be avoided, as this will reduce nitrogen fixation.
The sweetclover weevil can cause significant damage significantly in an area. This crop and the pest becomes more abundant whenever sweetclover acreage is increased. Refer to the Guide to Crop Protection for registered insecticides and their use.
Use of Sweetclover
Sweetclover is one of the most suitable crops for use as a green manure. In the second year it grows rapidly and can be incorporated early. Incorporation should be done at the bud stage, as most of the N fixation has occurred by this time (Table 4 and Figure 12). This allows time for recovery of soil moisture reserves and residue decomposition during the partial fallow period. Later incorporation should only be considered in cases where it is desirable to incorporate the maximum amount of organic matter. Examples of such cases are on moderately saline soils or soils that have poor structure due to very low levels of organic matter. However, excessive moisture depletion by alter growth may result in slower decomposition of the green manure and low moisture reserves for the succeeding crop.
Sweetclover is most beneficial on Gray Wooded soils. Where sweetclover is growth as a regular part of rotations on such soils, succeeding grain crop yields are similar whether the crop is used as forage or as a green manure. Where it is grown less frequently it may be more valuable as green manure than as hay. Where it is used as hay, care in harvesting is required to minimize leaf losses.
Sweetclover production for seed usually increases the cash value of the crop. However, seed prices can fluctuate widely from year to year. Sweetclover grown for seed reduces moisture available to succeeding crops, compared with sue as green manure or forage. Soil moisture recovery can be enhanced by leaving stubble as tall as possible or by leaving strips of standing crop to trap snow.
Much of the nitrogen from the crop is removed with the seed, therefore reducing the amount available to succeeding crops.
Figure 12. Average Nitrogen (kg/ha) in Tops and Roots of Three Legumes at Five Growth Stages on Black and Degraded Black Soils of Northeastern Saskatchewan.
Sweetclover fits well into short rotations with grain crops because it is a biennial. It is best adapted to use on problem soils such as degraded soils low in organic matter, soils where crusting is a problem or on saline soils. In many cases it fits well into rotations as a substitute for summerfallow.
A cereal crop should be grown following sweetclover. Oilseeds do not respond as well as cereals when grown immediately after sweetclover (Table 9). When grown as a second crop after sweetclover, oilseeds frequently show yield responses on degraded, low organic matter soils or those that crust.
Table 9. Yield Responses (lb/A) of Wheat and Canola Grown After Fallow
*N Fertilizer 32 lb/A on average.
Figure 13. Sweetclover Snow Trap Strips in the Southern Prairies
Sweetclover may be substituted for fallow on more productive soils. Most soils will benefit from the additional organic matter and nitrogen added. However, it is difficult to document yield responses by crops following sweetclover on such soils. Frequently yield depression occurs in crops following sweetclover compared to those grown on fallow. This is particularly so in drier areas and in dry years. Such losses may be recovered in improved yields in other years of the rotation or reduced costs for nitrogen fertilizers.
Leaving strips of sweetclover standing over winter will trap snow (Figure 13) and help soil moisture reserves to recover, however, they may cause problems tilling the field the following spring.
Perennial Forage Legumes
When perennial legumes are included in rotations, they fix nitrogen and add humus and nutrients to the soil. The choice of perennial legume will depend mostly on the soil zone and intended use of the crop.
Alfalfa is a widely adapted high yielding forage legume. It has good drought tolerance, moderate salinity and flooding tolerance and is winter hardy. Alfalfa will yield 1335-2225 lb/A per cut and two or more cuts are available when moisture supply is good. Sainfoin and birdsfoot trefoil are bloat-free alternatives. Sainfoin is short-lived and not very drought tolerant and seed costs are generally high. Birdsfoot trefoil is lower yielding than alfalfa and not widely grown in Saskatchewan. Alsike or red clover are best suited to the Parkland or acidic soils, but will produce less dry matter than alfalfa.
Alfalfa is the main perennial legume grown in western Canada. It is grown for hay, pasture, seed and in some areas as a major crop for the dehy industry. Alfalfa is adapted to a wide range of soil and climatic conditions. Production and persistence is favoured when alfalfa is grown on neutral to slightly alkaline soils and is severely limited by acid conditions (i.e., pH less than 6.0). Alfalfa growth is best on well-drained soils. It is intolerant of flooding and does poorly on soils with inadequate internal drainage. However, alfalfa is quite drought tolerant due to its deep root system. As with other perennial legumes, the best currently available varieties of alfalfa are listed in the latest Saskatchewan Forage Crop Production Guide. Alfalfa varieties are often characterized according to the nature of their root system, tap rooted or creeping rooted. Creeping rooted plants develop horizontal roots from the tap root which are capable of giving rise to independent plants. Creeping rooted varieties are generally more persistent, stress tolerant and grazing tolerant than other types of alfalfa. However, creeping rooted varieties have slower regrowth potential after harvest as compared to tap rooted types.
Red clover is another common perennial legume grown for feed and seed. It is shorter-lived than alfalfa and fits well into short-term rotations. It is adapted to a wide range of soils in the moister areas of the province and is more tolerant to acidic soils than is alfalfa. Red clover is not tolerant of salinity or extended periods of drought.
Other perennial legumes are grown to a limited extent in Saskatchewan. Alsike clover is a short-lived perennial adapted to low lying moist areas. It withstands considerable spring flooding and has the capacity to propagate itself from seed. It is well suited to acidic, organic soil. Birdsfoot trefoil is a potentially long-lived perennial forage which is very tolerant of waterlogged soils and can withstand several weeks of flooding and some acidity. It is not adapted to dryland areas. Although it can be used for hay in wetter areas, it is more commonly used as a pasture species because it does not cause bloat. Birdsfoot trefoil is very sensitive to competition, particularly during establishment. It should be sown either in monoculture or in mixtures with nonaggressive grass species. Sainfoin is another potentially long-lived perennial forage which does not cause bloat when grazed. In very dry areas it yields poorly, so is best adapted to the Dark Brown and Black soil zones. Sainfoin requires good drainage and is intolerant of flooding or waterlogging. It is also intolerant of acidic or saline soils.
The recommended varieties of perennial legumes, seeding practices and weed control are discussed in the latest Saskatchewan Forage Crop Production Guide.
To establish a good stand, perennial legumes should be inoculated with the appropriate inoculant and seeded shallow (1/3 to 3/4 of an inch) deep, into a firm seedbed. Provided there is sufficient soil moisture available, perennial legumes can be seeded in the spring until mid-June. They can also be seeded in fall just prior to freeze-up (after mid-October), but higher seeding rates may be required. Early spring seeding is preferred and may allow a light hay or silage crop harvest late in the establishment year. Seeding companion crops with perennial legumes usually reduces the subsequent forage yield, particularly when moisture is limiting. Decreasing the seeding rate of companion crops (e.g. to 1/2 to 1/3 or less of normal) and seeding the crops at right angles or in alternate rows will reduce competition. Companion crops are best removed early as hay or silage, leaving a tall stubble (6-8 inches) for snow trapping. Seeding companion crops with perennial legumes in the Brown soil zone may reduce the chance of obtaining a productive forage stand.
Figure 14. Comparison of Beneficial Effect of Alfalfa with Effect of N-fertilization on Grass yield on a Brown Loam Source: A.J. Leyshon, Agriculture and Agri-Food Canada, Swift Current
Weed Control and Fertilizer
The land for perennial legumes should be relatively weed-free (particularly of perennial weeds). Many weeds can be controlled with herbicides in both the seedling and established stands (refer to the latest Crop Protection Guide). The fertilizer used with the legume should be based on soil tests and expected production requirements. For example, the production of 2.5 ton/A of alfalfa will use approximately 35 lb/A of phosphate (P2O5), 150 lb/A of potassium (K2) and 15 lb/A of sulphur (S). Note that alfalfa is a high sulphur using crop and is particularly sensitive to sulphur deficiency.
In alfalfa-grass stands, nitrogen fertilizer is not needed as long as alfalfa makes up at least 25% of the stand. Grass roots intermingle with the alfalfa roots and can use the nitrogen that leaks from alfalfa roots and nodules (Figure 14).
Perennial Forages in the Rotation
If adequately inoculated, the legume will fix nitrogen. In a five year study at Melfort and White Fox, alfalfa accumulated about 90 lb/A of nitrogen by the bloom stage in the second year of growth (Table 4). In contrast to sweetclover, alfalfa and red clover had a greater proportion of nitrogen stored in the roots in the second year, and removing a crop still provided for the return of a substantial amount of nitrogen to the soil (Figure 12). In addition to nitrogen, perennial legumes add a considerable amount of organic matter to the soil. By the bloom stage in the year after seeding, alfalfa at Melfort and white Fox had accumulated approximately 1400 lb/A of dry matter in the top 10 inches of soil.
Over a 24 year period form 1956-1982, growing an alfalfa-grass mixture for 2 years in a 6 year cereal-forage rotation on a Gray-Black soil at Somme, Saskatchewan, increased the yield of grain on fallow and stubble by 8% and 15%, respectively. On a Black soil at Melfort, grain yields increased by less than 1%. However, the amount of nitrate-nitrogen in the soil in the fall was increased by 25-50% as compared to a straight grain cropping sequence. As a result, the protein content of grain grown in the grain-forage rotation was about one percentage point higher than the protein of grain produced in a straight grain cropping sequence. Other studies report similar effects of growing legumes in rotations.
Due to the cost of seeding perennial legumes, they fit best in rotations where they can be left down for three or more years and utilized for feed or seed. They are seldom used as a short-term plough-down crop. When utilized for hay, seed or dehy they can be profitable crops that improve the productive capacity of the soil. Substantial benefit will result, if properly fertilized perennial legumes are grown on degraded soils low in organic matter and on soils that tend to crust.
In working up a legume stand that has been established under dryland conditions for a few years, several weeks should be allowed for a partial fallow to kill the legume plants, decompose the sod and replenish the soil moisture reserves for the succeeding crop. The residue of red and alsike clovers breaks down quicker and the field is somewhat easier to prepare for cereal production than after an alfalfa stand.
On irrigated land at Outlook, growing a pure stand of alfalfa for 4 years added a considerable amount of nitrogen to the soil (Table 6). With irrigation, moisture is not a critical factor in future cropping and alfalfa could be worked up in fall and the land seeded to an irrigated grain crop the following spring.
Figure 3. Active Nodules on roots of Alfalfa (A), Red Clover (C), Fababean (F) and Pea (P) Source: Microbiology Laboratory, Agriculture Canada Research Station, Swift Current
Inoculation refers to the introduction of Rhizobium bacteria into the soil so that root hairs of seedlings will form nodules (Figure 3) that enable the legume to fix atmospheric nitrogen. The term `nodulation' is used to describe nodule formation.
The Need for Inoculation
Legume growers should look upon proper seed inoculation with high quality commercial inoculants as a very economical and generally effective means of optimizing crop production (see the section entitled Nitrogen Fixation in this bulletin). Many soils lack sufficient numbers of the specific rhizobia needed for growth and high yields of forage and grain legumes. The rhizobia that cause nodulation in alfalfa and clovers do occur naturally in most Prairie soils. However, some strains of the native soil population infect the roots but are not able to fix nitrogen, while other native strains fix nitrogen but often not as efficiently as the specially selected strains used in commercial inoculants. Inoculation corrects these deficiencies by sticking thousands of highly effective nitrogen fixing rhizobia to each seed immediately before planting.
Inoculant Types and Quality
An inoculant is made up of one or more strains of a Rhizobium species in a carrier material that supports and protects the live bacteria. The most common carrier is finely-ground peat moss with some lime added to prevent acidity. The peat has a very high water holding and buffering capacity which protects the rhizobia against unfavorable soil conditions. It is preferable that the carrier material be sterilized before adding the rhizobial cultures, otherwise the resultant inoculant products will likely contain more contaminant bacteria than rhizobia.
Inoculants are sold in Canada in three types of product formulations for on-farm use:
Commercially pre-inoculated forage legume seed has become widely available on the seed market. It is convenient as its use eliminates the extra time and effort required to apply inoculants on the farm, but it is more expensive and has a major disadvantage in that the viability of rhizobia decreases rapidly once they are applied to the seed. However, the quality of some preinoculated seed products, particularly coated seed, is now generally sufficient for adequate nodulation.
Figure 15. Performance Record of Inoculant Products in Canadian Quality Control Testing (1984-1993) Source: V.O. Biederbeck, Agriculture and Agri-Food Canada, Swift Current
All inoculants sold in Canada are registered under the Fertilizers Act administered by Agriculture and Agri-Food Canada. Inoculant products are regularly sampled by federal inspectors and tested by analysts from Agriculture and Agri-Food Canada to ensure that they meet rigid quality standards. Since its development in 1975 this control program has led to a marked improvement in the quality of commercial inoculants being marketed in Canada. As shown in Figure 15, the quality of inoculants has been consistently high while that of preinoculated seed products has been high since 1991. Each year Agriculture and Agri-Food Canada publishes the results of all regulatory testing and product rating in the Canadian Legume Inoculant and Preinoculated Seed Product Testing Report. Copies of the report are available from:
The inoculant package label contains the following information: legume types the inoculant is suitable for, name (species) of Rhizobium bacterium, quantity of seeds the package will inoculate, manufacturers lot number, federal registration number, expiry date, and directions for application. This ensures that good nodulation and nitrogen fixation will result if the inoculant is properly stored and correctly applied to the seed. Inoculants are live cultures of bacteria and should be kept refrigerated or at least cool, moist and out of direct sunlight.
Culture of several species of Rhizobium bacteria are available. For example, the species that nodulated alfalfa and sweetclover will not function on the clovers, nor on pea, lentil, bean, and other legumes. Thus each legume or group of legumes requires a unique species of Rhizobium (R) to form nodules and fix nitrogen. The commercial inoculants available are prepared for specific legumes and groups of similar legumes.
Inoculation with the proper inoculant is essential for good nodulation and high nitrogen fixation. Inoculants available for annual legumes, such as pea or lentil, contain either mixed or single strains of the same Rhizobium species. Mixed strains means the product contains several strains suitable for the crops listed on the label (e.g. pea and lentil).l These products tend to be lower in cost than single strain inoculants. Single strain products contain Rhizobium specific for one crop (e.g. lentil). Single strain inoculants can provide more efficient nitrogen fixation. Most legume inoculants sold in Canada now consist of single strains, with the strain selected in Canada.
Inoculation Using Powdered Inoculants
To be effective a powdered peat or clay-based inoculant must adhere to the seed to ensure that the rhizobia are close to the newly emerging roots. The old practice of dry application, i.e. dumping the inoculant powder onto the seed in the drill box and stirring the seed with a stick, is not recommended because most of the inoculant is wasted and various tests have shown that this method of application is not effective. Two methods are commonly used for applying powdered inoculants.
Water-Slurry Method - the inoculant is suspended in water and then mixed thoroughly with the seed until each seed is coated uniformly with the inoculant powder. Although water application is preferable to applying dry powder to the seed, it is not as effective as the sticker solution method.
Sticker Solution Method- this is the most effective method because it `glues' the inoculant to the seed. The adhesive material or inoculant sticker also serves to feed the rhizobia and to protect them from drying conditions on the seed. Several sticker products are available commercially from the companies that manufacture inoculants. Suitable stickers can also be prepared on the farm by making a 10% solution of corn syrup, table sugar or honey in water (Table 10). Powdered milk is also an effective adhesive agent, however, do not use milk replacer for livestock that contains antibiotics.
Table 10. Effect of Adhesive Agents on yield of King Grain Line X005 Soybeans.
Source: M.S. Elegba and R.J. Rennie, Can. J. Soil Sci., Volume 64:631-636 (1984)
Seed of forages and other small-seed legumes should be placed in a large container (cement mixer, tub, pails,) and sufficient sticker applied to slightly wet all seeds. Then half of the required amount of inoculant powder should be sprinkled on the seeds while mixing until the seeds are uniformly coated. To eliminate the need for spreading out and drying the inoculated seed and to avoid clogging of the seeder, the other half of the required amount of inoculant powder is then added to the partially inoculated seed and mixed thoroughly in the container. The fully inoculated seed can then be planted with normal seeding equipment. This type of seed inoculation may be done one to two days before the actual seeding date but only if the inoculated seeds can be stored in a cool place. However, prompt seeding of freshly inoculated seed is preferable. Detailed instructions are given on inoculant and sticker package labels.
Large-seed legumes, such as pea or lentil, can be effectively inoculated by metering and dribbling an inoculant sticking agent slurry into the auger intake as the seed is being augured. Seed inoculated by this method should be allowed to dry for a few hours in a cool, shaded area prior to seeding, in order to prevent bridging in the seed box.
There are now also several self-sticking inoculants on the market to make on-farm seed inoculation simpler and faster.
Other Seed Treatments
Legume seed treated with fungicides or insecticides requires special consideration because many chemical seed disinfectants are toxic to rhizobia. However, the fungicide Thiram, used for prevention of Certicillium wilt of alfalfa, has no negative effect on rhizobia survival. The seed treatment "Crown", registered for control of seed-borne Acochyta in lentil, and nitrogen fixing inoculant can be applied at the same time as long as the two are not tank-mixed before application. Read and follow label instructions.
When legume seeds must be treated with pesticides known to be toxic to bacteria then direct inoculation of the seed should be avoided and granular inoculant should be drilled into the seed furrow. Consult the labels of all pesticides to ensure they are compatible with the rhizobial inoculation of legumes.
Seed treatment with "JumpStart®" is fully compatible with rhizobial inoculants. Very little fertilizer can safely come in direct contact with inoculated legume seeds because the frequently low pH and high ion concentration in the soil solution around the fertilizer granules will kill the rhizobia, the seed and/or the seedling.
If legumes are being planted under any of the previously mentioned soil or chemical stress conditions, it is recommended that 2 to 4 times the usual amount of inoculant be applied.
How to Check for Nitrogem Fixation
To determine if nitrogen fixation is occurring, dig (don't pull) the roots of several plants at different spots in a field and examine the crown region for clusters of nodules. Slice open several nodules from each plant and if they are pink to beef steak-red on the inside (figure 16), then they are effective nodules. The red colour is due to leghemoglobin, an iron-containing pigment associated with active nitrogen fixation. The number of nodules and the rate of nitrogen fixation will increase with time after emergence and normally reaches a maximum just before the legume blooms.
The potential for nitrogen fixation is closely related to plant growth factors; thus any condition, stress or management practice that affects plant growth is likely to affect nodulation and nitrogen fixation.
Nodulation will be reduced in acid soils because most rhizobia are very sensitive to a pH below 5.5 and may die before nodulating the plants. Lime should be applied before seeding legumes where soil pH is low. Calcium, magesium and molybdenum are also very important in the nodulation and nitrogen fixation processes. There are special, commercially available, low-pH tolerant rhizobia strains in the alfalfa group which have been selected and tested in western Canada for their ability to survive and nodulate in acid soils.
High rates of available nitrogen can substantially reduce nodule formation and inhibit nitrogen fixation. Excessive rates of N fertilizer applied to legume-grass mixtures tend to reduce the legume component because of strong competition from the grass for light, water and nutrients.
Rhizobia do not survive well in dry or very warm soil. The first few days following planting are the most critical. Drying of the soil surface and/or soil temperatures above 20°C at seeding time will kill many bacteria and greatly decrease legume nodulation. Thus inoculated seed should be sown into a moist seedbed on cool days, if possible.
Figure 16. Interior of Effective Alfalfa Nodules
Legume crops encompass a wide range of species and associated seed sizes. Tthe objective in planting any crop is to place the seed at the optimum depth at a seeding rate which will provide the desired plant density. Seeding equipment must therefore be able to accurately meter seed of very different sizes and place seed at various specific depths. The seed should be placed in a seedbed which is sufficiently firm so that adequate contact exists between the seed and surrounding soil to enable germination and rooting to occur. The variation of seed size among crops in the legume family is outlined in Table 11.
Table 11. Legume Seed Characteristics
Some types of seeding equipment are able to successfully provide the seed metering and depth control requirements of all species within the legume family, while other implements are best suited for seeding only some of the legume crops.
Most of the annual grain legumes have a large seed size which can be handled by most commercially available seeding equipment. Of concern, however, is that damage to the seed (which results in germination problems) can occur during metering or delivery of the seed to the openers. Damage is possible during see metering with some drills, discers and air seeders and damage is possible during delivery in all air seeders with excessive fan speed and airstream velocity. More information on preventing seed damage during seeding can be obtained from Prairie Agricultural Machinery Institute (PAMI) in Humbolt.
Forage legume seed is relatively small and the seeding rate relatively low, and difficulty may be experienced metering at a low enough rate. On some equipment, a smaller diameter fluted feed sprocket may be necessary to lower the feeding rate enough when the drive shaft speed is unchanged. A small seed attachement (commonly called a "grass box") will provide this capability. Alternatively, a metering shaft gear reduction system can be used.
Forage legumes should be seeded shallow (1/3 to 3/4 of an inch) into firm, moist soil. this can be accomplished by direct seeding into untilled land (ensure hair-pinning is not a factor) or by using seeding equipment with depth control devices on tilled land. Forage stands can be established by broadcasting and harrowing in the seed on tilled land provided the seed is covered by enough soil and rainfall is received shortly after seeding.
An additional requirement when seeding grain legume crops on stony land is a land roller. Harvesting of pulse crops such as lentils requires cutting very close to the ground surface. Unless rocks have been pressed into the ground with a land roller, damage will occur to harvesting equipment and seed will be contaminated with foreign material, thereby downgrading it. Lentil should be rolled between seeding and up to the 5th node stage. Pea should be rolled between seeding and the 5th leaf stage.
Most harvesting equipment for grain production will also handle annual grain legumes, but in some cases, modifications will improve harvesting efficiency. Some commonly used modifications include floating cutter bars and pick-up reels on swathers or combine headers. Combine cylinder/rotor speed and concave clearance can usually be sufficiently adjusted to enable threshing of grain legumes under a wide range of moisture conditions without excessive seed damage or grain loss.
Annual grain legumes are much more susceptible to damage than most other grains. Therefore, it is important to use conveying equipment that minimizes damage to these high value crops. Prairie Agricultural Machinery Institute (PAMI) has completed a study on conveying equipment.
To enable dry-down pf pulse crops with indeterminate growth, desiccants are often sprayed on the standing crop after seed filling is complete. This requires spraying equipment which will minimize field damage from wheel tracks. Refer to the "Guide to Crop Protection" for registered desiccants.
Although legumes generally require a higher degree of management than cereals, with the proper inoculation and agronomic practice, they will improve soil quality.
Imperial - Metric Conersions
Some Legumes Grown in the Prairie Provinces
Produced by the Canada-Saskatchewan Agreement on Soil Conservation
This bulletin has been approved by the Saskatchewan Soil and Crop Management Subcouncil.
For further information contact the Agriculture Knowledge Centre at 1-866-457-2377.