Improving Soils With Legumes
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
|Annuals||chickpea, fababean, dry bean, field pea, lentil, lupin|
|Biennials||white blossom sweetclover, yellow blossom sweetclover|
|Perennials||alfalfa, alsike clover, birdsfoot trefoil, red clover, sainfoin|
Old and New Interest in Legumes
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
|Legume||Plant-N Derived from Atmosphere* (%)||N Fixed Symbiotically (lb/A)||N-Fertilizer Equivalent** ($/A)|
*Determined by 15N isotope techniques.
**1994 fertilizer prices.
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
|Legume||Average Seeding Rate * (lb/A)||Inoculant Cost**|
|($/A)||As a Proportion of Seed Cost (%)||As Fertilizer Equyivalent (lb N/A)|
*Based on current Saskatchewan Agriculture and Food recommendations.
**Average cost of self-sticking inoculant or inoculant plus sticker.
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:
- enhance the N-supplying power of soils
- increase the soil reserves of organic matter
- stimulate soil biological activity
- improve soil structure
- reduce soil erosion by wind and water
- increase soil aeration
- improve soil water-holding capacity
- make the soil easier to till
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
|Growth Stage and Date
|Legume||Dry Matter (lb/A)||Nitrogen in Plant (lb/A)|
|Early Bud (June 15)||Sweetclover||2029||552||2581||59||9||68|
|Full Bloom (July 15)||Sweetclover||4299||810||5109||74||10||84|
Source: K.E. Bowren et al., Can. J. Plant Sci., Volume 49:61-68 (1969)
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.
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.
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).
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.
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.
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.
Agronomy and Management
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.
- Reduce weed populations in preceding crops through selection of crops and herbicide use.
- Spray stubble land the preceding fall for control of winter annuals.
- Seed relatively weed-free fields.
- Conserve moisture by keeping any pre-seeding tillage shallow and by
- seeding as early as possible in the spring (immediately after preseeding
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.
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.
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.
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.
Produced by the Canada-Saskatchewan Agreement on Soil Conservation
B.J. Green (Saskatchewan Agriculture and Food, Regina)
V.O. Biederbeck (Agriculture and Agri-Food Canada, Swift Current)
V.O. Biederbeck (Agriculture and Agri-Food Canada, Swift Current)
H.A. Bjorge (Saskatchewan Agriculture and Food, Lloydminster)
S.A. Brandt (Agriculture and Agri-Food Canada, Scott)
J.L. Henry (University of Saskatchewan, Saskatoon)
G.E. Hultgreen (Prairie Agricultural Machinery Institute, Humboldt)
G.A Kielly (Agriculture and Agri-Food Canada, Swift Current)
A.E. Slindard (University of Saskatchewan, Saskatoon)
Members of the Saskatchewan Soil and Crop Management Subcouncil
R.W. McVicar (Saskatchewan Agriculture and Food, Regina)
M.E. Tremblay (Saskatchewan Agriculture and Food, Regina)
© 2015, Government of Saskatchewan
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