Does a legume green manure crop provide a "free" source of soil N?
Nitrogen fixed in a green manure crop is not a "free" source of additional N, but it can be an effective option for some cropping systems, such as organic crop production. There is a cost to buy the seed, inoculate, and plant it, and a cost to terminate the green manure crop at the right stage. There is also the added cost of not growing a marketable crop in that year, and greater depletion of soil moisture reserves in drier areas, compared to tillage or chemical fallow options.
Advantages of a legume green manure crop
There are many advantages to using a legume green manure crop in rotation, which include:
- N fixation - the actual amount of atmospheric N that is fixed and becomes available to subsequent crops depends on environmental conditions, soil fertility (other than nitrogen) and overall crop health. As a rule of thumb, background soil N levels greater than 30 lb./ac. can delay or reduce N fixation. Legumes must be properly inoculated with the correct strain of Rhizobium spp.
- Disease control - legume green manures provide a break in cereal and oilseed crop rotations to help minimize disease pressure.
- Erosion control - fallow operations, especially tillage, can leave the soil exposed to wind and water erosion. Legume green manure crops provide cover, and promote soil retention by helping to build soil structure.
- Later seeding - green manure crops are not grown to full maturity, and later seeding dates will not affect N fixation negatively unless soil moisture becomes too low. In areas where soil moisture is limiting, or where limited soil moisture storage may become an issue, legume green manure crops should be seeded early, and terminated before the end of June, to allow time for soil moisture recharge.
- Increased soil aggregation - legume crops help "build" soil structure over time, which increases aeration, water infiltration and root growth, and helps decrease the risk of soil erosion.
- Green manure crops remove carbon dioxide from the atmosphere and helps build soil organic matter after termination.
Disadvantages of a legume green manure crop
There are also disadvantages to using legume green manure crops, which include:
- Moisture use - where moisture is limiting, green manure crops can
utilize moisture that may otherwise be conserved during fallow. If moisture is not limiting, moisture uptake by green manure crops is less than moisture uptake in crops grown to maturity.
- Establishment costs - a marketable crop is not achieved with green
manure crops. Cost of producing a green manure crop should not exceed potential soil and N benefits.
- Rotation limitations - a green manure crop is another legume in the
crop rotation. To minimize disease problems, the use of other legume crops for grain production may need to be restricted.
In Saskatchewan, the most commonly grown legume green manure crops are:
- Annual: field pea, Indianhead (black) lentil, chickling vetch, and faba bean
- Biennial: sweetclover
- Perennial: alfalfa and red clover
There are benefits and drawbacks to growing each of the crops listed above. The large range in N fixation capability of each crop (Table 1) is mostly dependent on the crop growing conditions. Proper inoculation, combined with favourable crop development, will maximize N fixation.
Table 1. Nitrogen contribution of various types of legumes.
||Plant N derived from atmosphere (%)
||N fixed symbiotically *kg ha 1)
Adapted from Heichel, 1987 and Green and Biederbeck, 1995.
Field pea and lentil have good dry matter production and N fixation capabilities under a wide range of growing conditions. The residue quickly breaks down after crop termination, and field pea generally has greater N fixation and growth compared to lentil.
Chickling vetch is another good green manure crop option, recommended for dry conditions, and on lighter textured soil. It has the ability to fix more nitrogen, as well as more N in dry areas versus areas with heavier soils. (Table 2). Chickling vetch residue breaks down quickly, similar to pea and lentil.
Sweetclover is a biennial legume that can be seeded down with a crop in the year prior to the termination year. This reduces the need for a separate seeding operation, and helps suppress weeds in the fall and spring prior to termination. Sweetclover has a high N fixing capability and good dry matter production, but can use a lot of soil moisture. It is best to terminate it early, especially if soil moisture is low. Deep-rooted biennials and perennials (forage and red clover) are more suited to the moist Dark Brown and Black soil zones that generally have higher soil moisture reserves.
Faba bean has the capability to fix high amounts of N and produce a lot of dry matter, but if soil moisture is limiting (< eight inches), it will not provide as great a benefit as other legume crops more suited to dryland conditions.
Table 2. Fertilizer nitrogen replacement value (lb. N/ac.) of different legumes per 1,000 lb. dry matter produced.
||Good Moisture, clay soil
||Dr, sandy-loam soil
Source: University of Manitoba: Natural Systems Agriculture
Terminating the legume green manure crop
Most annual legume green manure crops should be terminated in the mid to late flowering stage. This achieves a balance between minimal soil moisture use and maximum N fixation. If the legume is not terminated and proceeds to set seed, a large amount of the fixed N will be translocated into the seed, and will have low availability for the next cropping season. Sweetclover should be terminated just prior to flowering, or in early flowering to avoid excessive moisture use. For drier areas, legume green manure crops should be terminated before the end of June, to allow as much time as possible for soil moisture recharge before the establishment of the next crop.
Is it better to use tillage or chemicals when terminating the green manure crop
This decision is based on cost, available equipment, and management system (no-till, conventional, organic, etc.). Tillage or chemical fallow are both effective at terminating a green manure crop, and show little difference in the resultant N benefit between the two.
Chemical termination retains higher amounts of anchored residue. Higher surface residue levels will reduce soil erosion risk and surface evaporation risk. Tall residue also allows for more snow trapping to help replenish moisture used to grow the green manure crop. Seeding into the residue left after chemical termination is possible if the green manure crop is terminated early enough, and seeding equipment with good residue clearance is used.
Annual green manure crops like peas, lentils and chickling vetch work well in any rotation that normally includes pulse crop varieties or fallow periods. To avoid disease and weed control problems, do not grow another pulse crop before or after a legume green manure in rotation. Cereals and oilseeds are recommended.
Sweetclover is a biennial, so growing it under a cover crop that is less competitive - usually a cereal (wheat or oats) - will reduce competition between crops and increase the viability of the sweetclover stand in the following year. It is important to terminate the sweetclover early as a green manure, in order to avoid high moisture use by the crop.
Is it economical?
Each producer should determine his/her own costs and weigh them against a realistic N benefit. There are a few key factors to consider:
- The largest cost is the seed:
- Source on-farm seed (test the seed for germination and disease levels) and use small seeded varieties to reduce costs;
- The cost of the inoculant to maximize N fixation; and
- The cost of tillage or chemfallow operations to terminate the green manure crop.
Soil testing is critical to measure the amount of nitrogen available for crop uptake following a green manure crop. Additional N may become available throughout the next growing season, as the residue continues to decompose and release mineralized-N.