Improving Soil Fertility With Green Manure Legume Crops

What can I do to reduce my fertilizer input costs yet maintain soil fertility?
Growing legumes and working the whole crop into land as green manure ploughdown is one strategy. Since legumes fix their own nitrogen from the atmosphere, green manuring can maintain or improve soil fertility without direct costs for fertilizer. Nitrogen fixed this way is not “free” as the costs for green manure nitrogen include seed and the seeding costs, stand termination and residue incorporation costs, and lost revenue, since there is no cash crop produced during the year of ploughdown.

How much nitrogen can be fixed by a green manure crop?
Under irrigation legume crops can fix well over 100 lb N/ac (Table 1). These high rates of symbiotically fixed nitrogen can only be achieved under low soil nitrogen conditions where inoculation is effective, and where crop growth is not limited by a lack of moisture.

Table 1. Nitrogen Fixation in Legumes Grown Under Irrigation*.
N Fixed Symbiotically
(lb N/ac)
Field Pea
Dry bean
*From R.J. Rennie as cited in Soil improvement with legumes including legumes in rotations.

Since fababeans fix the most nitrogen, aren’t they the best crop for green manure ploughdown?
Research indicates that if fababeans only have access to 8 inches of water or less, they do not fix as much nitrogen as other legumes. But even if moisture is abundant, fababeans may not be the “best” crop because fababean production costs are often high, making the nitrogen quite expensive. The best crop for green manuring has low production costs but still fixes enough nitrogen to meet the requirements for the following crop.

What are the advantages or disadvantages of the different legume crops for green manuring?
Several crops will work well for green manuring. The decision of which crop to grow will often have to be based on seed availability and seed costs as well as local environmental conditions. Some of the advantages and disadvantages of various green manure crops are summarized below.

Peas - Peas have good dry matter production and nitrogen fixation capabilities in a range of environments. Most varieties have a large seed size so seeding costs can be significant. Small seeded “silage” varieties do not offer savings as they are usually priced to be comparable to other peas on a $/ac basis. A wide range of field pea varieties can be used for ploughdown and common seed is available in most regions so shopping around can minimize seed costs. Lush plant material, incorporated at mid to late flowering, breaks down relatively quickly and can meet the nitrogen requirements of following cereal crops.

Lentils (Indianhead) - These lentils have a small seed size but the seed is not readily available and is often quite pricey. Total plant growth and nitrogen fixation with lentils is often lower than other legumes but is still acceptable in dryland situations. Another positive factor is that plant material breaks down quickly following incorporation.

Chickling vetch - Chickling vetch is a newcomer to the green manure world in western Canada. This crop has reasonably good nitrogen fixation capabilities and incorporated plant materials breakdown almost as quickly as lentils. In dryland experiments it has fixed enough nitrogen to meet the requirements for a following cereal crop. Seed is not readily available.

Fababeans - Unlike many other legumes, fababeans will fix nitrogen throughout the growing season. If conditions are good, fababeans will produce a lot of dry matter and fix high rates of nitrogen. However, this crop does not have very good nitrogen fixation capabilities if it has access to less than 8 inches of water. Another disadvantage is that it is large seeded with high seeding costs. Seed is also not readily available.

Sweetclover - Sweetclover’s biennial growth characteristic allows it to be seeded with the crop prior to the green manure ploughdown year so costs for a separate seeding operation are eliminated. Weed control options may be limited in this mixed crop year, but sweet clover can suppress weed growth while the crop is growing as well as throughout the fall and the following growing season. Seed is generally available at reasonable cost. A small seed size and low seeding rate also reduce seeding costs. Sweetclover has good dry matter production with high nitrogen fixing capabilities. Crop removal/ploughdown timing can be critical as sweetclover is reported to use more soil moisture than other green manure crops and this soil drying has suppressed the yield of subsequent crops.

  1. Biederbeck, V.O.; Bouman, O.T.; Looman, J.; Slinkard, A.E.; Bailey, L.D.; Rice, W.A.; Janzen, H.H. Productivity of four annual legumes as green manure in dryland cropping systems. Agronomy Journal. Sept/Oct 1993. v. 85 (5) p. 1035-1043.
  2. Blackshaw, R.E.; Moyer, J.R.; Doram, R.C.; Boswell, A.L. Yellow sweetclover, green manure, and its residues effectively suppress weeds during fallow. Weed Science. May/June 2001. v. 49 (3) p. 406-413.
  3. Bremer, E.; Rennie, R.J.; Rennie, D.A. Dinitrogen fixation of lentil, field pea and fababean under dryland conditions. Canadian Journal of Soil Science. Aug 1988. v. 68 (3) p. 553-562.
  4. Rennie, R.J.; Dubetz, S. Nitrogen-15-determined nitrogen fixation in field-grown chickpea, lentil, fababean, and field pea. Agronomy Journal. July/Aug 1986. v. 78 (4) p. 654-660.
  5. Rice, W.A.; Olsen, P.E.; Bailey, L.D.; Biederbeck, V.O.; Slinkard, A.E. The use of annual legume green-manure crops as a substitute for summerfallow in the Peace River region. Canadian Journal of Soil Science. May 1993. v. 73 (2) p. 243-252.
  6. Townley-Smith, L.; Slinkard, A.E.; Bailey, L.D.; Biederbeck, V.O.; Rice, W.A. Productivity, water use and nitrogen fixation of annual-legume green-manure crops in the Dark Brown soil zone of Saskatchewan. Canadian Journal of Plant Science. Jan 1993. v. 73 (1) p. 139-148.

Prepared by Doon Pauly, Alberta Ag-Info Centre, Alberta Agriculture and Rural Development.


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