Restoring the Soil - Canadian Foodgrains Bank

8Existing Green Manure/Cover Cropping Systems Around The World3. Existing Green Manure/Cover Cropping SystemsAround The WorldAdvantages of Green Manure/Cover Cropping SystemsThe proven advantages of gm/ccs are numerous:1. Increased organic matter and soil nutrients (Photo 9). Gm/ccs are capable ofadding as much as 50 metric tons/hectare (MT/ha) or more of organic matter (greenweight) to the soil each year (Photo 11). 3 This organic matter has various positiveeffects on the soil, such as recycling nutrients back into the soil, pumping nutrients upto the soil surface, and improving the soil’s water-holding capacity. It can also increasethe total amount of nutrients in the soil, improve its nutrient balance, increase thenumber of macro and microorganisms (very small animals in the soil, many of whichalso help a farmer’s crops grow better), make soil softer and easier to plow, improve theacidity of soil (i.e.: buffer soil pH) and sequester carbon. Organic matter makes soilnutrients, including those supplied by chemical fertilizers, more accessible to crops,though this impact has been greatly underestimated. In the case of phosphorus, thisis particularly important: in acidic soils, phosphorus may become four to five timesmore available to plants when surrounded by organic matter.12. Honduras. When most of us think of nitrogenfixingnodules, we think of tiny little spheres aboutone millimetre in diameter. This photo shows thenodules of just one mucuna plant grown underideal conditions with no competition from otherplants.2. Nitrogen fixation. Legumes (plants thatproduce their seeds inside pods) are ableto fix nitrogen (N) from the atmosphereinto a plant-usable form that accumulatesin plant tissues (Photo 12). Legumes canthereby add large quantities of nitrogento farmers’ soils. Most of the widely usedlegume gm/ccs are capable of producingmore than 50 kg N/ha, while a few gm/ccspecies fix significantly more. Mucuna canfix 140 kg N/ha, the jackbean up to 240kg N/ha and tarwi (Lupinus mutabilis),fava beans (Vicia faba) and Sesbania rostrataup to 400 kg N/ha. 4Even allowing for considerable losses ofnitrogen to the air (volatilization), the 140kg N/ha added by the gm/cc would cost atleast US$75/ha if purchased as a chemicalfertilizer. This increase in both nitrogen3Lathwell, Douglas J., “Legume Green Manures, Principles for Management Based on Recent Research” in TropSoilsBulletin No. 90-01 (Raleigh, North Carolina: Soil Management Collaborative Research Support Program, North CarolinaState University, June 1990).4National Association of Sciences (NAS) Tropical Legumes, Resources for the Future (Washington, D. C.: NAS, 1979) andTisdale, Samuel L., et al., Soil Fertility and Fertilizers, Fifth edition (New York: MacMillan Publishing Co., 1993).