Restoring the Soil - Canadian Foodgrains Bank

A Brief History of Green Manure/Cover Crops52. A Brief History Of Green Manure/Cover CropsGreen manures and cover crop systems have been used for at least 2,000 years. TheRomans wrote papers advising people on how best to use green manures. In the richercountries of Europe and the United States, green manures were by far the most commonmethod of maintaining soil fertility until the end of the Second World War.Some books from the 1930s include a tremendous amount of information aboutgreen manure systems in the tropics—information that today has been almost totallyforgotten. 2Since 1945, the ability of organic matter to effectively fertilize the soil has oftennot been emphasized or even mentioned in crop production manuals or agriculturetextbooks (Photos 1, 2, 3 and 4 show examples of the impact of organic matter inimproving crop yields).History shows beyond doubt that green manuring and gm/cc systems are capable ofmaintaining soil fertility for thousands of years. After all, they have done precisely thatin virtually all the world’s different soils, environments and farming systems (Photos 5and 6).In the tropics, the main method of maintaining soil fertility historically was by“fallowing.” This process involved leaving agricultural land to grow into forests orgrasslands for 10 to 15 years, until the soil had recuperated its natural fertility. Fallowing,like green manuring, maintained the fertility of the soils of whole continents forthousands of years. Since gm/ccs fertilize the soil in virtually the same way as fallowsdo, we can be quite confident (much more than we can be with chemical fertilizers)that gm/ccs are capable of keeping the soil fertile over the long term. Other methodsare used in the tropics for maintaining soil fertility. Animal manure is, of course, anextremely good material for maintaining crop fertility, but only rarely do smallholderfarmers have enough for more than about a tenth of their fields (Photo 7). Compostis probably the best of all soil amendments, but if we calculate the amount of laborit requires, we will almost always find that it is too expensive for basic grain cropsamong smallholder farmers, except in the case of rice (Photo 8). On the other hand,compost is usually far more useful than gm/ccs for farmers planting irrigated vegetables,since the cost of the compost is well-remunerated and the opportunity cost ofthe land used in planting gm/ccs is very high (Photos 9 and 10).2 See, for instance, Colombo, Charles, Use of Leguminous Plants in Tropical Countries as Green Manure, asCover and as Shade (Rome: Printing Office of the Chamber of the Deputies, 1936).