Restoring the Soil - Canadian Foodgrains Bank

12Existing Green Manure/Cover Cropping Systems Around The World13. The ecological advantages of trees. When we use tree gm/ccs as dispersed shade,we also gain the ecological advantages provided by trees: soil and crops are protectedfrom the tropical sun, soil moisture is conserved (because evaporation and transpirationrates are reduced), and desertification is curbed.When we compare the above advantages to those of composting, we find that in thevast majority of cases, gm/ccs will be far more attractive to farmers than composting.Gm/ccs provide weed control, food, and many other products and advantages, withmuch lower transport and labor costs than compost (Photo 8). The exception wouldbe those cases where farmers are growing very high-value crops and/or own a verylimited amount of land (less than half a hectare, for instance), so that there is virtuallyno space where the gm/ccs can grow.Each of the above advantages should be analyzed and weighed when choosing whichgm/ccs to use and promote. Experience shows that farmers are rarely attractedprimarily by the gm/ccs’ ability to increase soil fertility. More commonly, farmers aremost motivated by the gm/ccs’ potential for human consumption (usually the highestpriority of the above advantages) or their ability to control weeds. Therefore, gm/ccs should be promoted mostly by emphasizing these other advantages, not just theadvantage of increasing soil fertility. For instance, sometimes we refer to a gm/cc as a“food crop” or a “green herbicide.”Disadvantages of Green Manure/Cover CropsDespite all these advantages, gm/ccs are sometimes difficult to introduce to farmers.While working with the international development organization World Neighborsduring the 1980s, I tried for eight years to introduce the use of mucuna to farmers incentral Honduras, with very little success. This was because I didn’t understand someof the important disadvantages of gm/ccs. If we fail to overcome these disadvantages,we will not be successful at introducing gm/ccs. In a few situations, these disadvantageswill mean we should not try to introduce gm/ccs at all.1. The opportunity cost of the land. Farmers normally will not plant something thatonly fertilizes their soil if the land could instead be planted with either food crops orcash crops. Unless the gm/ccs also produce food, the land used to grow gm/ccs musthave no other valuable use (i.e.: it must have no “opportunity cost,” meaning the“cost” represented by not taking advantage of an “opportunity” to do something elsewith the land). Traditional green manure systems from temperate countries requirefarmers to monocrop green manures on their land before planting their regular crop,thus taking otherwise useful land out of production for a few months in order to producethe green manure. These systems did not recognize the need to use land that hadno other profitable use—which is probably why the introduction of green manuresinto the developing world before the 1970s was rarely successful.