Restoring the Soil - Canadian Foodgrains Bank

84Annex 2: The Evidenceseasonal winds called the “harmattan.” Thousands of tons of topsoil are carried acrossthe southern perimeter of the Sahara desert. To understand how much soil is moved,you only need to look at the windward side of many homes on the windward side ofSahelian villages. The build-up of topsoil against those walls is frequently over a metrehigh, and occasionally reaches the eaves of the house. In much of the Sahel, every lowlyingbush that has leaves year-round (including the nyama) has around 40 cm of soilbuilt up around its base. This represents a major transfer of nutrients from one field toanother, and a major influx of phosphorus to fields in which farmers are using nyamaor other bushes to fertilize their soil.With these dynamics as background, let’s return to the original question of theeffect of organic matter on the availability of phosphorus. Our present methods ofsoil analysis do not give us a very good answer. Some scientists who have looked atphosphorus levels in organic systems have admitted they cannot explain where all thephosphorus is coming from (Cheryl Palm is one of them).CGIAR (Consultative Group on International Agricultural Research) system scientistshave been telling us for over a decade that Africa is heading for a major crisis of soilphosphorus deficiency. They have not been clear about exactly when the problemwould hit, but certainly sometime within the 25 years following the start of theirpredictions. They normally give two major reasons for decreasing levels of availablephosphorus. One is the gradually decreasing levels of available phosphorus they havefound in the soil analyses they’ve done. The second is a calculation based on cropyields, from which they have calculated the annual loss to the soil of various nutrients.But there is reason to believe that somehow more phosphorus is available to plantsthan scientific studies would indicate. The CGIAR calculation based on crop yieldsmay be based on a major misunderstanding of the dynamics of smallholder agriculturein Africa. In the United States, calculating nutrient losses on the basis of crop yieldscan be quite accurate because American farmers consume virtually nothing of whatthey harvest, and even what they do consume goes into flush toilets and far away fromtheir farms. However, the vast majority of African smallholder farmers produce onlyenough food for 6 to 10 months of consumption. That is, most African villages arenet importers of food, and therefore net importers of phosphorus. Their food wasteall goes either onto the soil surface or into hand-dug latrines that go nowhere near asdeep as some of the roots of nearby bushes and trees. So quite likely, the total amountof phosphorus recycled into people’s fields is even greater than the amount that istaken out at harvest time.That leaves soil analyses of available phosphorus as the main credible piece of evidencefor the predicted African soil phosphorus crisis. Remember, scientists point to graduallydecreasing levels of available phosphorus in soil analyses. But do decreased levelsof available phosphorus correlate well with decreased levels of the total phosphorus inthe soil?Soil analyses, by and large, do not give us a very accurate assessment of the phosphorus