Restoring the Soil - Canadian Foodgrains Bank

Annex 2: The Evidence85content of a soil. First, they do not measure the phosphorus that is tied up chemicallyand therefore is unavailable to plants. Secondly, they do not measure the phosphorusthat exists in organic forms, whether that be in decomposing plant material or in soilfauna and flora. Furthermore, these inaccuracies may be quite large, leaving out alarge majority of the total phosphorus present in the soil.During a study I did in Africa in late 2009 for the Christian Reformed World ReliefCommittee, I traveled through significant areas of Malawi, Zambia, Kenya, Uganda,Niger and Mali, interviewing farmers wherever I visited. When doing such studies,I observe all the agricultural practices and conditions that I can as I travel along thehighways and dirt roads in order to triangulate that information with what I see insmallholder farmers’ fields and hear from the farmers. I saw tremendous evidencethat yields were diminishing, primarily because of nutrient deficiency. But nitrogenwas clearly most deficient in at least 95% (probably closer to 98%) of the tens ofthousands of fields that I observed. Nitrogen deficiency is due to a lack of organicmatter in the soil. All over Africa, farmers are mining the nitrogen out of their soils.Wherever one goes, one can observe the yellowish, stunted maize and millet fields thatindicate a nitrogen deficiency. On the other hand, the purple leaves and stems associatedwith phosphorus deficiency were visible on less than 5%, and probably closer to2%, of the fields.Something is obviously wrong with the CGIAR’s prediction of a major phosphorusbasedcrisis within the next decade or so. But why such a striking lack of phosphorusdeficiency symptoms? Deficiency symptoms in most grasses like maize and millettend to indicate only the single element that is most limiting. That is, if a nitrogendeficiency is more limiting to the plant’s growth than a phosphorus deficiency, youwill see only the symptoms of nitrogen deficiency on the leaves or stems of the plants.So my observation of fields around Africa did not indicate that there was no phosphorusdeficiency. Rather, it indicated that in a widespread, consistent pattern, the soil’sphosphorus deficiency (if present) was masked by a worse nitrogen deficiency.This consistent masking of whatever phosphorus deficiency there was—across a wholecontinent—leads me to believe there must be some relationship between the availableamounts of the two nutrients, causing the phosphorus deficiency to be consistentlyless limiting than the nitrogen one.If the original amount of phosphorus in the soil and subsoil minus the amount takenout by African farmers over the years were the main factor in determining the amountof available phosphorus, there would be no relationship between that quantity ofphosphorus and the quantity of nitrogen in the soil. There would, therefore, tend tobe many fields in Africa with the purplish signs of phosphorus deficiency. However,if the amount of organic matter in the soil were the main determinant of how muchphosphorus was available to plants, then a very clear relationship would exist. In theabsence of chemical fertilizer, the amount of nitrogen in a soil correlates quite closelywith the amount of organic matter, which in turn would correlate quite closely withthe amount of available phosphorus in the soil.