Restoring the Soil - Canadian Foodgrains Bank

Annex 2: The Evidence83this level of increase is quite common when we use gm/ccs.The one case I have followed most closely is the maize/mucuna system in northernHonduras (S1). It is a very interesting case, and quite well documented (see, forinstance, Bernard Trombley’s Cornell University doctoral dissertation). In this case,thousands of farmers working on poor tropical soils with a parent material low inphosphorus have been planting maize and mucuna on the same fields every year for40 to 45 years. Most of these farmers have never applied chemical phosphorus to thesoil. Yet, after 40 or more years of harvesting maize at rates of 1.5 to 2.5 t/ha, theystill are having no problem with phosphorus availability in the soils. In fact, Bernardfound no yield response to adding phosphorus to these soils.In most of my travels across more than 45 developing countries, I have observedgm/cc systems in use. Yields have doubled or tripled in case after case. Yet I havenever observed a serious phosphorus deficiency, even where farmers have applied nophosphorus for decades. The northern Honduras phenomenon is not an exception. Itis well within the normal range of cases.So what is happening? How can farmers take significant levels of phosphorus everyyear out of a soil that was never particularly rich in it anyway, and do so without runningout of phosphorus?One answer, of course, is that the incredible amounts of organic matter added to thesoil have increased the available phosphorus by 100%, or maybe even several hundredpercent. That would explain a good part of the mystery, but not all of it.I think the problem may well lie with one inherent assumption that undergirdseverything that has been written above: the assumption that these farmers’ fields areclosed systems. Most agronomists don’t even realize they are constantly making thisassumption. Up until now in our discussion, we have been assuming that the onlyphosphorus that could possibly exist in each farmer’s field is the phosphorus that wasthere 40 years ago, when they began using mucuna.But this assumption is wrong. It is, in fact, very wrong. I once visited a scientificresearch station in the Brazilian Amazon near the city of Belen. Farmers there werecutting down virgin forest, largely because of the weed pressure on all land that hadbeen farmed for more than a few years. Scientists found that birds and bats depositedan average of 10,000 viable weed seeds on each hectare of new farm land per year.How much phosphorus would be deposited in the guano along with those seeds? Wedon’t know, but the amount would be significant, especially given that a large majorityof bird species are carnivorous and eat insects rather than seeds. In addition to thosedepositions, phosphorus would be spread from forest to farm and from farm to farmby microorganisms, insects, earthworms, wild animals, domestic animals, etc.This transfer of nutrients does not only happen near the edges of virgin forests. Acrossthe Sahel, far larger transfers of phosphorus occur over larger areas because of the