Highland maize covers about 6 million hectares in the tropics, primarily in Mexico and the Andean Region of South America, and to a smaller extent in Eastern Africa. The diversity of highland production environments has given rise to myriad farmer-developed maize varieties, and adoption of improved seed is generally low, as are yields. Tropical soils are renowned for their low fertility. In addition, the agricultural practices historically associated with resource-poor maize farmers has contributed to the rapid depletion of the nitrogen, organic matter, and phosphate reserves in these fragile soils. Finally, much of the increase in maize production in the tropics comes from the opening up of new, increasingly marginal land, which means lower productivity and further degradation.
Low nitrogen is the major nutrient constraint to maize yields in the tropics.
The availability of phosphorus, the second most limiting nutrient, is highly dependent on soil acidity and residue management practices. Finally, acid soils generally reduce fertility on at least eight million hectares of maize lands in developing countries. Whereas overuse of fertilizers and environmental nitrification are widely cited as deleterious effects of agriculture, exactly the opposite problem is associated with maize production in developing countries. Low use of fertilizers has always represented a risk management strategy in response to the high spatial and seasonable variability of maize production environments in developing countries. This same variability has made it difficult to tailor and disseminate appropriate recommendations for fertilizer use, and underdeveloped markets and infrastructure in many regions has led to a reliance on world markets, increasing costs and price variation.