Numerous literature indicate that N is often the most limiting nutrient to agricultural productivity due to its requirements for plant growth in large quantities but also because soils, particularly in the tropics, are almost always N-deficient. However, it is more difficult to find studies that estimate the amount of N (ammonia, ammonium, nitrate..) in tropical soils (either absolute quantity or that relative to other nutrients needed by plants). I would very much appreciate if someone could provide information on this. Thank you in advance!
http://horizon.documentation.ird.fr/exl-doc/pleins_textes/divers16-03/17370.pdf
Dear Stephanie Fordeyn, In the tropics the long growing season has the potential to give multiple crops. In terms of nitrogen 80% of the atmosphere can provide all the nitrogen needs in legume based crop and animal systems. In terms of nitrogen crops can be limited by issues of insufficient water and unbalanced soils as the chief factors. The majority of tropical soils are acid and or alkaline which will limit their potential for most crops. The humid tropics have the worse soils which are acid older and have both toxicity and deficiency issues. This would be best addressed by liming where lime is available which is scarce agriculturally in many tropical areas. In the drier areas alkalinity and salts can be very limiting. Nitrogen and the major nutrients need to have pH adjusted to optimize nutrition. In terms of nitrogen fixation the micronutrients are very important many of the acid older soils are very deficient in Phosphorus. Balancing soils in terms of pH macro and micronutrients can provide a foundation of improved legume production. Improved legumes will help the grain production and improve the diet which is low in nutrition in many tropical areas. For a crop like maize the ability to measure the soil nitrate level when plants are knee high can identify if the soil condition has ability to respond to supplemental nitrogen application. If the some sample has 30 ppm or more there is little need for nitrogen supplementation and increase response would be expected at values below 30 ppm. Tissue analysis can also give clear ideas of the relative nutritional constraints under the field condition. In general terms about 1 to 3% of the total nitrogen can be mineralized for a cereal production crop. The role of organic amendments is they can serve to stimulate the mineralization of N which occurs from microbial activity. Organic amendment can provide nitrogen and carbon in many soils the low carbon content is very limiting for its negative effect on soil structure and inhibition of soil percolation which can be most limiting issue. Without sufficient soil organic matter the water dynamics will become the most limiting aspect of the crop potential.
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