A. Chemical fertilizers

The use of chemical fertilizers is essential for obtaining high yields in the highly weathered soils of the humid tropics. However, many small land holders and resource-poor farmers cannot afford costly fertilizers. Most soils in the humid tropics are so deficient in primary nutrients that it is imperative that strategies be developed for adding them from outside the ecosystem. Otherwise, sustainable cropping systems cannot be developed.

The most deficient nutrients in the soils of the humid tropics are N. P. and Ca as macro-nutrients and perhaps Zn as a micro-nutrient. There is some potential for enhancing N supply by biological N fixation. Other nutrients have to be supplied. The concept of low external input must clearly be examined in view of the limited nutrient reserves of these soils. However, the requirements for chemical fertilizers can be reduced drastically by decreasing losses, recycling nutrients, and through biological N fixation (Fig. 16).