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.