Introduction

Investment in forest tree planting is increasing annually, FAO surveys indicated that forest plantations in the tropics alone expanded from a total of 18 million hectares in 1980 to 44 million hectares in 1990. In tropical and subtropical countries the majority of trees planted are raised in containers. However, the most intensive research on producing good containerized planting stock, has been carried out in temperate countries. Nursery research has demonstrated that development of a fibrous root system is essential for good quality seedlings. Root absorptive efficiency is directly related to its surface area and fibrous roots provide greatest surface area. Development of fibrous roots is related to the porosity of the potting mixture which in turn is related to the organic fraction of the mixture. The organic fraction of a mixture helps resist compaction and retain water, while still maintaining porosity for movement of air and growth of roots.

Organic ingredients, such as peat and humus, are readily obtained in many temperate countries. Large nurseries in America and Europe can purchase commercial, ready-made potting mixtures, eliminating the need for nursery managers to make their own mixes. The most commonly used materials are peat moss, vermiculite, perlite, sand, and sawdust. In tropical and subtropical countries (such as China, India Nepal and Nigeria)' importing the vast quantities of potting mixtures or their components which would be needed is too costly. Therefore, virtually all nurseries must rely entirely on local ingredients for their potting media. Unfortunately, in most of the tropics and subtropics there is a shortage of organic matter as it is favored by farmers for their crops, fed to their animals, or burned as fuel.