Introduction

Numerous attempts have been made to solve the problems of site degradation due to shifting cultivation by focusing attention on improvement of the fallow period (Sanchez 1976). Indeed, that period wherein the physical and chemical properties of the soil are restored to a site has been called the key to the long-term success of shifting cultivation (Ewel 1976).

Accumulation of nutrients and organic matter under various types of both mature tropical forest vegetation and fallow crops has been described (Greenland and Kowal 1960; Nye 1961; Jaiyebo and Moore 1964; Juo and Lal 1977). Such studies show that both the type and age of a fallow crop may greatly influence the fertility status of a site by the end of the fallow period.

In recent years the leguminous tree Leucaena leucocephala (leucaena) has shown promise as an effective fallow improvement crop (Parfitt 1976; IITA 1980). However, there remains a lack of information on the effects of a leucaena fallow in shifting cultivation systems on such parameters as soil erosion and soil nutrient contributions.

This paper is presented in two sections. Section I presents a brief review of the literature dealing with fallow improvement in general and more specifically with leucaena as a fallow improvement crop. Section II outlines a fallow improvement system utilizing leucaena which was developed in the Philippines.