Introduction

Mark H. Powell, editor

With increasing population pressure in much of the tropics, farmers are often forced to expand agriculture into marginal areas. Here, they must try to produce crops and raise livestock under growing conditions that can be extremely difficult. Many of these poor sites are characterized by acid soils.

Strongly acidic soils - with a pH of less than 5.5 - present the farmer with several problems. They tend to be infertile, suffering in particular from low levels of calcium and phosphorus, and they may contain toxic levels of available aluminum and manganese. Many types of plants grow poorly on these soils, and others cannot grow at all.

The farmers who try to produce crops on acid soils tend to have small land holdings and limited financial resources. With little or no access to chemical fertilizer or irrigation, they are not likely to benefit from the high input agricultural technologies developed during the Green Revolution. Often, they are recent migrants from other farming areas, and they lack the specialized knowledge and experience required to produce crops on acid soils.

Governments and development organizations will have to devise technologies for sustainable productivity under the difficult conditions that acid soils impose and will have to introduce these to farmers. Production is limited to those plant species and cultivars that tolerate soil acidity. With limited land holdings and few, if any, purchased inputs, farmers must maintain yields by carefully ensuring the recycling of nitrogen and other nutrients.

Many nitrogen fixing tree (NFT) species are tolerant of acid soils. Planted in agroforestry systems, they provide a low-input technology that helps farmers overcome limitations to plant growth caused by lack of nitrogen. Only recently, however, researchers have begun to evaluate different nitrogen fixing species for agroforestry systems on acid soils. To bring together the results of recent and current research and to identify future research needs, the Nitrogen Fixing Tree Association (NFTA) and the Centro Agron¢mico de Investigaci¢n y Ense¤anza (CATIE) organized a workshop on Nitrogen Fixing Trees for Acid Soils, which was held on 3 8 July 1994 in Turrialba, Costa Rica.

Forty participants came to the workshop from 14 countries around the world. NFTA has published a proceedings volume that includes many of the papers presented at the workshop plus recommendations of the working groups for future research and development priorities. During the workshop, working groups drafted chapters for this field manual. The objective was to produce a practical field guide for selecting, evaluating, and growing nitrogen fixing trees on acid soils. I hope you will find this manual useful in your work and welcome your comments and suggestions.