By definition, any soil with a pH of less than 7 is acidic but the detrimental conditions for plant growth may not appear until soils have a pH of 5.0 or less. Low nitrate levels and detrimental effects of high hydrogen ion concentration on plant growth may occur in some soils. The most common problem is aluminum toxicity, which generally inhibits root penetration below 15-20 cm. Manganese toxicity may be associated with higher pH levels than Al toxicity. Soil acidity and aluminum toxicity constrain agricultural production in several ways.
Farmers are limited to planting crop species or cultivars that tolerate such conditions.
Soil acidity can also be a barrier to root development, limiting a plants ability to reach moisture in the subsoil. Nitrogen fixing trees are also grown for wood production, shade and support for crops, live fence posts, green manure, human food, as shelterbelts and windbreaks, browse or fodder for domestic animals, and as ornamentals. In establishing an agroforestry system on an acid soil, the first step is to identify suitable tree species. Once the best available species are identified, further investigation is needed to determine their value for agroforestry. In choosing nitrogen-fixing tree species, careful attention should be given to select species that are adapted to local environments and that provide farmers with products and services they need.