Breeding and selecting Leucaena for acid tropical soils

Abstract

In tropical South America 55% of the area (820 million ha), including 68% of Brazil (573 million ha) comprise very acid oxisols and ultisols of pH 5 or less. Tropical Africa and Asia also have substantial areas of these acid soils. Special problems are posed by very acid soils for the growth of legumes, including N₂ -fixing trees like leucaena. The properties of oxisols, especially of their subsoils, have been considered as a basis for selection and breeding of leucaena lines adapted to these conditions. Perhaps the principles discussed apply also to other N₂-fixing trees. In view of the low pH, the very low exchangeable calcium and high percentage aluminium saturation in the subsoils of oxisols (ultisols), pertinent aspects of plant nutrition were reviewed. The constraints imposed on plant growth by these Ca and Al levels were given particular attention. Because of the importance of Ca in root growth, its immobility in plants and its neutralizing effect on Al, ways of increasing downward migration of Ca into the subsoil were considered. The effect of Ca on nodulation was briefly discussed. Studies of Leucaena leucocephala cultivars, especially Cunningham, in the oxisols of the Colombian llanos and the Brazilian cerrados showed that retarded root and top growth was due primarily to Ca deficiency. The high exchangeable aluminium did not have a toxic effect on the roots, but it probably inhibited Ca absorption and intensified Ca deficiency. The work described has been successful in identifying acid soil tolerance and efficient root absorption of Ca, especially in limes of L. diversifolia and L. shannoni. Their acid soil tolerance has been transferred to fertile crosses with L. leucocephala. Selections from these crosses with good agronomic and forestry potential adapted to the South American oxisols are now possible.