Four Trees for Soil Amelioration

Population pressure is putting the squeeze on agricultural land worldwide. In response to this problem, fallow periods are being examined for their potential to become more time efficient. Planted tree fallows offer the dual advantage of regenerating the soil and providing wood products. Fast growing leguminous trees are prime candidates for planted fallows because of their capacity to fix nitrogen, control weeds, disease and pests, and provide mulch. However, because leguminous trees are so fast growing they can quickly sap the soil of nutrients if improperly managed.

A study in central Togo compared the impact of four trees (Cassia siamea, Albizia lebbek, Acacia auriculiformis, and Azadirachta indica) on chemical soil conditions under a planted fallow, a natural grass/herb fallow, and a bush fallow. (Neem and Cassia are not known to fix nitrogen). The results show that the effects of a tree fallow on soil chemistry can vary greatly depending on which tree you choose and, that the most productive tree is not necessarily the best for the soil.

The study was conducted in the semi-humid tropical lowlands of Koussountou, Togo, at an elevation of 356 m, where average annual rainfall is 1180 mm. During the dry season, November to February, monthly rainfall is less than 20 mm. The soils are Ferric Acrisols with a sandy loam topsoil over a loamy clay subsoil. The pH of the topsoil varies between 5.8 and 6.7.


In 1985, farmers planted the four species in 39 randomly located plots that ranged in size from 0.12 to 0.5 ha. The plots consisted of 13 Cassia, 7 Albizia, 13 Acacia, 3 Neem, and 3 mixed. Two traditional fallow plots, one grass/herb and one bushy, were used as non-afforested controls. Plots were not treated after planting. Tree harvest began in 1990.


Topsoil and litter samples were taken from five plots of the Acacia, Cassia, and Albizia treatments, three Neem, and the two control plots. Foliage samples were taken from several locations on the lower crown in three plots of all species.

The samples were analyzed for pH, organic carbon, N, P, cation exchange capacity (CEC), and exchangeable cations (Ca, Mg, K, Al and H). Foliar and litter samples were analyzed for a number of chemical constituents (C, N, S, Si, P, Ca, Mg, K, Al, Fe, Mn, Cu, Zn), tannins and phenols.

Treatments Compared

Acacia auriculiformis, which loses its leaves annually, proved to be a vigorous biomass producer in the trial plots. The soil in trial plots was completely covered by two 2-4 cm layers of partially fermented Acacia litter from previous years, overlaid by fresh litter. Acacia far surpassed Neem and Cassia in terms of growth, biomass, and litter production. Neem's performance improved in mixed stands.

Soil pH measured markedly higher under Cassia, Neem and Albizia than under grass and bush fallow. Neem and Cassia litter increased exchangeable Ca in the topsoil by 1300 and 650 kg/ha over the grass fallow. Under tree and bush fallows, soil organic matter (C and N), cation exchange capacity and exchangeable cations were greater than under the grass fallow.

Topsoil organic matter under Acacia, Cassia, and Albizia was greater than grass fallow by 40-60%. In Neem plots, N content increased by 115% or 1100 kg/ha, and P availability increased (Table 1). Only the Cassia plots shows higher extractable P than the bush fallow. For increasing soil organic matter (SOM), Neem fallow outperformed the other trees. Cassia and Neem lead to higher soil pH, followed by Albizia, then Acacia.

Acacia's low pH is associated with low exchangeable Ca. Highest Ca levels were found in Cassia litter, 39% greater than in Neem litter. Table 2.

Litter mineralization under Acacia is relatively slow due to the leathery consistency of the leaves, and perhaps a higher polyphenol content. The litter of the other species is relatively low in phenols and soft in consistency.

The Bottom Line

Ironically, the species that was most beneficial for the soil produced less biomass than the average of those tested. Further investigation into the limit between soil improvement and depletion by tree species is needed.

In this trial Cassia and Neem were associated with the highest soil fertility. Regular cutting and mulching of these particular species is another area for further study. Albizia showed the best mineralizable SOM, because of its C/N ratio, but only had an average impact on soil fertility. Acacia seems less suitable for soil amelioration than Neem or Cassia, but well suited for fuelwood production.

Drechsel, P., B. Glaser, and W. Zech. 1991. Effect of four multipurpose tree species on soil amelioration during tree fallow in Central Togo. Agroforestry Systems 16:193-202. Kluwer Academic Publishers.