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.
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.
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.
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.