Session 5 Social and Development Issues

Session Papers

Faidherbia albida on Mine Spoils in the Tin Mining Region of the Jos Plateau, Nigeria

A.D. Kidd¹, M.J. Alexander² and K.D. Phillips-Howard³

Abstract

A pilot study on the potential utility of Faidherbia albida for the improvement of mine-spoils suggested that the tree had a beneficial effect on spoil under the canopy and therefore could be used as an alternative to Eucalyptus spp. In a follow-up multidisciplinary study, F. albida showed much more variable effects on mine spoil and less obvious benefits. Farmer preference studies suggested that although the tree is regarded as useful by farmers, its use as an alternative for mine-spoil improvement would not be viable. This is because F. albida is not regarded as an "economic" tree by farmers and is ranked low in comparison with other trees in terms of its utility. Therefore its use in mineland reclamation is unlikely to be supported by these farmers.

Introduction

Tin mining on the Jos Plateau has produced an extensive disturbed landscape. Reclamation programs have filled the mining paddocks, levelled spoil, and planted Eucalyptus spp. However, only some 2-3 km² of the 316 km² damaged by mining has been formally reclaimed. Faidherbia albida grows extensively by natural colonization throughout the tin fields. The possible utilization of this tree on mineland by the Jos Plateau Environmental Resource Development Program (JPERDP) is the focus of this paper.

Soil Relations and F. albida

Research in Phase 1 of the Program (1982-86) gathered information on many different aspects of the environmental and human resources, settlements, and problems of the tin-mining region (Phillips-Howard 1989).

A strategy based on "level and fill" and planting with Eucalyptus spp was expected to improve the soil sufficiency for the land to be returned to productive agriculture (Wimbush 1963). The variable results of this eucalyptus planting meant that the JPERDP began to look for alternatives. This and encouraging work from elsewhere on the effect of F. albida on soil nutrient status (Charreau and Vidal 1965) led to a pilot study on a single tree in 1983.

Alexander (1989) reported that the tree significantly increased the pH, nitrogen, organic carbon, exchangeable base content, and percent base saturation in topsoil under the canopy. He concluded that F. albida had a beneficial effect on both the nutrient status and physical condition of the soils lying beneath its canopy.

Field and Laboratory Methods

To test this initial hypothesis four areas of mineland were chosen to examine the effect of F. albida on the soil. In each of these sites two F. albida trees were chosen for study. Diameter breast height (DBH) of the 8 trees ranged from 27-64 cm; canopy diameter ranged from 6-12 m. Two small sampling pits were dug near each tree. One pit was dug under the canopy about 1 m away from the trunk and the second was outside the canopy, at a distance of at least twice the diameter of the canopy from the trunk. Pits were located approximately southeast of the tree trunk, in the direction of prevailing winds during the time of leaf fall. Presumably, this reduced the chances of the site outside the canopy being affected by litter deposits.

Bulk soil samples were collected in Dec 1990 (midway through the dry season) from each pit at 0-5 cm and 5-20 cm depths. This corresponded to the sampling depths of the pilot study (Alexander 1989). There was no evidence of cultivation on any of the sites. Soil analyses were carried out using standard methods (Alexander 1989; Hesse 1971).

Results and Discussion

Vegetation growing immediately beneath F. albida, had about the same species composition as that growing outside the canopy, but was usually much thicker and taller. It also tended to have a greater ground cover (90-95%) compared with cover found outside the influence of the canopy (50-60%).

Of all the soil properties measured, pH appeared to show least variation between sites (average = 5.9). Organic carbon, total nitrogen and available phosphorus were all greater in the topsoil (0-5 cm) beneath the canopy as compared to soil outside the canopy. This was reversed for subsoil values (5-20 cm). None of these differences were statistically significant.

Exchangeable calcium, magnesium, and sodium were lower but statistically similar, in both the topsoil and subsoil beneath the canopy. Only exchangeable potassium was significantly higher (P < 0.01) in the topsoil beneath the canopy as compared to topsoil outside the canopy. A similar, but not significant, trend was found in the subsoil samples for cation-exchange capacity (CEC) and base saturation.

The impact of F. albida on mine-spoil soil fertility is variable and not as obviously beneficial as suggested by Alexander's (1989) pilot study. Only improvement in topsoil potassium was noted. However, Plateau soils tend to have sufficient potassium for crop growth.

Social Issues and F. albida

Field Methods

Studies involved a combination of individual semi-structured interviews, group discussions, and preference ranking with both male and female farmers in the areas where the soil samples were taken. Preference ranking involved 1) asking which trees grow in and around their farms, and 2) pairwise comparison of each tree species. Farmers were asked which of each pair they preferred and why. Subsequently a ranking was found and the reasons for preferences were listed (Table 1).

Table 1. Farmers' order of preference for trees found on farms in Du village, Jos, Plateau State, Nigeria, 1990.

Results and Discussion

Known as gao in Haousa and sina rogo in Berom (the predominant tribe in the tin-mining region), F. albida is locally regarded as useful as a raw material for firewood, fencing, making mortars, medicine, fodder (both pods and leaves) and as a source of shade in the dry-season. Moreover the tree "produces fertility" (taki in Haousa) and "makes the soil soft" (laushe in Haousa).

Grasses are observed by farmers to grow well under gao. This led one group of farmers to cultivate yams under and around the tree on old tin workings, believing that the presence of grasses indicated sufficient soil fertility for yams production. Planting was initially on a small scale, as an experiment (gwada in Haousa), which, if successful, would lead to expanded cultivation, or, if not, other crops will be tried.

One dry-season farmer said he was very pleased with growth of tomatoes around a gao which he attributed to the taki that the tree puts into the soil. He had lopped all the branches at head height (later used for firewood) to avoid getting thorns in his foot and to reduce shade on his crops. Many farmers, however, associated luxuriant vegetative growth under gao with poor production of Digitaria exilis, or hungry rice (known locally as findi, or fonio), and tomatoes. Other farmers noted that the trees' roots can make cultivation difficult.

F. albida is not regarded as an "economic" tree by local farmers. Rather, it is seen as a liability on their farms. Farmers do not plant the species or protect it. Some trees are felled, despite fear of sanctions from the State Forestry Department. Dry-season farmers probably gain little beneficial effects of leaf fall that occurs early in the rainy season. Instead, they are troubled by shade effects.

F. albida is perceived as having the best impact on soil fertility and E. camaldulensis as having the worst. Farmers say that crops always do badly adjacent to E. camaldulensis and vegetative cover there is always very poor. However, one farmer stated that, although F. albida gave the most taki and that E. camaldulensis was detrimental to the soil, he preferred the five tree species found on or near his farm in the following order: mango, guava, E. camaldulensis, F. albida, and E. torelliana. Mango and guava were ranked top because they produce fruit. E. camaldulensis was preferred over F. albida because of its ability to produce more firewood. E. torelliana was least preferred as it was seen as a poor producer of firewood without the redeeming features of F. albida with respect to production of fodder and medicine.

The two species consistently ranked last were F. albida and E. camaldulensis. The main reason for preferrence of other species was that they provide human food. The most preferred species were those that produce edible oil. It seems reasonable to infer that a reclamation strategy using F. albida would probably not receive much support from farmers.

Conclusion

Following the pilot study, investigations were carried out to clarify the potential utility of F. albida for the improvement of mine-spoil soils. The study took a human ecological approach included both natural and social scientific perspectives. Soil studies found more variable effects of F. albida on mine-spoil than the pilot study had inferred and less obvious benefits, except for significant improvement in topsoil potassium levels.

Benefits associated with greater efficiency of water use in crop/tree systems in semi-arid areas are probably not so applicable on the Jos Plateau. The climate of the Jos Plateau is less harsh than that of the Sahel. This means that the region is more conducive to growth of a wider range of economic plants.

Consequently, the comparative advantage of F. albida are not so apparent. Further, F. albida is not regarded as an "economic" tree by farmers and its utility is ranked low in comparison to other trees. Therefore its use in mineland reclamation is unlikely to be supported by these farmers. It is important to search for other alternatives for soil improvement on mineland that fit in better with farmers' resource priorities.

References

Alexander, M.J. 1989. The effect of Acacia albida on tin-mine spoil and their possible use in reclamation. Landscape and Urban Planning 17:61-71.

Charreau, C., and Vidal, P. 1965. Influence de l'Acacia albida Del. sur le sol: nutrition minérale et rendements des mils Pennisetum au Sénégal. Agronomie Tropicale 6-7:600-626.

Hesse, P.R. 1971. A textbook of soil chemical analysis. London, UK: UK Murray. 520 pp.

Phillips-Howard, K.D. 1989. Synthesis of research findings and recommendations of Phase I. JPERDP (Jos Plateau Environmental Resources Development Programme) Interim Report 13. Durham, UK: Department of Geography, University of Durham. 26 pp. (Limited distribution.)

Wimbush, S.H. 1963. Afforestation of restored tinmining land in Nigeria. Commonwealth Forestry Review 63:255-262.

Footnote__________

1 Jos Plateau Environmental Resources Development Program, Department of Geography and Planning, University of Jos, PMB 2084, Jos, Nigeria.

2 Department of Geography, Durham University, South Road,Durham, UK.

3 Department of Geography, Durham University, South Road, Durham, UK (University of Jos, PMB 2084, Jos, Nigeria.)

Kidd, A.D., Alexander, M.J., and Phillips-Howard, K.D. 1992. Faidherbia albida on mine spoils in the tin mining region of the Jos Plateau. Nigeria. Pages 177-180 in Faidherbia albida in the West African semi-arid tropics: proceedings of a workshop, 22-26 Apr 1991, Niamey, Niger (Vandenbeldt, R.J., ed.). Patancheru, A.P. 502 324, India: International Crops Research Institute for the Semi-Arid Tropics; and Nairobi, Kenya: International Centre for Research in Agroforestry.