|Agro-forestry in the African Humid Tropics (UNU, 1982, 162 pages)|
|Considerations for the future development of agro-forestry|
Akinola A. Agboola
Department of Agronomy, University of Ibadan, Ibadan, Nigeria
G.F. Wilson and A. Getahun
IITA, Ibadan, Nigeria
University of Ibadan/llTA, Ibadan, Nigeria
Gliricidia septum is a small leguminous tree of tropical American origin. It is currently used on farmers' fields in several locations in Oyo State, western Nigeria, where it seems farmers value this introduced woody legume in restoring soil fertility. Despite its widespread use here, and farmers' acceptance, there is hardly any study or account of this woody legume, especially in this West African subregion. This paper reports the major observations made during a field survey in the Ibadan area. Leaf protein content was calculated to be 23.6 per cent and soils under G. septum fallow had higher nutrient status than comparable sites under natural bush fallow.
With increasing population, the traditional shifting cultivation system tends to break down, for the natural bush fallow becomes too short to fully restore soil fertility. A substitute system is therefore essential, and a planted tree fallow is one possibility. Leguminous woody plants such as Cajanus cajan, Tephrosia candida, and Leucaena leucocephala are fastgrowing and improve soil fertility in a shorter time than does the natural regeneration. Farmers in the Ibadan area claim that the small leguminous tree Gliricidia septum has the potential of maintaining and even improving land productivity under continuous arable cropping. The purpose of this study was to investigate that claim.
Gliricidia septum was introduced into Nigeria during the colonial days by the Department of Forestry. Its original use was for fences, but now it is also used as supports for yam vines, erosion control, shade in forestry and agricultural nurseries, feed for livestock, fuelwood, and as a soil improvement agent.
A field survey was made in a number of locations within Ibadan, including Polytechnic, Eleyele, Ahmadiya, Nihort, Jericho, the Forestry Research Institute, Apata, and Ring Road. Interviews and personal observations were the methods employed to determine how Gliricidia septum is used in the farming systems. Soil and leaf samples were collected, digested with H2SO4, and autoanalysed. Nitrogen obtained was multiplied by 6.6 to estimate protein content. In all, 14 soil samples were collected at 0-15 cm and were analysed for pH (Beckman pH meter with soil:water ratio 1:1), percentage of organic carbon (Walkley-Black method), percentage of total nitrogen (micro-Kjeldahl method), available phosphorus (Bray-1P), extractable cations (by 1 N NH4OAc), and total acidity (extraction by NaOH with phenolphthalin as indicator).
The farmer interviewed at Polytechnic said that the land had been continuously cultivated with yams for four years, and Gliricidia was used as support for the vines. It was then left under Gliricidia fallow for five years. Two soil samples were collected, one bulk sample from the fallow area (sample 1 ) and the other (sample 2) from an adjacent site cultivated with yams, vegetables, and maize.
Eleyele was a farm belonging to the Ahmadiya secondary school. Gliricidia was planted two years ago as a support for yams. Two soil samples, one (sample 3) from the farm and the other (sample 4) from nearby natural bush, were collected.
On a farm near Ahmadiya school, the land was reported to have been continuously cropped for three years without fertilizers. Two soil samples were collected, the first (sample 5) from this farm, where Gliricidia was being intercropped with maize and cassava, and the second (sample 6) from an area free of Gliricidia.
No farmer was found to be interviewed in Nihort. However, observations indicated that Gliricidia had been growing for one to two years, and it was assumed to have been used as yam stakes. One soil sample (7) was collected from the area under Gliricidia.
The farmer in Jericho claimed he had been using the land for the past 12 years to cultivate yam, maize, and vegetables. He said that he planted yams ten years ago using Gliricidia as stakes and that he prunes the Gliricidia periodically so that there is space to plant maize and vegetables. One soil sample (8) was collected.
Gliricidia was found to be on two fallows at the Forest Research Institute: one fallow was two years old and the other three years. Soil samples (10, 11, and 12) were taken from the sites under Gliricidia. One sample (9) was taken from the same area but from a different plot where Gliricidia is intercropped with yams, maize, and vegetables as follows: yam is planted in November with Gliricidia stakes serving as supports for the yam vines; maize follows in March after the stakes have been pruned; finally cassava is planted in July.
The farmer at Apata has cultivated the land continuously for three years, intercropping Gliricidia with maize, cowpeas, and cassava. He pruned the shrub before each planting. One soil sample (13) was collected. The location on Ring Road was a thick bush of Gliricidia, and many earthworm casts were observed. No farmer was interviewed but a soil sample (14) was collected. Leaves were collected from all sites and analysed for protein content.
Potential for Agro-forestry
The following facts about the legume emerged during the survey:
TABLE 1. Results of Soil Analvses for 14 Soil Samples
|Sample No.||pH||Organic Carbon (%)||Total N (%)||Available P (µg/gm)||Exchangeable Cations ( µg/100 gm )||Total Acidity (µg/100gm)||CEC (Meq/100 g)|
Results and Discussion
The results (table 1 ) indicate that soil under Gliricidia fallow is better than soils where Gliricidia has not been grown in terms of pH, organic matter, available phosphorus, and cation exchange capacity. They also show that the level of soil fertility increases with the length of the fallow period. Also the protein content of G septum leaves is relatively high, with values ranging from 20.65 to 27.39 per cent. The mean value of 23.6 per cent compares favorably with other woody legumes like Leucaena leucocephala (14.2 per cent) and Cassia spp. (12.6 per cent) (NAS, 1979).
IITA (1980) reported on the effects of planting maize and cowpeas between rows of Cajanus cajan, Gliricidia septum, Leucaena leucocephala and Tephrosia candida. The crops were intercropped in alleys 2.25,3.75, and 6.75 m wide. The results indicated that G. septum at an alley width of 3.75 m resulted in a maize yield of 5,055 kg/ha and a cowpea yield of 586 kg/ha. These were the highest figures recorded among all the yield data. No information was obtained on the amount of nutrients supplied by the shrubs in this experiment. In view of its potential in aiding continuous arable cropping as evidenced by these observations, Gliricidia deserves greater research attention. Further work should emphasize: