Voacanga Africana farming system in the Assin South District: Socio – economic and soil nutrient implications

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Assessing the impact of conversion of forest to agricultural land use and its induced changes on soil fertility properties is essential in addressing problems of ecosystem transformation and sustainable land-use. Adopting a case study approach, this study was conducted to assess Voacanga africana farming system in the Assin South District: socio-economic and soil nutrient implications in the Central region of Ghana. The specific objectives of the study were to identify management practices of farmers in Voacanga production systems and any potential for improvement, identify socio-economic importance of Voacanga in sustaining livelihoods of rural households and challenges along the production–marketing chain. The study also assessed the impact of Voacanga africana cultivation on soil physico-chemical properties and its potential for soil carbon sequestration along a chronosequence of secondary forest, 5, 10 and 15-year-old Voacanga farms. Data for the study was obtained through the administration of structured questionnaires involving 60 farmers, key informants interviews and field observations. Changes in soil physical (particle size distribution or texture and soil bulk density) and chemical (pH, soil organic carbon, total nitrogen, phosphorus, exchangeable cations and ECEC) properties were assessed through the collection of replicated soil samples at 0-10 cm, 10-30 cm and 30-50 cm soil depths and subsequent laboratory analysis of the samples. The study identified farmers to be using spacing of 4.2 m x 4.2 m, 6 m x 6 m and 8.4 m x 8.4 m in Voacanga system. Weeds were controlled by manual and chemical methods with pruning been the main tree management practice. Increasing demand for Voacanga seeds and expected income from the sale of seeds is the most important economic factor motivating farmers to go into Voacanga cultivation. The main constraints facing farmers were poor marketing, farm theft, lack of technical information, lack of access to credit, problems of land acquisition and inadequate labour. The constraints facing local buyers on the other hand were lack of direct marketing links with companies, absence of standardised marketing board, insufficient labour for seed processing, transportation difficulty and lack of trust worthiness of some farmers. With respect to impact on soil properties, particle size distribution differed significantly along the chronosequence with clay percentage increasing with depth. Soil bulk density (kg m-3) similarly differed significantly among land-uses with Voacanga plots generally having higher bulk densities compared to secondary forest. Significantly, low soil pH was observed among land-uses with pH ranging from 5.1 to 6.6 in 10-year-old at 30-50 cm and 0-10 cm soil depths respectively. While mean soil organic carbon (SOC) concentration was not significantly affected at the studied depth (0-50 cm), total SOC stocks increased significantly at 5 and 15 year-old plots along the chronosequence. Total nitrogen increased with increasing age of Voacanga plantation at 0-10 cm soil depth but decreased with soil depth. Soil exchangeable Mg and K were low compared to Ca. Effective cation exchange capacity increased along different ages of Voacanga plantations. Overall, soil quality did not deteriorate over the studied duration. Under a changing climate, Voacanga system has the capability to sequester carbon. The study recommends that in future studies carbon stable isotopes technique can be included as a tracer of soil organic carbon, long term research has to be conducted into the economic analysis of yield and agronomic practices to generate information on management practice for Voacanga system.  
A Thesis submitted to the Department of Agroforestry, Kwame Nkrumah University of Science and Technology in partial fulfillment of the requirements for the degree of Master of Science in Agroforestry.