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Rheinischen Friedrich-Wihelm-Universität Bonn (2007)

Spatially explicit modeling of sorghum Sorghum bicolor (L.) Moench) production on complex terrain of a semi-arid region in Ghana using APSIM

Kpongor, Dilys Sefakor

Titre : Spatially explicit modeling of sorghum Sorghum bicolor (L.) Moench) production on complex terrain of a semi-arid region in Ghana using APSIM

Auteur : Kpongor, Dilys Sefakor

Université de soutenance : Rheinischen Friedrich-Wihelm-Universität Bonn

Grade : Doktor der Agrarwissenschaften 2007

Résumé
An increasing human population and decreasing fallow periods have resulted in a rapid decline in soil productivity in the semi-arid region of Ghana, which is characterized by low-input subsistence agriculture. Soils are inherently poor and contain little to support crop production. Attempts by smallholders to increase production have resulted in the concentration of nutrients in the homestead fields through the use of animal manure and crop residues from the distant bush farms. This has contributed to spatial variability in soil nutrients and soil organic carbon (SOC). The study area was classified into land-use trajectories based on a rural rapid appraisal technique with the aid of the farmers in the community and by remote sensing quick-bird imagery. The influence of land-use trajectories on soil nutrient stocks was evaluated. Spatial distribution of soils and soil properties and the factors influencing their distribution were assessed in a landscape of 1.5 km2 selected within the study area. Data on soil chemical and physical properties collected were analyzed with geostatistical techniques for their spatial dependency. The Agricultural Production Systems sIMulator (APSIM), a crop simulation model, was calibrated for sorghum (Sorghum bicolor (L.) Moench) and evaluated for yield response to inorganic nitrogen (N) and phosphorous (P) fertilizer treatments in two farm types (homestead fields and bush farms). Land-use trajectories are revealed to have influenced the nutrient stock of the soils in the study area. Furthermore, the impact of farmers’ management activities on nutrient stocks was significant. Though a non-parametric test revealed distinct soil types, considerable variability could be observed within individual soils based on their chemical and physical properties. The distribution of soil parameters in the selected landscape was influenced by the soils, farmers’ management practices and topography. APSIM predicted the grain yield response of sorghum to both N and P application with an overall modified internal coefficient of efficiency of 0.64. A gradual decline in grain yield was observed over the 29-year simulation period in both the homestead fields and the bush farms, with yields being much lower in the latter. If crop residues were returned to the fields, half the mineral N fertilizer was needed in the homestead fields to produce the average grain yields produced on the bush farm with full fertilization. Temporal variability in grain yield was consistently higher with the removal of crop residues, irrespective of farm type. APSIM is responsive to both organic and inorganic fertilizer applications in the study area and also highlights the essential role of crop residues and inorganic fertilizer in influencing the temporal variability in sorghum grain production and hence the impact of farmers’ management practices on food security. This is evident in the rapid decline in soil organic carbon accompanied by a decline in grain yield after 29 years of cropping. The use of inorganic fertilizer and incorporation of crop residues (organic matter) are critical for attaining food security in the study area.

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