The Government of Rwanda would like to improve farming livelihood through the sustainable production of high quality coffee. This would be a win-win situation because improving farming livelihoods also provides increased export earnings. In order to fulfil this, the Government of Rwanda needs to design and support policies which sustain this growth. This can be achieved through an improvement programme of soil fertility and reduction of soil erosion, among other strategies. The programme should identify suitable areas for Arabica coffee production and assess sustainable land management technologies to improve soil fertility and reduce soil erosion to create conditions for optimizing production of high quality coffee. This PhD project contributes to those objectives. Experimental field measurements were conducted to assess the effects of mulch application on improving soil properties and reducing soil erosion, thus positively impacting yields. Assessments were made for three agro-ecological zones of Rwanda, namely the highlands, the central plateau and the shores of Lake Kivu.
The thesis covers a GIS Spatial analysis of coffee production systems at national level (chapters 2 and 3); at system and farm levels. It assessed the effects of best practices on soil erosion and soil fertility (chapters 4 and 5); and then it recommended best upscaling practices for a country level (chapters 3, 4, 5 and 6). Chapter 2 also reviewed coffee growing conditions, systems and soil management practices in Rwanda. Data showed that growing conditions are confined to an altitude range of 1,400 - 1,900 m a.s.l., an annual rainfall regime of 1,500 - 1,600 mm, a temperature class of 18 – 22 °C, and an average sunlight amount of 2,000 - 2,400 hours per year. Coffee is mainly grown along the shores of Lake Kivu, on the plateau in the central part of Rwanda, and in the Mayaga region located in the Southern-East part of the country. In these areas, coffee is mainly cultivated on moderate slopes and soils with low fertility. Coffee requires deep and finely-textured soils, which are well drained, aerated with moderate acidity (i.e. pH of 4.5 - 6.0) and moderate-to-high fertility. Many of the coffee farming lands in Rwanda do not accommodate these ideal conditions, e.g. steep slopes or scattered coffee plots intercropped with annual crops. Steep slopes, in particular, deplete soil fertility through soil erosion. The thesis looked at how mulching can assist with erosion control, improve soil properties and boost yields in such farming situations.
Chapter 3 assessed suitable areas for sustainably growing Arabica coffee, potential production zones, their productivity levels and the prediction of potential coffee yields. Results showed that coffee production zones cover about 32,000 ha, or about 2.3% of all cultivated lands in the country. About 21% of the country has a moderate yield potential, ranging between 1.0 and 1.6 t coffee ha-1, mainly around the Central Plateau; 70% has a low yield potential (< 1.0 t coffee ha-1) mainly in the Eastern semi-dry lowlands and very cold highlands of the North-West of the country. Only 9% of the country has a high yield potential of 1.6 to 2.4 t ha-1, particularly around the shores of the Lake Kivu. The results have some variances depending upon the location and variable conditions in terms of agro-ecological zones.
Chapter 4 discusses the positive effects of different types of mulch. Again, the success of each area showed to be site-specific, with significant effects using a mixture of mulches composed of crop residues applied at a rate of above 20 t ha-1, where soil erodibility was reduced significantly (up-to 91%). Clay content and silt particles are also noted to play a part. In fact, the association of soil organic carbon with clay and silt particles may be related to the amount of organic carbon that can be mobilised to saturate the clay and silt fractions on a particular soil texture.
Chapter 5 reports the positive effects of mulch on availability of soil nutrients to improve soil fertility and coffee yields. The amount of nutrient contents released after mulching was regulated by the amount of mulch biomass, the quality and type of mulch used, the climatic conditions and soil properties. Significant effects were obtained with a mixture of different types of mulch applied at a rate of above 20 t ha-1. Success results were also site-specific and additional application of inorganic fertiliser was recommended to obtain coffee yields of above 1.9 t ha-1. The inorganic fertiliser rates differ for each agro-ecological zone and soil type. The sustainability of coffee productivity could be ensured by intensifying the use of mineral fertilisers, but there are limited financial resources.
Chapter 6 presents a synthesis of our research findings and conclusions of the previous chapters. It highlights sustainable best land management practices that help to improve and optimise coffee productivity in smallholder farming systems. This chapter also presents key policy strategies and recommendations to improve coffee productivity of smallholder farmers in Rwanda. Besides the research results, conclusions and recommendations, the chapter highlights also a number of topics that remain to be addressed in order to understand more clearly the effects of mulching on soil fertility improvement and reduced soil erodibility, such as: (i) predicting coffee productivity through mapping of soil fertility indicators, (ii) modelling soil erosion of scattered smallholder coffee farms at micro-catchment level, and (iii) assessing the quality and the effects of the organic mulches applied in small scale coffee farming systems, accounting for related costs and benefits for the farmer.