Investigating Food Waste Diversion and Compost Use in Urban Agriculture for Soil Management in Mwanza, Tanzania

Field: Environmental Science (Physical Sciences)

African countries are urbanizing at a rapid rate. Studies on African agriculture have focused on rural food production, but research on urban agriculture may be key to ensuring urban food and soil security. In this dissertation, four studies were conducted to evaluate pathways for integrated soil fertility management, considering both social perspectives and nutrient management. The aim of the first study was to understand the social component of urban agriculture in the rapidly developing city of Mwanza, Tanzania. Semi-structured interviews were conducted using judgement and snowball sampling with urban farmers (n=34). Questions included their reasons for engaging in urban farming, their cultivation practices, perspectives on soil management, and the constraints of urban farming. Qualitative analyses using coding software showed that urban farmers range in age and gender, as well as in experiences and cultivation practices. However, all participants reported farming manually using hand tools. Maize was the preferred crop among urban farmers during the rainy seasons, though a disease affecting maize crops has added uncertainty to continued maize cultivation. Where irrigation is possible through ponds, wells, springs, streams, or the lake, irrigated crops are primarily leafy greens and vegetables, including cabbage (Brassica spp.) and amaranth greens (Amaranthus var.). Reasons for cultivating ranged from family sustenance (44%), to selling for income (15%), and even tradition, enjoyment, or exercise (39%). A majority of respondents (82%) would like to improve their soils. Most use manure to improve their soils and many (32%) believe that animal manure is the way to improve soil. Additionally, most (62%) urban farmers have not tried any form of food waste compost but responded positively to try it if they had access to and were taught how to use it. The study concludes that urban agriculture is an integral social and economic aspect of Mwanza City. The objective in the second study was to investigate the current waste management practices in the hospitality sector of Mwanza, Tanzania and propose a composting method for urban organic (food) waste that would be suited to the city’s current waste management strategy. Questionnaires were used to survey restaurants (n=30) and hotels (n=20) around Mwanza, Tanzania on current waste management practices and opinions on organic waste. Over 50% of the waste generated by the hospitality sector in Mwanza, Tanzania is organic, and for a majority of restaurants (60%) and hotels (65%), organic wastes are already separated from other wastes before collection. However, the wastes are mixed in preparation for collection. Direct observations of the city’s waste management strategy gave insight into waste collection, waste transfer, and final waste disposal. Wastes were collected by waste workers primarily using trolleys and barrows. Mixed wastes were gathered at waste transfer points within the urban city centre in large, blue containers. The containers, once full, were transported by trucks to the land dump in a rural ward. A rapid composting experiment was then designed considering space, cost, and skilled labour constraints, and conducted over a one-month period. The proposed rapid composting method reduced the volume of organic waste by more than 80% within one month. The resulting products were a compost with a C:N ratio of 14:1 that can be used as a soil amendment, albeit with higher application rates to meet crop nutrient needs, and black soldier fly larvae, which can be used feed for ducks, chickens, pigs, and other livestock. In the third study, the effects of organic and inorganic amendments on soil and crop growth were evaluated and compared over a short-term growing period. A complete randomized design was used to assign, in triplicate, poultry manure, inorganic nitrogen fertilizer, two types of food waste compost, and control treatments. Soil samples were taken after amendment application (before planting) and after harvest and analyzed for physical (bulk density and soil water holding capacity), chemical (organic carbon, nitrogen, phosphorus), and biological parameters (soil microbial biomass-carbon). Amaranth greens (Amaranthus var.) and cabbage (Brassica oleracea var. capitata) were planted for two short growing periods, and their growth heights were monitored and compared between the treatments. Organic amendments (poultry manure and food waste compost) improved the water holding capacity of the soil by 14 to 19% and enhanced microbial biomass 1.7 to 4 times the inorganic nitrogen fertilizer treatment. Additionally, crop growth under organic amendments was comparable to crop growth under inorganic nitrogen fertilizer. In fact, inorganic fertilizer, when applied incorrectly, has detrimental effects on plant germination, increasing risks for urban farmers who do not receive proper instructions for use. The fourth study explores future trends in urban soil organic carbon, comparing the effects of organic and inorganic amendment use for the next 100 years. CENTURY soil organic model was used to simulate historical conditions and land management leading to the current conditions using geographical information, local weather data, and soil parameters to align the model with Mwanza, Tanzania. Characteristics of four amendments (food waste compost, poultry manure, ash, and inorganic nitrogen fertilizer) and their application rates and frequency were added as parameters to simulate future trends in soil organic carbon. The greatest changes in soil organic carbon in 29 years under annual application were in food waste compost (+4002 gC/m2) and poultry manure (+3203 gC/m2) compared to ash (+2381 gC/m2), and lowest with inorganic fertilizer application (+518 gC/m2). Without any amendment the change in soil organic carbon in 29 years was -1154 gC/m2. Additionally, organic waste amendments enabled highest accumulation of resistant carbon fraction (slow C), which is lost over time after natural land conversion to agricultural land use. Therefore, organic waste amendments can increase soil organic carbon through urban agriculture and counter the impacts of land conversion. The recommendations from the research are that land zoning policies in developing cities, such as Mwanza, in Tanzania, and more broadly in sub-Saharan Africa, should support urban farming practices and use of organic waste amendments to maximize the benefits for food security and reducing environmental impacts of cities. Additionally, the tested rapid composting method demonstrated that, as a decentralized, small-scale system, it has the potential to be implemented at waste transfer points located around the city. While it may not have the capacity for processing all of the organic waste produced in the city, it can contribute to training waste workers as a pilot-scale system, and the skills can transfer to a larger-scale, centralized organic waste diversion and processing strategy.