Valuing Waste – A Multi-method Analysis of the Use of Household Refuse from Cooking and Sanitation for Soil Fertility Management in Tanzanian Smallholdings

Abstract

The starting point of this work is the intention of two farmers’ initiatives to disseminate locally developed and adapted cooking and sanitation technologies to smallholder households in Karagwe District, in northwest Tanzania. These technologies include improved cooking stoves (ICSs), such as microgasifiers, and a system combining biogas digesters and burners for cooking, as well as urine-diverting dry toilets, and thermal sterilisation/pasteurisation for ecological sanitation (EcoSan). Switching to the new alternatives could lead to a higher availability of domestic residues for soil fertility management. These residues include biogas slurry from anaerobic digestion, powdery biochar from microgasifiers and sanitised human excreta from EcoSan facilities. Such recycling-driven approaches address an existing problem for many smallholders in sub-Saharan Africa, namely, the lack of soil amenders to sufficiently replenish soil nutrients and soil organic matter (SOM) in soils used for agricultural activity. This example from Tanzania systematically examines the nexus of ‘energy-sanitation-agriculture’ in smallholder farming systems. The short-term experiments demonstrated that all soil amenders that were analysed could significantly enhance crop productivity. CaSa-compost – the product of co-composting biochar with sanitised human excreta – quadrupled grain yields. The observed stimulation of crop yield and also plant nutrition is attributed to improved nutrient availability caused by a direct increase of soil pH and of plant-available phosphorus (P) in the soil. The assessment of the lasting soil implications revealed that CaSa-compost and biogas slurry both show the long-term potential to roughly double yields of maize. Corresponding nutrient requirements can be adequately compensated through residue capturing and subsistence production of soil amenders. The potential of CaSa-compost for sustainable soil fertility management is superior to that of standard compost, especially with respect to liming, replenishing soil P and restoring SOM. Biogas slurry, however, yields inferior results in all aspects when compared to compost amendments.