Resource Recovery from Waste - Investigating the Use of Digestate as an Organic Fertiliser for Nutrient Poor Soils

Agyabeng Fofie, Esther and Semple, Kirk and Martin, Alastair and Dodd, Ian (2025) Resource Recovery from Waste - Investigating the Use of Digestate as an Organic Fertiliser for Nutrient Poor Soils. PhD thesis, Lancaster University.

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Abstract

This thesis investigates the use of digestate, a by-product of anaerobic digestion, as a sustainable organic fertiliser in nutrient-poor soils, particularly in sub-Saharan Africa (SSA). Digestate has been researched for its nutrient content, notably its richness in nitrogen, which can replace or complement mineral fertilisers. Although digestate's effectiveness has been studied globally, limited research has been conducted in SSA. Through desk research and laboratory experiments, this thesis aimed to: (i) review the organic resources and impacts of digestate, (ii) characterise digestate and evaluate its use for the early growth of micro -Tom tomatoes in a laboratory greenhouse, (iii) test the impact of digestate on soils, and (iv) assess the effect of irrigation and different digestate application rates on plant productivity and yield of micro-Tom tomatoes. Key findings reveal that plants treated with digestate exhibit higher above-ground biomass than those in unamended soil, demonstrating improved nutrient uptake. Increased application rates of digestate also correlated with higher retention of available nitrogen in the soil. However, no significant differences were observed in microbial biomass, soil pH, electrical conductivity, or between various water application rates and digestate application strategies. Further detailed results from three experimental scenarios illustrate varying effectiveness. In the first scenario, digestate increased crop yields by 55-65% compared to unamended soil, though it was less effective than urea fertiliser. The second scenario showed that both digestate and nitrogen-phosphorus fertilisation raised nutrient levels, albeit without significant differences between application rates. The third scenario confirmed no significant impact of fertiliser type or water regime on yield outcomes. This study concludes that while digestate can enhance soil fertility and crop yields, its performance does not consistently surpass that of traditional mineral fertilisers.