Investigating Horticultural Applications of Liquid Digestate

Escott, Tommy and Dodd, Ian and Haygarth, Philip (2023) Investigating Horticultural Applications of Liquid Digestate. Masters thesis, Lancaster University.

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Abstract

Background. Sixty percent of global crop production depends on inorganic fertiliser, supporting nearly half of the world’s population. However, this pro- duction accounts for 1.6% of global CO2 emissions and is based on increasingly unreliable, finite resources. Thus, there is an urgent need to find sustainable alternatives. Digestate, a by-product of anaerobic digestion, can be used whole as a fertiliser, or separated into solid and liquid fractions. Liquid digestate is far more voluminous than solid digestate, making it difficult to store. While digestate has been used in broadacre agriculture for decades, its use as in horticulture is not widely studied. 1Up Natural, a British biogas company, produce separated digestate fertilisers from a plant-based feedstock, for retail in domestic horticulture. On account of liquid digestate’s high volume, it is pertinent to evaluate it as a replacement to synthetic fertilisers in horticulture. Methods & Results. To assess liquid digestate’s application as a fertiliser, a series of progressive pot-based trials were designed. Firstly, liquid digestate was compared to industry-leading synthetic fertilisers at half and full manufacturer’s recommended concentration. Synthetic fertiliser outperformed liquid digestate at the recommended concentration, which was attributed to differing nutrient levels. Secondly, liquid digestate was matched for nitrogen against a synthetic fertiliser. Again, synthetic fertiliser increased plant growth relative to liquid digestate, likely due to limiting influence of non-matched nutrients. Finally, liquid digestate was applied at different concentrations in a sand-culture experiment, to establish its suitability as a sole nutrient source. Liquid digestate induced NH4+ toxicity and deficiency in other nutrients. Conclusions. Based on these experiments, it is apparent that liquid di- gestate must be altered to be suitable as a sole nutrient source or alternative to synthetic fertilisers in low-nutrient mediums. However, liquid digestate may be suitable for application in high-phosphorus soils to prevent excess phosphorus application.