Eggshell hydrochar application to enhance soil fertility and promote plant growth

Eggshells, a common kitchen waste, are produced and discarded in hundreds of thousands of tonnes worldwide without utilisation. The production and application of hydrochar from eggshell waste can recycle this biomass and serve as a soil amendment to enhance soil quality. However, the impact of eggshell hydrochar on plant growth is still not well understood. This study aims to investigate the effects of eggshell hydrochar at various application dosages on soil fertility and plant growth. Completely Decomposed Granite (CDG), a prevalent soil type in Hong Kong, was used as the test soil. Eggshell waste was collected and pyrolyzed at 200°C with a biomass-to-water ratio of 1:2 to produce the hydrochar for testing. CDG was mixed with eggshell hydrochar at concentrations of 0%, 5%, 10%, and 25% (w/w) for planting Brassica rapa seeds. During the 20-day plant growth period, the germination rate was recorded. The shoot height and leaf area were measured using ImageJ based on photographs. Root surface area was assessed at harvest using an EPSON scanner to evaluate root development. Additionally, the plant-available nutrients, including phosphorus (P), potassium (K), nitrogen (N), and dissolved organic carbon (C), were determined to assess soil fertility under different hydrochar application dosages. The water-holding ability of the soil treated with eggshell hydrochar was analysed. The results provide valuable insights into the application of eggshell hydrochar as a recycled soil amendment to improve the soil-plant system. This research presents a sustainable solution for low-carbon strategies in eco-geotechnics.