Effects of Hydrothermal Carbonization Conditions on the Characteristics of Hydrochar and Its Application as a Soil Amendment: A Review

Hydrochar (HC) has received increasing attention due to its potential impact on soil amendment. The process parameters of hydrothermal carbonization (HTC) have a significant impact on the characterization of HC, and HC as a soil amendment has effects on soil properties. Therefore, this work summarizes the effects of feedstock type, temperature, residence time, and solid–liquid ratio on the characteristics of HC, and analyzes the effects of HC on soil by HC addition. The feedstock type determined the characteristics of the HC. Temperature had the greatest effect on HC properties, while residence time had a similar but smaller effect than temperature had. The residence time did not affect the characteristics of the HC when HTC was carried out to a certain extent. Solid–liquid ratio determined the characteristics of HC, but research in this area is lacking. Moisture is a crucial factor in plant growth. Compared to other soils, HC significantly increased the water-holding capacity (WHC) in sandy soils. HC is rich in nutrients and micronutrients, making it potentially useful as a fertilizer. The toxicity of HC may inhibit plant growth but may be ignored for poor soil. This review aims to provide recommendations for HC as a soil amendment and to identify areas where further research is needed.