Soil improver
Any substance dug in to improve soil structure. This is generally organic matter, such as animal manure, garden compost, or leafmould, but could be an inert substance such as lime or gypsum. Synthetic soil stabilisers (e.g. polyvinyl alcohol, cellulose acetate, and polyacrylamide) are used commercially to prevent soil capping (formation of a surface crust on the soil when drying after heavy rain).