Another word for this is ‘soil conditioner'. The term ‘soil improver' in the UK also includes materials used as surface mulches such as bark, used to prevent weed growth and to conserve soil moisture.
Soil improvers are generally used to improve the soil fertility by increasing the soil organic matter level. They improve the workability of the soil over time, for example making heavy clay soils more manageable and they can be used to improve the water holding capacity of light soils. This is becoming increasingly important in many parts of the country with recent dry years and restrictions on water use. Some soil improvers, such as manure or spent mushroom compost-based products, also contain significant amounts of plant nutrients.