Top soil is the most nutrient rich layer of any naturally occurring soil, being the product of decomposition and broken down organic matter that has been reduced to a fine blend.
It shouldn't be the most nutrient rich part of a maintained garden however, which will contain compost and mulch, but it is great for filling out space or reinvigorating spent patches.
Use it to mix into beds/borders, leveling out or filling holes and it's usually a good idea to put some down before any turf is laid.
This 30 litre bag is full of Westland's highest quality, screen topsoil. Ready to go straight out of the bag.