Burnt turf the type used in old fires produces ash thats a great source of potash and trace elements for feeding you plant or for use in a potting compost. Its high in potash and can also be used as a top dressing for lawns. As a top dressing for lawns you need to spiking the lawn first with a garden fork if the area is not to large this will help the nutrients to travel down under the main layer of turf and encourage the roots to root deeper