The importance of planting well can not be too strongly emphasized. The better you plant, the more you will be rewarded. These high quality composts are similar to the compost we grow our plants in. They are much better than most of the so called ericaceous composts sold in garden centres which have been milled to pieces and have no structure.
We recommend 20 litres of compost per 5 or 7.5 litre plant on most soils. Then double it for larger plants and double again on poorer soils!