I quite liked the God compost, probably good stuff. Three of the four plants like acid soil, rhododendron, Acer and slightly acid roses. Hydrangea will cope in both Acid and Alkaline soil giving either a blue or pink flower depending on the soil. Unless your soil is acid pH under 7, you may need to raise the acidity to lower the pH, you could introduce ericaceous compost Improving the soil, means improving the soil structure and will depend upon the soil conditions and may sometimes require grit, sand, organic matter compost, and sometimes additives to lower or raise the pH.