I have a beautiful hydrangea that was given to me by a neighbor, who grew it from a cutting of her own. Originally it was a deep pinkish-red; now the flowers are shading toward magenta to dark purple. It is growing in a mix of sand and well-composted road cuttings (got those free from when the Public Utility folks trim stuff back from the road so drivers can see better.) Quite a ways down, the soil base is sandstone and very heavy yellow clay. Are there any natural (or safe commercial) conditioners I can add to the soil to feed this hydrangea and support its pink-red color?