If you have a wet clay soil although its great for growing certain types of plants such as fast growing perennial plant such as Gunnera its not so good for a number of more delicate plants. One solution we often use is to create raised beds by adding grit sand and spent mushroom compost. Simply apply a layer and dig it into the existing soil to create a raised area, the area can in effect be a mound or it can be raised with a low wall created from stone or timber planks. The spent mushroom compost its great for breaking up clay soil as it contains lots of lime which breaks up the clay particles