Bed Prep Is Critical!
Soil is the foundation of all great gardens. The smaller the plant will be at maturity, the more critical it is that you give it ideal soil. That’s essential for annuals and perennials, vegetables, groundcovers and small shrubs.
It’s much more difficult (and expensive) to do "heroic" soil preparation for large shrubs and shade trees. In those latter cases you simply need to concentrate on types that are perfectly suited to your native soil.
Organic matter is the best way to improve any soil, whether sandy or clay. It helps sand hold moisture and nutrients, and it breaks clay into a more permeable structure.
Organic matter comes in many different forms. It’s best to use a blend of many different types so that you can get the combined and long-lasting effects of them all. Your choices include finely ground bark mulch, Canadian peat moss, compost, rotted manure and even freshly shredded tree leaves (listed in decreasing order of longevity in your soils).
Flower and vegetable gardens should receive 4 to 6 inches of organic matter, and it should be tilled 12 inches into the existing soil. Groundcover beds will prosper with 2 to 3 inches tilled into the top 8 inches of soil.
Texas A&M has also had very good results using expanded shale as a soil amendment in clay soils. It takes the place of the less successful washed brick sand. My recommendation would be that you work 1 inch of shale in with the organic matter, but only for clays. Neil