You may already know this, but sandy and clay soils are VERY different. Near our house we have both soil types and I killed more than a few plants before I figured out how to improve my planting success rate. The most effective strategy I have found for cultivating healthy plants was to know the soil (experience and testing) and employ planting depth to manage the natural drainage.
Sandy soils may lack a broad spectrum of nutrients and very often drain much too quickly for plants to capture as much water as they need to get established during their first critical growing season. So, when I encounter sandy soils I plant "full-depth" with the top of the root ball resting at the same level as the existing soil surface. Then I build a ring of compacted soil around the hole to capture and retain more water like a catcher's mitt. Once the plant is installed and carefully backfilled to eliminate air pockets I place a high-grade mulch 2" thick inside the ring to slow down evaporation.
Clay soils are usually rich in nutrients, but they drain very slowly. If you plant at full depth then the hole you place the root ball in will likely fill with water, hold it too long, and drown the new planting. To mitigate this problem I build mounds of good soil and never plant the root ball deeper than 50% below the surface of the existing clay soil surface. In fact, I have had good success without digging a hole at all! I place the root ball on the existing ground, and mound new dirt up to the top of the root ball and in a wide circle at least 4x the width of the rootball. Finally I cover the whole shebang with shredded hardwood mulch that will not wash away as easily as bark nuggets.
The symptoms of over-watering and under-watering often look the same. Only with careful management of the soil's drainage can you reliably mitigate these hazards.
I hope this method helps somebody! Let me know what you think.
