Preparing the soil in any garden, particularly a sustainable garden, is the most important thing. It’s about striking a fine balance between really good drainage and water retention. There are 3 soil types - sandy, clay and loam in the middle. Loam is the perfect soil, well drained and it holds the water. Sand just drains water too fast and doesn’t hold the water. Clay soil just gets waterlogged.
To improve sandy and clay soils add as much organic matter as you can, as it’s better and greener to cope with the soil that you have rather than removing and replacing all the soil. By adding organic matter such as compost and/or manure it not only helps balance your soils water retention, it also adds much needed nutrients to aid in the growth of your plants.
It’s also a good idea to incorporate the use of water crystals in the soil that store and release water over a period of time.