Amending and Enriching the Bed
I recommend a combination of organic and inorganic amendments when amending beds. Different soil types require different amendment combinations, whether your soil is clay, sand or caliche.
Organic compost, or composted leaves, garden and grass clippings, composted manure, kitchen vegetable scraps, shredded newspapers, humic acid, alfalfa pellets, cottonseed hulls or whatever composted material is the best. It is better to use a combination of composted plant and animal (composted manure) matter, not using any more than half of the organic matter as composted manure in any one season.
Depending on the size of your flower bed or border, you may want to consider buying good quality compost in bulk, rather than by the bag. And please keep in mind the nutritional content of compost varies, depending on the mixture of composted materials. Temporary nitrogen deficiencies can be created by the addition of too much high carbon material such as sawdust or wood chips. Carbon is the main food of soil organisms, and nitrogen is necessary for the soil organisms to ingest the carbon. However, after the microbes die and release their nutrients into the soil, the balance is restored.
If you have heavy clay soil, inorganic amendments should be added for better long-term drainage in addition to the organic amendments. I have added sand to clay to provide better tilth and aeration, but never alone -- always with plenty of compost. Since then, other inorganic amendments have become available that are better than sand in retaining nutrients, water and providing pore spaces for air. In 2002, my xeriscape garden was filmed by for a short 5 minute video entitled How to Prepare Soil for a Xeriscape Bed for the Southwest Yard and Garden Show, produced by New Mexico State University, Las Cruces. At that time, I wasn't using the inorganic amendments, but as soon as I learned about them, I switched from sand to Turface® and expanded blue shale.
They are calcined clay (sold under the name Turface®), zeolite (Ecolite™) expanded blue shale (Tru-Gro™) and diatomaceous earth (Axis™). A calcined clay amendment for sandy soils is marketed under the trade name Profile. These inorganic amendments provide aeration, water and nutrient holding capacities in compacted soils. This results in long-term increased drainage of your soil, along with a reduction of water use and feeding (less leaching of nutrients beyond the root zone). Other inorganic amendments are rock dust, lava sand, glass sand, granite sand, greensand, and humates. Of the inorganic amendments, coarse sand is the least effective, and also least expensive. Work amendments in to a depth of 6-12 inches.
Greensand, granite and lava sand will slowly add trace minerals to your soil. If you choose sand, use sand that does not include lime for our alkaline soil.
Your choice of plants determines how much compost you add to the bed; however, a minimum of 3 inches organic compost should be used. Cost of materials may be a constraint.
High water-use plants are generally high feeders. In high water-use beds, amend with 4-8 inches of composted organic matter. Incorporate this into a depth of at least 12 inches. The more compost used, the deeper it needs to be mixed into the existing soil.
Medium water-use, or mesic, plants require from 3-6 inches of composted organic matter, preferably worked into the soil to a depth of 12 inches.
Low water-use beds don't require as much; xeric plants are not heavy feeders. In any case, your soil should be worked down to a minimum of 4 - 6 inches; deeper to the top 8 - 12 inches of soil for better results. Xerophilic plants -- plants that are adapted to dry conditions -- often can survive lower temperatures if the soil is kept dry in the winter. Wet soggy soil with kill your plants faster than drought, especially in the cold, moist months of winter.
Drainage is Important
Good drainage will also keep your plants from becoming waterlogged and dying if we are treated to an extended rainy spell. The roots of plants need oxygen in the soil; proper drainage provides spaces for the oxygen. Under waterlogged conditions, plants die from lack of oxygen; the water is not able to drain out quick enough. Plan to use enough compost or compost and inorganic amendment so you can create mounds, which add visual interest and also create different micro niches and better drainage.
These tiny differences in drainage, aspect and slope will enable you to plant a wider variety of plants successfully. I like to look at preparing the soil the same as a builder looks at laying a foundation of a home. If you take the time to do it properly, far fewer problems will be encountered later.