The amount of amendment added during planting depends entirely on the type of soil you have and what type of plant you are planting.
Xeric & native plants will not require much amendment upon planting, but it is better to add too much than none.
Each plant has a specific soil recipe that they prefer. Note plant preferences while considering what type of amendment is right for your project. Different combinations will create a unique and occasionally detrimental effect.
Some amendments are better suited for specific goals and used in enclosed environments like pots.
Clay is the predominant soil composition we deal with in the Southwest, which is why the art of pottery is associated with our region.
While it can present difficulties (extremely heavy soil with minimal drainage or aeration), with a bit of extra work you can still create your perfect paradise.
Many of our native plants are accustomed to clay and may even prefer it due to its ability to hold moisture during drought periods. Many of our favorites, including Lavender, can grow successfully with minimal amendment to the natural soil. *NEVER add sand to clay soils to increase drainage: Clay + Sand = Cement!!