Caliche: Love It Or Leave It
Recently, I had the good fortune to attend a geology workshop at the Appleton-Whittell Research Ranch. The course was taught by local geologists Jan Rasmussen and Sandy Kunzer. It was as interesting a two days as I've had in a long time. You really have to appreciate how geologists think. Something that happened 60 million years ago is a “recent event”. Makes you wonder what they would mean if they said they would be a “little late” for a meeting. By plant standards, a 200-300 year old tree is “ancient”. One of the subjects covered briefly was an understanding of how caliche is formed. Caliche is that beloved layer of whitish dirt-rock that occurs near or on the surface of much of the arid Southwest. Sometimes people refer to clay as caliche or claim that it is acidic. It's neither. Caliche is chalk-like and very alkaline.
Caliche starts to form when rain, which is acid by nature, falls on limestone. The acid rain dissolves some of the calcium in the limestone and runs off into the ground. This deposit builds up over time and becomes caliche. A geologist might refer to it as calcium carbonate. The question is what to do with it if you want to plant in that area.
The problem is too big to try to change the soil by amending it. Some local planting guides will recommend digging through it. Sounds good on paper. Unfortunately, some caliche layers can be very deep. I've looked down leach field trenches (10'-12') and not seen the end of it. I don't believe you are going to dig through that.
Probably the best idea if you need to plant in that area is to use alkaline tolerant plants. Cliffrose (Cowania stans) is a shrub that thrives in white dirt. That is where it prefers to grow. Emory Oak, Evergreen Sumac, One Seed Juniper, and Beargrass all occur naturally in caliche soils.
Sometime the rules are broken. Years ago I planted an apple tree near the crossroads in Sonoita. Almost pure white dirt. It should have struggled and shown deficiencies. Instead it has thrived and bears almost every year. From a nurseryman's perspective it doesn't make sense. Maybe I should go ask a geologist.