It's always underfoot, takes a beating from our summer sun, and often provides material for jokes among gardeners.
"It" is our desert soil, and it's completely foreign to any soil gardeners may have worked with before.
Our arid Arizona soils, technically referred to as aridisols, are nearly indestructible, making gardening difficult.
One of the most obstinate aspects of dirt in the Sonoran Desert is the concrete-like caliche (pronounced kuh-LEE-chee). The name comes from the Latin root for calcium, and it is referred to as the calcic horizon by soil scientists. Calcium carbonate cements particles together, forming a rock-hard layer among sandier deposits of mineral soil. These layers of hardpan can be as much as 6 feet thick in Southern Arizona.
Caliche provides Southern Arizona gardeners with plenty to talk about: How deep it is found, how thick a single layer may be and how many layers exist are all factors that vary from place to place.
For several reasons, caliche is a hardship to gardeners. The most obvious is that tender plant roots can't penetrate the unyielding layers without help.
But there are even more complicated forces foiling our flora. Wherever a layer of caliche forms, water pools and drowns plant roots. That abundance of water leaves behind a high concentration of mineral salts toxic to plants. And finally, the high concentration of calcium carbonate results in a higher pH, restricting the availability of iron to plants.
The best way to deal with caliche is to remove it from the growing area. Arizona Cooperative Extension's Master Gardener Manual recommends digging a hole large enough to accommodate the extended roots of a mature plant and replacing the caliche with a more hospitable soil mix, using one part compost to two parts topsoil. For a small tree, this means digging a hole 5 feet deep by 5 feet in diameter.
If eliminating the caliche is not an option, try at least to punch a hole through the caliche to provide adequate drainage to plants. This improved drainage will also help to mobilize the salt buildup in close proximity to the caliche. Test the drainage first by adding at least 4 inches of water to the hole; if 4 inches of water drains out within four hours, your plants will have suitable drainage.
Other soil issues for gardeners are the lack of organic material, alkalinity, or a high pH, and limited availability of nitrogen, phosphorous and iron. Native aridisols grow native plants relatively slowly, so organic matter is built up only in small quantities. When monsoon rains flood the desert, organic debris rapidly decomposes, leaving little for gardeners to work with.
Adding organic matter such as vegetable compost or composted manure is a good way to supplement native soils with nutrients and to lower the pH. It also helps our arid Arizona soils to hold on to precious water.