Often called hardpan, caliche is a four-letter word to Arizona gardeners. The pale layer is similar to concrete, poured by a whimsical giant with a twisted sense of humor. It spreads over large areas or remains localized to your backyard lawn. It often lurks inches or feet beneath the surface, only coming to light as your turf struggles against dehydration, waterlogged soil or nutrient deficiencies.
HOW CALICHE FORMS
Wondering where that layer of caliche comes from? First, calcium combines with carbon dioxide dissolved in the soil’s water to form insoluble calcium carbonate (CaCO3). Over time, soil particles become cemented together by calcium carbonate to create solid deposits known as caliche. Your digging may unearth random chunks of caliche mixed in with soil and rocks or concrete-like layers of caliche several inches to several feet thick.
As calcium-carrying rainwater or runoff flows over and through the ground, it contacts carbon dioxide in existing groundwater or soil-borne moisture. The calcium and carbon dioxide bond and the water evaporates, leaving the minerals behind. These deposits of calcium carbonate crystallize in the soil, bonding with buried rocks and neighboring crystals as it forms a dense, hard amalgamation of gravel, rocks, sand and lime. Not every dry climate has caliche, but conditions are right in the Sonoran Desert.
WHY CALICHE IS BAD
Caliche creates several problems for gardeners. First, roots cannot penetrate through it. Shallow root systems are susceptible to drought stress as well as uprooting during windstorms. In addition, plants must survive on the limited nutrients found in the soil above the caliche layer.
Second, water cannot soak through a tight caliche layer. This results in poorly drained, wet soil, which doesn’t provide the aeration that roots require. Think of a root ball in a planting hole set above caliche in the same way that a plant sits in a container without drainage holes. When water is unable to penetrate through the soil, salts cannot leach beyond the root zone. Over time, salt buildup kills plants.
Caliche impedes root formation and may stunt plant growth. Even lawns are not immune, despite relatively shallow root systems. The nonporous formation also prevents water drainage, a serious problem during heavy summer monsoon rainfall. This creates poorly draining basins that drown plant roots or encourage disease. The poor drainage prevents water from leaching salts from the soil, causing damaging salt buildup. Additionally, caliche has a high pH, creating alkaline conditions that prevents plants from absorbing magnesium, iron and other nutrients.
Gypsum is a mineral compound celebrated for its ability to break up dense clay soil. Caliche, however, is immune to gypsum. Gypsum is calcium sulfate, and adding this calcium salt to caliche’s calcium carbonate may make matters worse. West Texas A & M University states, “Any soils with caliche will not benefit from the addition of gypsum, though in rare instances, sulfur might help.” Sulfur acts slowly, however, and may take years to alter soil texture.
Dealing with caliche depends on your situation. If your caliche problem is minor and your lawn is not large, digging out the caliche and replacing the area with soil may solve the problem. Solid caliche over a large area, however, is nearly impossible to remove, unless your landscaper has an arsenal of nuclear weapons at the ready. Instead, the Arizona Master Gardner Manual suggests adding 8 or more inches of topsoil over the soil before seeding or laying sod.