Caliche is a hardened deposit of calcium carbonates that cements clay, gravel, sand and silt into nearly impenetrable layers from a few inches to several feet deep. Caliche is common in Arizona, New Mexico and the high desert in the United states and in central and western Australia. Layers of caliche are usually found on the surface or just below it, but layers are also found deeper.
Caliche appears randomly in desert soils. It is ordinarily a white or cream-colored layer in the soil, sometimes in lumps mixed in the soil. To determine if a soil is compacted caliche, dig a hole 1 foot deep and fill it with water twice in one day; if hole is full of water 24 hours after the second filling, it is likely caliche. If your first choice of a planting site is blocked by caliche, you might be able to change locations to find acceptable soil.
Caliche is often so solid that plant roots cannot penetrate it. It can also block water, leading to soggy, poorly aerated roots and an accumulation of salt that can reduce plant vigor. Caliche often causes a high soil pH, resulting in a lack of iron getting to plants; plant leaves turn yellow while their veins remain green. Soggy soil found above a blocking layer of caliche makes the iron problem worse.
Gardeners sometimes dig holes in caliche three to four times the size of the root balls to accommodate single transplants. A hole must be drilled in the caliche below the planting hole so it can drain, otherwise the water can back up, causing root rot and other fungal diseases.
When faced with a large, deep layer of caliche, some gardeners dig it up and haul it away so they can fill the space with soil they can cultivate. This may require a jackhammer. A less arduous solution is to build a raised bed above the impermeable layer of caliche. The bed, made of soil that allows the penetration of nutrients and water, should be at least 2 feet deep, and the caliche underneath it should be bored with drainage holes. The soil that replaces the excavated caliche or the raised bed should be deep enough to accommodate the roots of mature plants, not just seedlings.
Salts builds up in plants growing near caliche, because calcium is needed to displace sodium, and the calcium in caliche is insoluble. Do not use softened water to irrigate plants surrounded by caliche or growing above a deep layer of caliche, because water softeners add sodium to the water.