GV?Gardeners: What exactly is caliche?

Nearly all of us, even non-gardeners, have at some time tried to dig a hole in Southern Arizona soil. It is nothing like opening the soil in Ohio, Missouri or Wisconsin!

The first thing we are told is that we are dealing with a substance called caliche. Others will say that there is little caliche in the Green Valley area, but it is really “hardpan.” Still other knowledgeable sources refer to these substances as one and the same. It can get very confusing, particularly for the novice gardener who simply wants to plant a row of shrubs.

Generally, hardpan is compacted desert soil, usually made worse by developers’ heavy equipment. It is the color of native soil, usually brown or reddish. If soaked with water, hardpan will soften to allow digging.

Within this compacted earth, often many stones will be found; some small, others quite large. Soaking doesn’t help that factor, so the only thing to do is use a pry bar or shovel tip, and remove the stones as you dig. This can be a laborious and tedious job!

If you do have caliche in the soil, you will think that dealing with hardpan is a “piece of cake.” Caliche is defined as a hardened deposit of calcium carbonate cemented together with other materials such as gravel, sand, clay, and silt. It occurs worldwide in arid or semi-arid regions. It is quite prevalent in parts of Australia, the high plains of Western U.S., and the Sonoran desert.

Caliche is light-colored and usually found on or near the soil surface.

However, it also exists in the deeper subsoil. It is generally found in layers which can measure from a few inches to a few feet thick. There may be multiple layers present within one location.

The formation of caliche is on-going, created when minerals leach from the upper soil layers and accumulate in the next layer down. At first it is deposited in grains, then small clumps, then forms a discernible layer, and finally a thick solid bed.

Caliche is an impermeable layer that prevents water from draining completely. Without proper drainage, plant roots are starved for essential oxygen and are prevented from spreading through the layer. This limits the nutrients, water, and space for normal development of the roots; therefore reducing the over-all vigor of the plant. Further, salts can build up in the soil.

Presence of a caliche bed creates high pH of the soil which restricts plants’ ability to use many of the micro-nutrients present. The most common resulting problem is iron deficiency. This condition can be identified by yellowing of youngest leaves while the veins remain green. Application of chelated iron according to product label can reverse this problem.

What to do if you run into a caliche layer or bed while preparing to plant? This material appears randomly in the soil, so perhaps by finding another location in the yard, the problem may be skirted. Otherwise, it is necessary to break apart and remove as much of the layer as is practical.

Caliche acts as a barrier, so you must break through. A practical tool for this job is the old-fashioned pick axe. If unable to remove the entire layer within the planting area, then dig drain holes, or “chimneys” through the layer to provide adequate drainage.

After preparing the planting hole, check drainage by adding water. The level should then drop 1-inch per hour. To replace the caliche removed, use a mixture similar to the surrounding native soil. Make sure the hole is large enough to allow space for future root development. Remember that when roots cannot spread to provide a strong anchor, trees may blow over in strong winds.

Calcium carbonate is mined for use in the manufacture of plastics, rubber, paper, pharmaceuticals, agricultural products, and of course, concrete. Look across the valley to the northeast, and see the large white spot growing on the mountain. This may be a valued commodity in the marketplace, but is not as appreciated when it is bonding together layers of caliche in the garden!

Mary Kidnocker is a Master Gardener who writes frequently about the subject from a local perspective. Her columns are featured Sundays. She is president of the Green Valley Gardeners.