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Coping with caliche

Author: Cathy Cromell
Issue: November, 2008, Page 222
Illustration by Michael Gellatly
When digging becomes difficult, desert gardeners are quick to blame the Southwest’s notorious caliche. However, not all “hard labor” signifies you’ve struck this rock-hard substance. Sometimes that “thunk” sound is just your shovel striking extremely compacted desert soil—known as hardpan—that becomes even harder after heavy construction equipment compresses it.

You can distinguish the difference between caliche and hardpan by color: caliche is light (whitish-gray or cream), whereas hardpan usually is the same brownish color as the surrounding soil. Another method is to soak suspected areas with water. A layer of hardpan will soften with repeated soakings, allowing you to dig deeper a few inches at a time. Caliche will never soften.

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. 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.

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.

Finally, calcium carbonate is a high pH substance; pH is a measure of acidity (low pH) and basicity (high pH). Although iron is available in the soil, high pH may inhibit a plant’s ability to absorb it, causing iron deficiency. This condition is more likely to impact non-native plants than natives.

-  If soil is compacted, test the area’s drainage before planting. Dig a hole 1 foot deep. Fill with water twice during the day. If water remains 24 hours after the second filling, drainage is poor and should be dealt with before planting. Because caliche appears randomly in desert soils, the solution may be as easy as moving your planting site elsewhere.
-  If there are no other site options, break apart and discard as much caliche as possible before planting. You may need to rent a jackhammer to crack through it. If it is impractical to remove a large and deep expanse of caliche, create narrow drainage holes—or “chimneys”—through the layer (see illustration.) Retest drainage after your efforts. When planting a tree, make a chimney hole on each side. To gain maximum benefit of water and air penetration for expanding roots, the chimneys should be about 5 feet out from the trunk.
-  If breaking through caliche seems to be impossible, add a 2-foot layer of soil over the projected planting area and expanding outward to where roots eventually will grow. Use soil that is similar to the surrounding area. Soil must cover the entire circumference of the root zone based on the plant’s mature size, not its size when it is transplanted. Roots will expand one-and-a-half to four times beyond the mature canopy in their quest for water and nutrients, so this can be a significant and expensive amount of soil buildup.

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