Caliche, Drainage and Irrigation in Surprise

What caliche is

University of Arizona Cooperative Extension's Managing Caliche in the Home Yard describes caliche as soil whose particles are cemented together by calcium carbonate. It can appear as light-colored lumps from under an inch to several inches across, or as a solid layer from a few inches to several feet thick, and it can range from loose to rock-hard. Extension calls it a common problem in southern Arizona soils, while noting that not all desert soils have it.

There is no published caliche map for individual Surprise lots, so the only way to know is to dig. A shovel that stops dead a foot down on a pale, chalky layer is the usual sign.

Why it matters for irrigation

Extension lists three effects, and each one shows up in an irrigation system.

Roots stay shallow. Solid caliche can be impenetrable to roots, so the plant has to draw water and nutrients from a small volume of soil above the layer. Shallow-rooted plants suffer drought stress quickly and can be poorly anchored in strong wind.

Water perches. Water applied to the surface cannot move through a tight caliche layer. It sits on top, saturating the soil around the roots and starving them of air. In a drip bed, this can look like a plant that is drowning and thirsty at the same time.

Salts accumulate. Soils that drain poorly because of caliche tend to become saline, since salts cannot leach out of the root zone. The City of Surprise's Landscape Watering Guidelines recommend watering twice as long once or twice a year to leach salts. That only works if the water has somewhere to go.

Extension also notes that the high pH of calcium carbonate can make iron unavailable, which shows as pale yellow or white between the veins of the newest leaves. That is a soil chemistry issue, not a watering one, and more water will not cure it.

Test drainage before planting

Both Extension publications give a simple test. The caliche guide says to partially fill the planting hole with water and check that the level drops at least four inches in four hours. The Care of Desert-Adapted Plants guide gives a looser rule: fill the hole and expect it to drain in 12 hours to a day. Either way, a hole that still holds water the next morning will hold irrigation water too.

When the test fails, Extension's advice is to break through the layer:

• Dig the planting hole completely through the caliche where practical, keeping it no deeper than the root ball and three to five times as wide.
• If that is not practical, dig a smaller drainage hole or chimney through the caliche beside the root ball, not directly under it. A drain under the root ball forces excess water through the roots.
• Remove solid chunks of caliche from the backfill.
• If drainage cannot be opened, add soil to give about two feet of rooting depth across the root zone and set plants slightly above grade.

Setting irrigation over caliche

Once plants are in, the goal is still the city's depth targets: 24 to 36 inches for trees, 18 to 24 inches for shrubs, 8 to 12 inches for groundcovers and cacti. Over a shallow layer, those numbers may not be reachable, and pushing longer runtimes only adds perched water.

A few practical adjustments help:

• Probe, do not guess. Extension suggests a soil probe, long screwdriver or rebar. Over caliche, the probe may stop at the layer rather than at dry soil, which tells you where the water is going.
• Shorter cycles with a soak between. If water pools at the surface, splitting a run into two shorter cycles gives it time to soak in, the cycle-and-soak approach in the EPA WaterSense Sprinkler Spruce-Up checklist.
• Watch for a wet surface over dry roots. Water spreading sideways over the layer can wet gravel well away from the plant while the root ball stays dry.
• Keep emitters off the trunk. Place them over the root zone and move them outward as the plant grows.

Plan the system around the soil

A new irrigation system is the best time to deal with caliche, before plants and tubing are in place. Map where the layer is shallow, group plants by water need, and give each group its own valve so a poorly draining bed does not set the schedule for the rest of the yard. City Master Gardeners hold Saturday office hours at the Water Education Center next to the Xeriscape Demonstration Garden for plant and soil questions.

Next step in Surprise

For a new layout, or a bed where plants keep failing, see irrigation system installation in Surprise. The irrigation system installation page explains the work, and the Surprise service area page and all irrigation services list related help. Related reading: Watering New Desert Plants in Surprise and Reading the Surprise Landscape Watering Guidelines.