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Phoenix Soil Guide: Caliche, High pH, and What They Do to Your Landscape
Last updated: July 30, 2026
Quick answer: The defining soil problem across the Phoenix area is caliche — a layer where soil particles are cemented together by calcium carbonate. University of Arizona Cooperative Extension describes it as appearing either as light-coloured lumps, from under an inch to several inches, or as solid layers from a few inches to several feet thick. Where it is solid it is often impenetrable to plant roots and it restricts water movement, which perches water above it, starves roots of air, and lets salts build up in the root zone. Because calcium carbonate is a basic, high-pH substance, it also makes iron unavailable to plants — the cause of the pale yellow or white leaves so common on Phoenix landscape plants. Fixing planting technique matters far more here than fixing fertilizer.
What is caliche?
University of Arizona Cooperative Extension defines it directly: “Caliche is layer of soil in which the soil particles are cemented together by calcium carbonate (CaCO3).” It ranges from loose to genuinely rock-like, appearing either as light-coloured lumps ranging from less than an inch to several inches across, or as solid layers from a few inches to several feet thick.
It forms slowly and continuously. As Extension explains: “Calcium is continually added to the soil, mainly dissolved in rain water. This calcium combines with carbon dioxide dissolved in soil water, forming insoluble calcium carbonate deposits.” Whether and where it forms depends on rainfall, how water infiltrates, and how the soil drains — which is why two lots on the same street can differ.
Why does caliche kill landscape plants?
Three separate mechanisms, and most struggling Phoenix plants are dealing with more than one.
It blocks roots. Extension states that “Solid caliche layers may be impenetrable to plant roots.” A plant that cannot root below the layer is extracting water and nutrients from a much smaller volume of soil than its size suggests, so it is permanently closer to drought stress than it looks.
It blocks drainage. “Impenetrable caliche layers restrict water movement.” Water applied at the surface reaches the layer and stops, perching there. That perched water displaces air in the root zone, and roots need air. This is why a plant in a caliche-bottomed hole can drown in a desert.
It causes salt build-up and iron deficiency. Because water cannot drain through, salts cannot leach away and instead accumulate in the rooting zone. And since calcium carbonate is “a basic (high pH) substance,” iron becomes unavailable to the plant. The visible result is chlorosis: newest leaves turning pale yellow or white between the veins while the veins themselves stay green.
My plant’s new leaves are yellow with green veins. Is that caliche?
That pattern — pale yellow or white tissue between green veins, showing on the newest growth first — is the classic iron-deficiency symptom Extension associates with high-pH, carbonate-rich soil. It is frequently misread as a nitrogen problem and treated with more fertilizer, which does not address the cause. The iron is usually present in the soil; the pH is making it unavailable.
How do I plant correctly in caliche?
Break through it. Extension’s guidance is to penetrate completely through the caliche layer when making a planting hole. A hole that stops at the layer is a container with no drain.
Get the hole shape right. Dig “no deeper than the root ball and up to 3 to 5 times as wide.” Wide and shallow beats narrow and deep: roots spread laterally, and a hole dug deeper than the root ball lets the plant settle.
Test the drainage before you plant. Extension gives a usable field test: fill the hole with water and watch it. A water level that drops at least four inches in four hours indicates adequate drainage. If it does not, planting there without fixing drainage is planting into a bucket.
Remove the chunks. Solid pieces of caliche broken out of the hole should be removed from the soil rather than backfilled with it.
Where you cannot break through, build up. Extension’s alternative is a raised bed providing two feet of total soil depth over the entire rooting zone — not a mound over a hole, but genuine depth across the area the roots will occupy.
Sulfur can help. Extension notes soil sulfur applied at roughly one-half to two ounces per cubic foot, depending on soil type, to improve iron availability and drainage.
Does this change how I should water?
Yes, and in a way that surprises people. Where drainage is restricted, the failure mode is not only under-watering but over-watering — because the water has nowhere to go. Frequent light watering on top of a caliche layer keeps the root zone saturated and airless while still failing to leach accumulated salts downward.
Deeper, less frequent irrigation applied where drainage genuinely exists is the better pattern, and drip is the delivery method that makes it controllable. Our Phoenix irrigation page covers system design, and Phoenix watering restrictions covers what the city currently asks of homeowners.
What should I do before starting a landscape project?
Dig test holes across the area, not one. Caliche is not uniform — depth and thickness vary within a single yard, and a design that assumes what the first hole showed will fail wherever the layer sits shallower. Knowing where it is, and how thick, is what determines whether a given plant belongs in a given spot, whether a raised bed is needed, and how much of the project cost is going to be excavation rather than plants. Our Phoenix landscaping cost guide covers how site conditions like this move a quote.
Frequently asked questions
What is caliche? University of Arizona Cooperative Extension defines it as a layer of soil in which the soil particles are cemented together by calcium carbonate. It occurs as light-coloured lumps from under an inch to several inches, or as solid layers from a few inches to several feet thick.
Can plant roots grow through caliche? Often not. Extension states that solid caliche layers may be impenetrable to plant roots, which limits the soil volume a plant can draw water and nutrients from.
How do I know if my soil drains well enough to plant? Extension’s test is to fill the planting hole with water: a water level that drops at least four inches in four hours indicates adequate drainage.
How wide should I dig a planting hole in Phoenix? No deeper than the root ball, and up to three to five times as wide.
Why are my plant’s new leaves yellow? Calcium carbonate is a basic, high-pH substance, which makes iron unavailable. The symptom is pale yellow or white colouring between the veins on the newest leaves.
What if I cannot dig through the caliche? Extension’s alternative is to build up instead: add soil to provide two feet of total depth over the entire rooting zone.
Does caliche cause salty soil? Indirectly, yes. Because caliche restricts water movement, salts cannot leach out of the soil and instead build up in the rooting zone.
Sources
- University of Arizona Cooperative Extension — Managing Caliche in the Home Yard (publication az1281; Jack Kelly & James Walworth, March 2002): extension.arizona.edu