Reading an alkaline soil test
pH, free lime, salts, and iron chlorosis in Mountain West soils, and why adding iron or sulfur often does not do what the bag claims.
A yellow leaf in June is not a diagnosis. On calcareous ground it is often iron chlorosis, and the iron was in the soil the whole time. Colorado State University Extension describes the symptom as interveinal chlorosis: the leaf yellows and the veins stay green, showing up first on new growth because iron does not move well inside the plant.
In severe cases the leaf goes pale, the margins scorch, and a limb or the whole plant declines. The fact sheet is also clear about the cause in this region. Many Colorado soils are high in lime, which holds pH above 7.5.
Iron is abundant, including in the red rock people point at, and it is chemically tied up in forms roots cannot take. Pouring on more iron sulfate repeats the tie-up. The bag disappears and the leaf stays yellow.
That is the center of Western soil practice. Test first. Change the plant or the water before you try to change the geology.
What to send to the lab
Use a land-grant lab. In Colorado that is CSU’s Soil, Water and Plant Testing Laboratory.
Utah gardeners have USU’s analytical lab. A home pH kit can tell you that you are above 7. It will not tell you whether free lime is high, whether salts are the real problem, or whether sodium is high enough that gypsum would do anything.
Collect several cores from the root zone of the bed you care about, mix them, and label the bed. A sample from the lawn and a sample from the vegetable garden should not share a bag. They are often managed differently and they should be.
Read the report for four things before you read the fertilizer suggestion.
pH. Above 7 is alkaline. Above about 7.5, CSU treats iron chlorosis as a likely problem for susceptible plants. Vegetables vary. Asparagus, beets, and cabbage tolerate high pH better than blueberries, which do not belong in this soil at all.
The fact sheet names acid-loving plants that are a mismatch here: blueberries, azaleas, rhododendron, flowering dogwood, heather. Wanting them is not a soil-amendment plan.
Lime or a carbonate test. If the lab says free lime is present, the soil is buffered. Elemental sulfur can lower pH in a low-carbonate soil over time. In a soil full of calcium carbonate it is shoveling against a hillside.
CSU notes that sulfur is often not a practical fix in that case, and that alkaline irrigation water can undo the attempt. PlantTalk Colorado says the same thing in fewer words: adding sulfur may not work, and watering with alkaline water works against you.
Salts, often reported as electrical conductivity. Western soils and Western manures can be salty. A high reading means you leach with good water if drainage exists, and you stop adding composted manure until the number falls. It does not mean you add gypsum automatically. Plants in a salty bed look burned at the edges and stunted, which people mistake for drought.
Sodium. This is the gypsum question. Gypsum, calcium sulfate, is a response to sodic soil, where sodium has dispersed the clay. It is not a general treatment for “clay” or for “West.” If the sodium is ordinary and the problem is calcareous clay, gypsum adds calcium and sulfur you may not need and leaves the structure to organic matter and timing. The clay page covers that work.
Iron chlorosis is also a water problem
CSU is explicit that overwatering shows up as iron chlorosis. Wet, cold, compacted spring soils make the symptom worse, and the fact sheet says chlorosis is more common after wet springs and where gardeners overwater early.
Dry soils cause trouble too, because roots take nutrients up in solution. The cultural fix, before any product, is drainage, less frequent deep watering once plants are established, and an end to daily sprinkling that keeps the root zone cold and airless.
Mulch that prevents compaction helps. Mulch piled on the trunk does not. See mulch.
A plant that is simply the wrong species will not be saved by a better irrigation schedule.
CSU’s table of woody plants with high susceptibility includes maples, pin and red oak, aspen, birch, honeylocust, apple and crabapple, peach, pear, cherry, and a number of conifers.
PlantTalk Colorado repeats the maples, the oaks, aspen, and the fruit trees, and says that above pH 7.5 the best control is to avoid the sensitive tree. The tree page is the planting decision. The fruit pages are the decision about whether a susceptible apple is still worth growing for the crop, knowing you may see yellow leaves in a wet June.
Products, in the order they actually help
Foliar iron sprays can green a leaf for a few weeks. CSU and PlantTalk both describe that limit. Spray in the cool of the day, keep the spray off concrete if the product stains, and do not expect a large tree to be treatable this way.
Soil-applied chelates vary. Some hold iron only in a narrow pH band and do nothing in a very alkaline bed. Read the label for the pH range.
If the product does not say it works above 7.5, believe it. Trunk injection is an arborist’s tool with its own wounds. It is not a Saturday project with a hardware-store spike.
Lowering pH with sulfur is the long project, and only where the lab says carbonate is low. Even then, work in pounds the lab or the extension note calculates for your square footage, not a scoop from a video.
Recheck pH in a year. A vegetable bed can be managed more easily than a tree pit because you can add organic matter and choose tolerant crops. A tree pit full of sulfur next to a calcareous subsoil is a teacup in a quarry.
Compost does not “neutralize” a calcareous soil in any useful sense. It improves structure, water holding, and biology, which can reduce the stress that makes chlorosis worse. It is worth adding. It is not an iron program.
Details of how compost behaves in dry air are part of the clay and bed pages: finished compost, moderate amounts, no hope that three inches will move pH from 8 to 6.
Vegetables on high-pH ground
Most common vegetables will grow at pH 7.2 to 7.8 if drainage and organic matter are decent. The exceptions are the acid lovers, already named, and potatoes in the sense of common scab, which likes dry alkaline soil.
Even moisture at tuber set is the cultural response, covered in roots and potatoes. Beans and tomatoes will show iron chlorosis when the bed is cold and wet. Warm the soil with a raised bed, water less often and more deeply, and do not conclude you need a blueberry-style acid program.
If a vegetable bed is truly failing and the soil test shows high salts, the fix is leaching and a pause on manure, not a chelated iron foliar on a plant whose roots are burning. Read the number you paid for.
Checklist
- Test each bed, not the whole property as one bag.
- Read pH, carbonate or lime, salts, and sodium before you buy an amendment.
- Treat interveinal yellowing on new growth as a chlorosis clue, then look at water and species.
- Skip blueberries, azaleas, pin oak, and the other acid lovers.
- Do not lime Western soil.
- Buy gypsum only if sodium is high.
- Use foliar iron as a temporary cosmetic on a tree you intend to keep, not as a diet.
- Choose tolerant plants first. The species list is a cheaper fix than a generation of sulfur.
The design consequence is a planting plan sorted by water and by chemistry, which is what xeriscaping is for. A single irrigation zone that waters a maple and a penstemon the same way will chlorose one and rot the other.
Sources
- CSU Extension, Iron Chlorosis of Woody Plants.
- PlantTalk Colorado, 2121 Iron Chlorosis in Trees and 2115 Iron Chlorosis.
- CSU Soil, Water and Plant Testing Laboratory, agsci.colostate.edu/soiltestinglab.
- USU Extension yard and garden, for Utah lab and horticulture guidance: extension.usu.edu/yardandgarden.