Iron Chlorosis & Calcareous Soils

Iron chlorosis due to high pH soils is a significant problem in Missouri and Kansas. Though MO and KS soils normally contain adequate amounts of iron, a high pH makes that iron unavailable to the plant. Iron plays a major role in the production of chlorophyll. Thus, a lack of iron reduces the amount of chlorophyll and results in yellowing of leaves. Iron chlorosis weakens, and in severe cases, may kill a susceptible plant. A soil test is needed to determine the pH of your soil. See for information on taking a soil test.

A popular recommendation for high pH soils is adding sulfur to reduce pH. This works well for many soils. But some soils are calcareous. Calcareous soils are those that contain actual particles of calcium carbonate (limestone). Calcareous soils can be difficult to practically impossible to acidify because the sulfur must neutralize all the free limestone before the pH is affected. In many cases you would need well over a pound of sulfur per square foot just to neutralize the free lime. To tell if your soil is calcareous, add a drop of vinegar to dry soil. If you see or hear it fizz, then you have a calcareous soil and changing the pH over the long-term will be practically impossible.

So what do you do if you have a calcareous soil? That depends on the situation. In vegetable gardens and annual flowerbeds, products can be worked into the soil when there are no plants present. Oregon State University suggests mixing 5 pounds of sulfur per 100 square feet into the soil before planting. The idea is to form little pockets of acidity so that enough iron is available for the plants during the year of application. Note that this must be done each year. Another possibility is to use iron chelates. Iron chelates hold the iron in such a way that the plant can get to it. However, not all iron chelates will work in high pH soils. For soils with a pH above 7.2, use a chelate that contains FeEDDHA (iron ethylenediamine-di-(ohydroxyphenylacetate). This can be found in the products Sequestar 6% Iron Chelate WDG, Sequestrene 138 and Millers FerriPlus. Chelates can either be mixed into the soil at planting or sprayed on the foliage early in the season. Reapply as needed.


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