KENNEWICK -- Back in July, I promised to followup with information on solving the problem of iron chlorosis due to alkaline soil conditions.
I know many of you have been on pins and needles waiting so here we go ... .
In an ideal world, we gardeners would have a soil test done before planting our landscape. A soil test would tell us the pH (acidity/alkalinity), as well as potential lacking nutrients needed for healthy plant growth. In that same perfect world, we would avoid planting plants not well suited to our local alkaline soils, such as rhododendron, blueberries, silver maple and pin oak.
In reality, few of us check our soil pH before planting or selecting our landscape and garden plants. With that said, what can we do to mitigate the problem of iron chlorosis due to alkaline soil?
First, gardeners should check to see if high pH is the problem. Similar symptoms of chlorosis can be produced if the plants are overwatered, underwatered, growing in compacted soil or have failing root systems. This means reviewing cultural practices and having the soil tested.
The addition of elemental sulfur to the soil after planting is not recommended. This can help if done before planting, but not after. Sulfur does not dissolve. It's ground into fine particles and mixed into the soil where only the soil in the immediate area of each sulfur particle is acidified. Sulfur applied at the surface doesn't work its way down into the root zone where it's needed.
Experts recommend several options for addressing the problem of alkaline soil. First is mixing equal parts of ferrous sulfate and elemental sulfur and putting the mix into holes placed in the root zone of trees and shrubs.
Over time (months and years) this mix will acidify the soil close to the holes and the effect may last several years.
(Examples of products with iron sulfate plus sulfur are Copperas, Jirdon Super Iron Green, Hi Yield Soil Acidifier Plus Micros, and Fertilome Soil Acidifier Plus Iron.)
A costlier route is adding iron chelates to the soil in the same manner. An iron chelate is a material where the iron molecule is protected by a larger molecule. This larger molecule keeps the iron in an available form until it is broken down by soil microbes and soil alkalinity.
The response to the iron chelates is quicker, but will not last as long as the sulfur/ferrous sulfate mix.
(Look for iron chelates with the FeEDDHA molecule. This includes Monterey Sequestar Iron 6 percent, Sequestrene 138 and Millers Ferriplus. Other iron chelates for soil application are available, but don't work well when the pH is above 7.2.)
If a quick fix is desired, there are iron chelate materials that are applied as sprays to the leaves. The leaves green up quickly, but this is primarily a cosmetic treatment and does nothing to help solve the underlying problem of high pH.
Leaf response from chelate sprays can be uneven and the "greening up" may not last all season, especially if there is new growth after application.
Some garden stores may recommend the use of aluminum sulfate to acidify the soil. Repeated use of this product can lead to aluminum toxicity and isn't recommended. Materials injected into the trunk of a tree are also ill advised since they create wounds that can result in wood rot.
The iron applications I've mentioned are worth a try, but if they don't work, consider replacing the chlorotic plants with ones that are more tolerant of alkaline soils.
* Marianne C. Ophardt is a horticulturist for the Washington State University Benton County Extension Office.