By Nadia Zane
A stroll through your garden of late may have revealed some plants with yellowing foliage. There are several possible causes, but iron chlorosis, a condition in which a plant deficient in iron cannot produce sufficient chlorophyll, is common at this time of year. Chlorophyll is responsible for the green in plants and capturing light for photosynthesis, the process in which plants produce food for themselves.
If you have already dashed off to purchase iron, then keep the receipt, because I have a surprise…you probably don't need iron fertilizer for your iron-deficient plant!
To understand how this could be, it's important to know what's happening in the soil. Iron is one of 17 elements essential for plant growth and metabolism. Iron is typically plentiful in California soils, but certain conditions often make it “unavailable” to plants:
Healthy leaf (top) and leaf suffering from iron chlorosis (bottom)
Alkaline soil, meaning soil with a pH of 7.0 or higher, holds on to iron (and many other nutrients), making it inaccessible to plants. Most plants prefer a pH around 6.5.
Soggy and/or cold soil limits microbial activity and the important services they provide in getting nutrients to the plants. Microbes need porous soil and a certain temperature range to thrive, which is why chlorosis is more common in winter. Compacted or heavy soils are especially prone to a lack of porosity in cold, wet weather.
Nutrient imbalances, especially an overabundance of zinc, copper, and manganese, can make iron less available. Causes vary, but can be due to over-application of certain fertilizers.
When determining if you do, in fact, have iron chlorosis, consider factors such as watering, the time of year, and then look at the pattern of yellowing on the leaves. Iron chlorosis presents as green veins with the interspaces being yellow, typically on the newer leaves. Other possible causes include Zinc and Manganese deficiencies or exposure to herbicides containing simazine or diuron. See the UC Integrated Pest Management website for pictures of various patterns of chlorosis:
www.ipm.ucdavis.edu/PMG/C107/m107bpleaftwigdis.html
So what can you do? The key is to focus on improving soil health, which can help lower the pH and increase iron availability. Adding compost to the soil and mulching the surface increases biological activity and improves porosity. If the chlorosis appears only in winter (and isn't severe), then simply wait until the soil dries out, and the problem may correct itself. Be sure not to over-water at other times of the year, not just because it's wasteful, but because it can also induce iron chlorosis.
Selecting plants that tolerate alkaline soil can make things easier. Plants native to arid regions, including California natives and other Mediterranean-climate species, have a better tolerance for alkaline soils than acid-lovers such as Hydrangea, Gardenia, Rhododendrons/Azaleas, Camellias, Hollies, Magnolias, and Blueberries. If you must grow acid-loving plants, be sure to apply plenty of compost and mulch, or try growing them in containers where you can control the soil pH by purchasing acid potting mix.
You may go to a garden center and be told that simply adding sulfur will effectively lower the pH. This is correct if your soil does not contain “free-lime” (calcium carbonate), which cancels out the acidifying effects of sulfur. If you have hard water, then you are more likely to have free-lime in your soil. You can test this by taking a small sample of dry soil and adding household vinegar. If it fizzles, then you have free-lime, and you can return that bag of sulfur along with the iron (I hope you kept receipts for both). Keep in mind that highly alkaline soil (above 7.3 pH) cannot be amended enough to grow acid-long plants without ongoing chlorosis issues.
Applications of iron sulfate or chelates to the soil can correct deficiencies if your soil is sufficiently acidic (6.5 or less) and actually needs iron. Remember that simply dumping extra iron into you soil is not going to fix a deficiency if the soil conditions make it unavailable! Foliar applications are good for a quick-fix, though the effects disappear quickly too, meaning you may need to reapply several times a year.
Be sure to avoid Ironite®, an iron supplement containing toxic levels of arsenic and lead. It has been illegal in Canada since 1997, and is under investigation in California. Aluminum sulfate is another amendment to avoid because of the potential for aluminum toxicity. Gypsum has often been touted as a way to lower pH, but this is not true. It increases porosity (drainage) in clay soils with an imbalance of magnesium and calcium, but if your clay soil is “tight” (i.e. holds on to nutrients; not releasing them to plants) for other reasons, gypsum won't do you much good.
With all these complicated explanations and “don't do this” commands, it's actually all very simple: add organic matter such as compost, only apply as much water as is needed to maintain moderate growth, and use plants that are OK with the soil you have.