Iron

Iron is not something most people think about when it comes to gardening, but it should be.

The chemistry that happens in the ground beneath your feet is amazing and complex. It is a delicate balance that either works like a finely tuned instrument, or more like a room full of toddlers armed with heavy spoons and pot lids. You get the idea.

Iron deficient lemon tree shows yellowing leaves (Victor M. Vicente Selvas) Public Domain

​Fertilizer isn’t just NPK

Most gardeners are very familiar with the NPK found on most bags of fertilizer. These letters represent the three primary macronutrients needed by plants: nitrogen, phosphorus, and potassium, respectively. Of course, what you mostly get when you buy fertilizer is filler. A 20-pound bag of 10-10-10 fertilizer actually contains 10%, or 2-pounds, of each element. The remaining 14-pounds is just filler. Plants also use calcium (Ca), magnesium (Mg), and sulfur (S) as secondary macronutrients. Plants also need these micronutrients: copper (Cu), manganese (Mn), zinc (Zn), boron (B), and iron (Fe).

​Nutrient movement within plants

Some plant nutrients are highly mobile. This means they can be moved around, within a plant, to where they are needed. Nitrogen is so mobile that you can identify nitrogen deficiencies in the soil by looking at the leaves. Older leaves will begin to turn yellow as nitrogen is pulled away to make fresh, bright green, new leaves. Iron, on the other hand, is relatively fixed once it enters a plant. Older leaves will stay green, but new leaves will be pale or look bleached.​​

​Iron deficiency symptoms

Plants grown in iron deficient soil often turn yellow and they cannot thrive because they are starving. This is because iron is needed to produce chlorophyll and in plant respiration. [Note: plant respiration is not the same thing as mammalian breathing - plant respiration refers to breaking down stored food reserves to release usable energy into the plant.] Yellowing (chlorosis) due to iron deficiencies normally begins in the areas between leaf veins, which stay green. Young leaves may look bleached. Symptoms are more pronounced in acid-loving plants, such as blueberries, raspberries, and camellias. Over time, leaf size will be reduced, and dead (necrotic) patches will appear along leaf edges (margins) and between veins. Leaves will also die and drop prematurely. Shoots and canes can also die back.

Iron and soil pH

Often, you will see the word ‘chelated’ on a soil amendment that includes iron. This is because free iron molecules can become unavailable to plants when pH levels are not between 5.0 and 6.5, or when phosphate concentrations are out of balance. This balancing act has to do with the way micronutrients interact with each other in the soil. If they bind to one another, plants can’t get to them. This is particularly troublesome in our highly alkaline, compacted clay soil.

Other causes of iron being unavailable to plants include soil that has been waterlogged (due to flooding or a leaking sprinkler), or too many other nutrients. High levels of copper, manganese, phosphorus, calcium, or zinc will bind to the iron, making everything unavailable to plants. This is a case where adding fertilizer just makes things worse, rather than better.

Case in point

I have a beautiful yard and I love to garden. We moved here in 2012. There were several fungal disease problems present, as a result of leaky and poorly placed sprinklers. The soil is heavy clay and compacted. Borers, scale insects, and aphids were problematic. We stopped using the sprinklers and switched to soaker hoses. Then we started adding compost and mulch to the landscape. And I sent a soil sample to a lab for testing. The results were educational, to say the least.

My soil had an excess of every nutrient, except iron. My soil’s iron levels were far below the recommended level, making every other nutrient unavailable to my plants. As a result, the plants were not healthy enough to fight off pests and diseases as well as they would have been otherwise. Without that soil test, I might have added even more fertilizer, making things even worse for my plants. I can’t stress this enough: get your soil tested by a reputable, local lab. The information is invaluable.

Counteracting iron deficiencies

If your soil is low on iron, you can jump-start your plants’ health with a foliar spray of iron. Spraying leaves with chelated iron or ferrous sulfate allows plants to absorb the mineral directly through leaf tissue. Once the iron is inside the plant, other nutrients can be used. This treatment will need to be repeated as new leaves emerge. Also, it should not be done during hot weather, and it might stain your patio. Longer term solutions include:

• selecting plants that will thrive in the soil (and microclimate) you already have
• acidifying the soil with iron sulfate, elemental sulfur, or ammonium sulfate (which also contains nitrogen) - do not use aluminum sulfate, which can be toxic to root systems
• reducing soil compaction with aeration and mulch
• trunk injections of iron by a licensed professional
• fertilizing the soil with chelated iron

These treatments should only be done AFTER a laboratory soil test has indicated that iron levels are deficient. Over-the-counter soil tests are not effective enough to justify any adjustments.