Pumping Iron: Why Plants Need Iron in Their Diet

Iron is one of the essential nutrients required for plant health and development. It is needed by plants in relatively low amounts, relegating it into the class of micronutrients (as opposed to those nutrients needed in large quantities known as macronutrients). For comparison, growers only need one to 1.5 pounds of iron for one acre, whereas they would need 80-200 pounds of nitrogen (a macronutrient) for the same area.

Role of Iron in Plants

Like all nutrients, iron serves multiple roles within a plant’s functions. Most notably, as a component of enzymes and protein used by plants during photosynthesis, it is involved in the manufacture of chlorophyll and responsible for the function, structure, and maintenance of chloroplasts (component of plant and algae cells). It also plays a role in the process of respiration, an essential function of life.

Iron is most useful and useable to plant when it occurs in soluble form. It is most soluble in the pH range of 5 to 6.5. When the pH is outside of this range, the resulting deficiency not only results in an aesthetically displeasing plant, but it can have economic impacts as well. Crop plants without sufficient iron do not yield as much at harvest time, and iron-deficient food crops often contain less nutrition per unit than their iron-laden counterparts.

Detecting Iron Deficiencies in Plants

Iron deficiencies most often present themselves as a yellowing (chlorosis) of the plant’s leaves. Many other nutrient deficiencies also cause yellowing leaves, so it is important to note where and how the chlorosis appears. A plant lacking iron will show its deficiency first in the younger leaves; in other words, the new growth.

Read More: Why Are My Leaves Turning Yellow?

When the deficiency first appears, the entire leaf will not be yellow, but rather the interveinal surface will still appear green. As the problem continues or progresses, eventually the entire leaf will turn yellow, even white in some plant species. New growth, if it continues to be produced by the plant, will be stunted.

Eventually, brown lesions may occur on the leaves and leaves will start to drop, starting with the newer growth. In woody species such as trees and shrubs, whole branches will begin to die. Eventually, the entire plant can succumb to iron deficiency and die in extreme cases.

By contrast, nitrogen deficiencies present themselves as chlorosis on the plant’s older leaves first. Here, the new growth takes the nitrogen from the older leaves. Iron, on the other hand, is not so mobile within the plant and leaves that have sufficient iron will hang on to it.

Iron deficiency can also be difficult to distinguish from the symptoms of zinc deficiency. Zinc deficiency presents almost identically to iron deficiency in both color and location on the plant.

The subtle difference between the two conditions is that the interveinal lesions on the leaf caused by a lack of zinc will be slightly more rounded and the ones caused by a lack of iron will be more angular. If unsure which deficiency is present, take a sample to an experienced horticulturist or your local cooperative extension for identification.

A more difficult to ascertain but nonetheless present symptom of iron deficiency is to be found in the root zone. For established plantings like trees or shrubs, it is not practical and may do more harm than good to dig out the plant to examine the roots. However, in mass plantings such as with food crops, this information is valuable.

Plants absorb usable iron towards the root tips. Depending upon the root structure of the plant, this may be in the last inch to few inches of root growth. An iron deficient plant will likely have a stunted root system, which makes uptake of iron difficult or impossible.

Another thing that plays a role in iron deficiency is pH. Even if there is sufficient amounts of iron in a soil, too high of a pH (6.5 or higher) will tie up the available iron and chlorosis can occur. If the pH is too low, manganese will outcompete the iron and it will not be available for the plant to use either. Plants located in heavy clay or compacted soils that don’t drain well are also at risk for iron deficiency.

Applying Iron Plants

The most effective way for a deficient plant to absorb iron is through a foliar application of liquid iron. This may be applied as chelated iron or iron sulfate. (A chelate is an organic compound that, when combined with an elemental metal, makes it more readily available for an organism’s use.) Liquid applications bear results the quickest of all the possible remedies, with improvement usually visible within two weeks’ time. It’s also often effective for up to one year. Liquid applications, however, will only improve the leaves that they are sprayed on.

New leaves can still show the same deficiency that caused the problem to begin with. It should also be noted that applying iron sprays to plants during periods of hot weather can burn the foliage of the plant. So, apply in the early morning or wait for an overcast day when rain is not predicted. Iron sprays are also capable of staining non-target areas that are subject to over-spray. This includes asphalt, concrete, and the sides of buildings.

For trees and large shrubs that are iron deficient, iron implants can be injected into the trunk or root flare. It can be as simple as inserting a capsule into the plant, or may involve a complex network of tubes delivering the iron. Essentially, iron is released slowly over time into the sap stream, sending the needed nutrient throughout the entire plant. The effects can last for two growing seasons or more before another injection is required.

While there are do-it-yourself implants containing powdered iron, this approach is usually only available to professionals in the plant care industry. Also, given the expense of a treatment, this is generally reserved only for valuable specimens or large trees.

Changing the soil pH can also help make more iron available to plants. Before attempting to do so, however, it is important to know if high soil pH is indeed the problem you need to fix. Most garden centers and home improvement stores carry inexpensive soil pH test kits. For a nominal fee, most cooperative extensions perform this service as well. Upon determining that lowering the soil pH is indeed the correct approach, granular and powdered forms of iron are available as pH-lowering amendments.

Chelated iron is available in a dry form and can be spread along the drip line area of the affected plant. The most common products to use are elemental sulfur or iron sulfate. Elemental sulfur is usually much more economical and efficient than iron sulfate, as it takes about six times more iron sulfate than elemental sulfur to treat an area. Aluminum sulfate is also suggested by some as it works faster than either elemental sulfur or iron sulfate.

However, unless your soil is lacking it, the addition of aluminum will build up to toxic levels and be detrimental to plant health. Current wisdom suggests that the only relevant purpose for aluminum sulfate is to change the color of some types of hydrangea flowers from pink to blue.

Some other, mostly outdated, literature also suggests using sulfuric acid to lower a soil’s pH. While it will do so, it is generally not worth the health risk to handle sulfuric acid, especially when there are far easier and less expensive amendments available. Results are variable, depending on pH and the amendment, and this procedure may have to be applied on a yearly basis.

The only permanent solution to treating iron-deficient plants in a location that is not suitable for the uptake of iron is to replace them with a species of plant that is tolerant of the pH on site. Plant varieties vary greatly in regards to their susceptibility to iron deficiency.

Some particularly susceptible plants include members of the rose family and ericaceous plants like azaleas, rhododendrons, kalmia, pieris, heathers, and blueberries.

All other solutions, even adjusting the soil pH, are temporary and need to be done again. Some amendments can adjust the pH for a few months; others as long as a few years. Either way, it will have to be done again.