7/20/2012 - By Harrell's

As one of the 17 essential plant elements, iron is commonly applied in various turf and landscape settings.  In the plant, iron plays a role in photosynthetic electron transport, respiration, chlorophyll formation, and numerous enzymatic reactions.  Due to its use in chlorophyll formation and photosynthesis, iron is often applied to enhance greening without increasing plant growth.  However, when the decision is made to apply iron, which iron source should you use?  At least five different types of iron fertilizers are commonly used.  Each have specific characteristics which should be considered when selecting an iron source.  The objective of this article is to briefly describe the forms of iron in soils, to define five types of iron fertilizers, and to provide sufficient information which will help you understand which iron source is best for your situation.

In order to understand the differences between iron fertilizers, we must first look into the soil to understand how Fe changes between two primary forms, Fe₂ (ferrous) and Fe₃ (ferric).  Both forms of iron exist in nearly every soil of agronomic importance and both forms are taken up by plants.  However, in most soils, Fe₂is more soluble than Fe₃ and, thus, soil conditions which favor Fe₂ formation will likely provide sufficient Fe for many plants without additional fertilizers.  Conversely, soil conditions which favor Fe₃ formation may lead to iron deficiencies which can be corrected by iron fertilization.

So, what conditions favor Fe₂ and Fe₃ formation?  In short, you will want to be aware of two conditions: soil moisture and pH.  As soil pH increases, the concentration of Fe₂ and Fe₃ in soil solution decreases.  A good pH to remember is 6.0.  Below 6.0, Fe₂ in soil solution will likely be in sufficient quantity to meet plant demands.  As pH increase above 6.0, the plant will be more and more likely to exhibit an iron deficiency.  Not only does pH directly influence iron solubility (Eq. 1), but it also indirectly influences it by the presence of bicarbonates.  Bicarbonates may react with iron which will create a precipitate, FeCO3.  This precipitate renders the iron unavailable for plant uptake and, thus, you may observe iron deficiencies in high pH soils.  The reaction involving bicarbonates may also be observed at lower pHs especially when irrigation water contain bicarbonates.  This phenomenon may be observed when using reclaimed water.

Equation 1.  Iron becomes more available as hydrogen ion activity increases.

 As soil moisture is increased, soil O₂ levels decrease.  It is this basic concept that manages the conversion of Fe₂ to Fe₃ in waterlogged soils.  As O₂ levels decrease, less oxygen is available to maintain the oxidized Fe₃ form and, thus, Fe₂ is more likely to form.  With more Fe₂ being available in wet soils, you may now realize why iron chlorosis is rarely observed in waterlogged soils and commonly seen in well aerated soils.  So let’s wrap up our incredibly exciting chemistry lesson for today!  Acidic and well-aerated soils favor Fe₃ and, thus, iron deficiencies in plants.  Low pH and water logged soils favor Fe₂ and rarely require any additional iron applications.

Ok, ok enough with the chemistry lessons.  I know you want to make your plants green, green, green so let’s get on to the fun stuff!  Don’t worry, don’t worry I will refer back to the chemistry part in a short bit.

Many common iron sources used in fertilizers are listed in table 1.  While it may seem appealing to choose the iron source with the highest iron percentage, ironically the lower percentage source often provide the greatest response.  This is “irony in iron”.

 Iron oxide

The highest percentage source, Iron oxide (rust), is nearly completely unavailable for plant uptake above pH 6.0.  Ironically, while it is more available below pH 6.0, iron deficiencies rarely occur below this pH because the existing iron in the soil is more soluble in acid soils.  Thus, based strictly upon agronomics, iron oxide provides very little plant benefit.  Furthermore, iron oxides are known to produce stains on solid surfaces which may create an aesthetic nightmare! (Fig. 1)

Iron sucrate

Iron sucrate (iron oxide’s sweeter cousin) is produced by blending iron oxide with sugars to form an iron containing organic complex.  Generally, molasses or some other inexpensive sugar is used.  Iron sucrate has limited water solubility and, therefore is less prone to staining than iron oxide.  Comparative research is limited; however, some research suggests that sucrates may be more effective on high pH soils than oxides.  Staining may occur with iron sucrate.

Iron sulfate

Perhaps the most common iron fertilizer used in specialty markets is iron sulfate.  Many iron sulfate fertilizers contain ferrous iron which is more plant-available than ferric, depending on pH.  When possible, it
is better to apply ferrous iron rather than ferric sources of iron.  However, with all granular iron sources, it is important to remember the following rule of thumb:  In alkaline soils, most iron in granular iron fertilizers will be converted to the unavailable form, Fe(OH)₃, within only a few days.  So if you applied iron sulfate rather than iron sucrate or iron oxide on an alkaline soil, you will only get a few days of available iron.  But at least you got a few days rather than zero days from the other sources!  Like most granular iron sources, staining may occur on concrete, especially in wet conditions.

Natural Organics

Natural products may also contain small amounts of iron.  Iron humate, biosolids, and compost are a few sources that are commonly applied.  Generally, the form of iron in natural organic compounds is more plant available than the synthetic compounds previously mentioned.  The iron is either naturally present in the organic substrate or is added during the manufacturing process.  These compounds typically contain small amounts of iron so a large amount of product may be needed in order to produce an iron response.  Staining of concrete may occur from natural organics but, in general, staining is less common than oxides, sucrates, or sulfates.

Chelates

If you are determined to turn your plant green, then iron chelates are the product you are looking for!  Most plants naturally produce small amounts of chelates on their own.  This allows plants to obtain sufficient iron to sustain life in soils of less than optimum pH.  However, when the plant needs some help, we may enlist the use of a variety of nature or synthetic chelates (Table 2.).  A chelate is a soluble, organic compound that bonds with metals such as Fe, Mn, and Mg and increases their solubility and, thus, their plant uptake.  In essence, what iron chelates do is protect the iron from precipitation to an unavailable form.   With 18 carbons (Table 2.), iron EDDHA is the most stable iron chelate and, thus, is the most resistant to precipitation at nearly every pH range you will encounter.  Iron DTPA is useful up to about pH 7.0 and iron EDTA is only stable up to around 6.5.

Iron plays a crucial role in plant health and development.  Aesthetically speaking, iron is able to produce green leaves and healthier looking plants.  I would love to tell you there is one perfect iron fertilizer that you should use in every situation.  However, as you have read, iron is dynamic and the availability of iron is largely based upon pH and soil moisture.  Whether you are using an inexpensive source, such as iron oxide, or a more valuable source, such as iron EDDHA, you should consider the characteristics of each source as well as soil pH and moisture in order to select the best source for your situation.