The TURF[TECH]REPORT–Iron’s Function in Plant HealthFebruary 2, 2018
Iron is one of the most recognizable micronutrients in plant science. Although needed in small quantities, its impact on plant health is profound. Iron is a key component of various enzymes involved in the synthesis of chlorophyll (dark green pigmented molecules that drive photosynthesis in plants). Iron transports oxygen, as well as carries electrons throughout the plant. When turf grass is suffering from an iron deficiency, its physical appearance is negatively impacted. Turf grass that exhibits a rich, deep green color can only be achieved when iron is abundant in the soil. Interestingly, iron comes in many forms. In this article, we’ll discuss the various forms of iron available in the market so that you can choose which form is best for your turf.
Iron Uptake in Plants
Iron uptake typically occurs under the soil surface at the root tips. Iron is transported in the form of various organic complexes or chemical compounds, but rarely in its pure atomic form. The common forms of iron that plants uptake are the oxidized states, Fe2+ (ferrous form - soluble) and Fe3+ (ferric form –insoluble). Uptake occurs in various ways, including a chelation mechanism whereby plants release siderophores (molecules that bind and transport iron which enhance iron’s solubility.) Iron is most efficiently taken up by new roots or root hairs, so it is important to maintain a healthy and active root system for optimal plant health.
Type of Iron
After examining the chart to the right, it may seem most appealing to use the iron fertilizer with the highest percentage of Fe, but that is not necessarily the case. Let’s break it down and figure out which iron fertilizers are best for plant and turf health.
IRON SULFATES (FERROUS SULFATE, FERRIC SULFATE): Iron sulfates are some of the more commonly used granular iron fertilizers in the market. Many iron sulfate fertilizers contain ferrous iron sulfate which is more readily available for the plant than ferric sulfate fertilizers (which are dependent on soil pH). For example, if the soil is alkaline, the iron sulfate in the iron fertilizer will convert to an unavailable form of iron. It’s more beneficial to apply ferrous iron over ferric iron fertilizers. Staining may occur on concrete if not swept away after application, especially during wet conditions.
IRON SUCRATE (FERRIC SUCRATE): Another well-known granular iron fertilizer, iron sucrate is produced by combining iron oxide with sugars. This type of iron has limited water solubility so it is less prone to staining. This fertilizer is desirable for areas that have large potential staining areas. Compared to iron sulfates, iron sucrates are less prone to staining when quickly swept off concrete.
IRON OXIDES (FERROUS OXIDE, FERRIC OXIDE): Iron oxides present some of the highest percentages of iron. It may seem logical to choose any one of these as an iron source for turf, however iron oxide is completely unavailable for plant uptake above a pH of 6.0.
IRON CHELATE (can be a foliar spray): This is a form of iron that can easily be absorbed by turf, resulting in a quick response when applied to soil. Chelated iron is responsible for the quick green up observed within a few days after application. Since most plants already produce small amounts of chelates, this additional application will allow the plant to obtain iron more efficiently, especially in soils that are experiencing less than optimal pH levels. Iron chelates protect iron from converting into a less available form. There are three forms of chelated iron:
- DTPA iron: Largest molecules of the iron chelates. Stable up to a pH of 7.0, can bond with multiple metals and other compounds to help transport iron into the plant easier.
- EDDHA iron: Mid-range molecule size of the iron chelates. Most stable of the iron chelates; most resistant to converting into a less available form of iron at almost every pH range.
- EDTA iron: Smallest molecules of the iron chelates. Stable up to a pH of 6.5; can bond with multiple metals and other compounds to help iron transport into the plant easier. Will more readily release its iron atom once transported to its destination within the plant.
NATURAL ORGANICS: Natural organics such as biosolids, iron humate and compost contain small amounts of iron. Iron found in natural organic compounds tends to be more available to the plant since it is naturally present in the organic substrate. Larger amounts of organics need to be used due to these compounds typically containing small amounts of iron. Staining still may occur on concreted, but less likely compared to sulfates, sucrates and oxides.
Optimal Iron Levels in Soil
Note: mg/kg is equivalent to parts per million (ppm)
Iron in Soil – Factors that Influence Iron Availability
According to the Nutri-Facts facts article, “It’s not Surprising–Iron Is Required by Plants,” most soils contain thousands of pounds of iron but much of it is not readily available for plant uptake; this is why iron deficiencies in turf are so common. As mentioned previously, iron aids in the synthesis of chlorophyll, so it’s no surprise that plants deficient in iron suffer from a condition called “chlorosis.” Chlorosis is commonly diagnosed by a pale green color, yellowing sheathes, sharp distinction between its interveinal issues and its leaf veins. Often these symptom first appear when younger leaves begin to yellow. Severe iron deficiency symptoms can manifest by turning the entire plant white. There are several different soil factors that can influence and lead to iron deficiencies, such as:
SOIL pH IMBALANCES: Iron becomes readily available in a soil where pH is 6.0. Liming acidic soils is anecessity in order to optimize turf health but an overload of applications can cause iron deficiencies.
LOW LEVELS OF ORGANIC MATTER: Many plants are unable to take up nutrients from the soil without the assistance of organic compounds produced by beneficial microbes. These small living organisms consume organic matter (dead plant residues) and through digestion, turn this matter into nutrients a plant can absorb, thus fostering the kind of environment that promotes plant growth and health. Low levels of beneficial soil microbes may indirectly lead to low levels of available iron for plant consumption.
NUTRIENT IMBALANCE: The proper distribution of primary (N, P, K), secondary nutrients (Ca, Mg, S) and micronutrients (Fe, Mn, Zn, Cu, B, Mo, Cl, N) in the soil (and ultimately in the plant) is fundamental in order to fully reap the benefits of proper turf management practices (fertilizer, control products and soil amendment applications). A balance between all these nutrients is essential in order to maintain a soil conducive to optimal turf health. For example, excessive phosphorous (P) levels can trigger iron deficiencies. Focusing on feeding the soil can enhance plant/lawn health and nutrient uptake.
Iron deficiencies in turf are sometimes difficult to pin-point because multiple factors (including but not limited to other nutrient deficiencies or imbalances) can overshadow them. The absence of iron may also be mistaken for pest infestations, disease or herbicide damage. If chlorosis-like symptoms occur, test the soil to establish existing soil fertility, pH, and composition.
Managing Iron Deficiencies
Iron deficiencies can be managed with a short-term application of an iron-based foliar spray or with nitrogen fertilizers, although the best mode of action is prevention. New roots and root hairs become active during the iron uptake process. When outside factors interfere with root development (such as poorly balanced soils or imbalanced nutrients), iron uptake is disrupted. Identifying the true cause of an iron deficiency with a soil test can create a better understanding of why iron is depleted in turf while preventing the problem from occurring again. Focusing on (1) feeding the soil and (2) maintaining a healthy and active root system are paramount in helping to keep turf free from any potential iron deficiencies.
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