Understanding Iron in Hydroponics

Pale and pasty new growth, green veins on a yellow background, stunted plants and slow growth — these are all signs of what may be an iron deficiency or uptake issue. While there can be a couple of different reasons for these types of symptoms, one of the most common is iron – a problem that is not necessarily due to a lack of iron in the nutrient solution.

Iron (Fe) is a trace element essential for plant growth and development. However, in hydroponics iron is often not well understood in terms of supply and uptake. In particular, hydro-organic and aquaponic systems often struggle with finding suitable forms of iron, and deficiency symptoms are not always caused by low iron in the root zone. To complicate matters further, the early foliar symptoms of iron and manganese deficiency look remarkably similar and even experienced growers can find it hard to differentiate between the two.

Iron and Plant Nutrition

_{Advanced iron deficiency can cause an overall yellowing of the new growth.}Iron is the micronutrient required in the largest qualities in hydroponic nutrient formulations. It is an essential component of proteins and many vital enzymes contained in plant chloroplasts as well as electron transfer proteins in the photosynthetic and respiration chains. Because iron is involved in the synthesis of chlorophyll and is essential for the maintenance of chloroplast structure and function, a lack of iron rapidly shows up as the development of pale new foliage.

Iron is an immobile element within the plant tissue, so it can’t be redistributed from old foliage to new, thus young developing leaves and tissue need a continual supply of iron from the root zone. Typical average foliar levels of iron in leaf tissue are around 50-350 PPM, however, this is somewhat dependent on species and growing conditions. In hydroponic nutrient solutions, iron may be run at levels of between 0.5-6 PPM. The actual requirement for iron is dependent on several factors such as plant species, growing medium, and growing conditions including temperature and plant growth rate. Growers in high-light climates, for example, run iron levels in the nutrient solution much higher than those in lower light, cooler climates to allow for these differences in plant requirements.

(Read also: The Do’s and Don’ts of Foliar Feeding)

Iron deficiency symptoms occur first on the youngest leaves with interveinal chlorosis (yellowing between the leaf veins that remain green), however, in the early stages, yellowing may be uniform on the leaves or even appear as a pale green coloration. Under severe deficiency conditions, leaves may take on a very pale or even white appearance with necrotic spots and distorted leaf margins on some plant species. Under severe iron deficiency, growth and flowering are inhibited and the reduction in chlorophyll synthesis can lead to severe plant stunting due to a lack of assimilate production from reduced photosynthesis.

Iron Sources and Chelates

_{An organic hydroponic plant showing typical iron deficiency symptoms.}Some iron is commonly found in water supplies, however, this is present as iron hydroxide and is not available for hydroponic plant uptake. Iron in this form can cause issues in irrigation systems where it can block drippers, emitters, and filters and levels greater than 5 PPM can cause toxicity symptoms in some sensitive plants, particularly if the pH level in the root zone is below 5.5. For this reason, low levels of iron in a water supply analysis can’t be used for plant nutrition in the same way that other minerals such as calcium and magnesium can be. All of the iron required for a hydroponic crop must be provided in the correct form for plant uptake, otherwise deficiency symptoms will rapidly show and growth will be compromised.

In conventional hydroponic systems iron is supplied as a type of chelate such as iron EDTA (6-14 percent Fe), EDDHA (6 percent Fe) or DPTA rather than as iron sulphate, which is unstable in solution and tends to form iron hydroxides, which are insoluble. This reaction can be prevented by protecting the metal iron inside a chelating agent or ligand. During plant uptake of the chelated iron, the chelating agent is dissociated and remains in the nutrient solution. Chelation allows the iron to remain available for plant uptake at a wide range of pH values, however, maintaining correct pH levels assists with iron remaining available for use.

_{Early stages of iron chlorosis on a hydroponic tomato plant.}The choice of which iron chelate to use is dependent on the pH of the nutrient solution. Iron EDTA and DPTA are stable up to a pH of 6.5, whereas iron EDDHA should be used where pH levels rise above pH 7.0. Fe-EDDHA is commonly used in substrate systems often where a high alkalinity water supply is present and where pH is more difficult to directly control and raises above pH 7.0. Fe-EDDHA is also more suitable for use in systems such as aquaponics where pH levels are typically maintained above 6.5. In organic production, organic chelating agents may be used. These include citric acid, humates, amino acids, and fulvic acid complexes.

(Read also: How Amino Acids Aid Plant Growth)

Diluted iron chelate solutions can also be used as a foliar spray, applied as a fine mist with a good quality surfactant mixed in to help with spreading and sticking to the leaf surface. While these are not used to supply the plant’s entire iron requirement, foliar sprays are useful for the correction of deficiencies and where the root system may have been compromised and unable to take up sufficient iron for growth.

What Causes Iron Uptake Issues?

While a lack of iron, or the incorrect form of iron in a nutrient solution, will directly cause iron deficiency, there are several other factors that can prevent or restrict iron uptake in the root zone. The most common of these is excessively cool conditions, particularly with warm-season crops that have a limited ability to take up iron under low temperatures.

Oversaturation of the root zone due to excessive irrigation or a substrate that holds too much moisture is another cause or iron deficiency symptoms due to limited iron uptake. When the growing medium becomes waterlogged, this excludes oxygen from the root zone that is required by the root tissue for iron uptake. Often, low temperatures are combined with overly saturated root zones in winter cropping areas and this amplifies the problem with iron uptake.

_{Iron is involved in the synthesis of chlorophyll and is essential for optimal photosynthesis.}Any other factor that has a negative effect on root health also restricts iron uptake, the most common of these are root rot pathogens such as Pythium. The reduction in healthy root volume and activity caused by disease results in a lack of iron uptake. Often, pale coloration of the new foliage or yellowing in the plant tops is an early sign of root pathogen infection and warrants a root inspection. Despite the use of iron chelates that allow iron to be more stable at a wide range of pH values, a pH that is too high can also induce iron chlorosis.

In conventional hydroponic systems where pH is poorly controlled due to a hard water supply, organic hydroponic systems and aquaponics where pH may run naturally high, levels above 7.5 may still develop iron deficiency symptoms despite chelates being used.

(Read also: Hard Water or Soft Water: Does My Garden Care?)

Unexpected iron depletion is another cause of iron deficiency symptoms — this may be just due to inadequate iron supplies in the nutrient solution to start with or due to high rates of plant uptake. However, some nutrient treatments can also break down iron chelates. These include the use of sterilization treatments for pathogen control such as UV and ozone. With UV treatment it is possible to prevent iron problems by increasing the levels of iron chelate and using the most stable chelate forms which are Fe-EDDHA, followed by Fe-DPTA. Ozone treatment can also break down iron chelate and may cause precipitation of manganese. Both these compounds may require adjustment or monitoring where ozone is in use.

Iron availability and uptake by plants can be a little more complex than many other elements, however, a few simple rules can help prevent iron chlorosis issues. These include keeping the root system healthy, well oxygenated, and at the correct moisture level; ensuring temperatures in the root zone are optimal for the species being grown; and, most importantly, check that the form of iron in the nutrient solution is in a correctly chelated form and at the right level for the crop and growing environment. If iron chlorosis is suspected, a leaf analysis from an analytical lab can be used to confirm levels and foliar sprays are useful to correct mild deficiencies.