What are Iron Chelates?
Iron chelates are compounds that stabilize iron ions and protect them from oxidation and precipitation and make iron more available to the plant. They consist of, Fe3+ ions, a chelating complex such as EDTA, DTPA, EDDHA, amino acids, humic/fulvic acids and citrates, and sodium (Na+) or ammonium (NH4+) ions.
Iron is mostly applied as iron chelates. Un-chelated iron applied to the soil as a fertilizer is often ineffective as it quickly transforms to the Fe3+ ion which precipitates. All plants need iron to grow, but some plants have a higher need for iron, these are known as ‘ Acid loving plants’ and include Roses, Hydrangeas and other flowers.
What are the different iron chelates
The most common iron chelates include Fe EDDHMA, Fe EDDHA, Fe DTPA, Fe HEDTA and Fe EDTA. They are stable at different pH ranges as the chart below indicates and differ widely in price. EDTA is the most common chelating agent for other micronutrients.
pH ranges at which chelating agents are stable
Which is the most preferred iron chelate
Fe 6% EDDHA is the favorite Iron chelate for Kenyan flower growers, although Fe DTPA is cheaper and perfectly suitable in many circumstances.
What are the important considerations when choosing an iron chelate?
The main considerations when choosing a chelating agent are:
1. The operating pH of the irrigation water and the growing medium
|Chelate formulation pH Range at which it is effective
|Fe EDDHMA 3-11
|Fe EDDHA 3-10
|Fe DTPA 1.5 - 7.5
|Fe HEDTA 1.5 - 7
|Fe EDTA 1.5-6.5
2. The cost – always balance the cost benefit when considering iron chelates.
3. The type of cultivation – iron chelates are broken down by light so in hydroponic systems and recycling often extra iron has to be added, and it is preferable to use even a small amount of the more stable EDDHMA form.
4. Fe EDTA has a high affinity for calcium and it is advised not to use it in calcium rich soil or water.
Are Chelated fertilizers compatible with other fertilizers?
Iron chelates interact with other fertilizers and are therefore traditionally mixed in the A tank, with the calcium nitrate. High phosphate will lock up the iron and make it unavailable. Other metallic ions, e.g. copper, zinc, manganese, will replace the iron in the chelate complex and make it unavailable. They are not compatible with sulphur and will dissociate at high pH and in sunlight.
What are the symptoms of iron deficiencies and toxicity?
Iron is essential for the production of chlorophyll, and a deficiency manifests itself as a yellowing of the younger leaves, while the veins remain a dark green in contrast to the leaf. The leaves turn whiter and whiter, the growing tips die, and eventually the whole plant dies. Chlorophyll is essential for the plant to make sugars from the sunlight, so even a small deficiency that will not be so obvious in the leaves, can affect production and quality. In this case a plant tissue analysis will identify the problem.
Iron toxicity tends to be non-symptomatic, but causes ‘plugging’ of the plants nodes, reducing uptake of other nutrients, lowering production and quality. This can only be identified by plant tissue analysis.
How can a grower monitor and manage Iron deficiencies in crops?
Iron deficiencies are the result of iron not being available rather than not being present, and this is determined by many factors. It is important to do regular soil analysis and to control the pH of the drip solution. Make sure that the soil is not compacted or flooded. Plant tissue analysis will pick up any ‘ hidden hunger’ that is preventing a crop realize its full production potential. A weekly iron based foliar spray will quickly resolve an iron deficiency (locate a product on http://shambaza.com/