Unmasking Iron Deficiency
Unmasking Iron Deficiency
When leaves on a crop turn from green to yellow, the finger of blame often gets pointed to nitrogen (N) deficiency, or less often to magnesium (Mg) deficiency. However, many times the real culprit is a lack of iron. Soil tests and tissue tests often miss detecting iron deficiency, because although it may be present in the soil or within the plant, it isn’t available for use.
Plant-available iron levels are difficult to determine for two reasons. First, iron is easily bound up chemically in the soil when the pH is less than 5.6, and most agronomic crops are grown at ranges between 6.5-8.0. Although a soil test will show that iron is present in the soil, most of it can’t be absorbed by the plant. Second, iron is relatively immobile even once it gets inside the plant; once it gets into a leaf, it stays there and does not circulate even if other leaves are deficient.
“When you’re doing a conventional tissue test, you’re sampling around the new growth,” says Rick de Jong, International Business Development manager for Agro-K. If you’re not sampling old and new leaves separately, you’re going to miss it. Or the tissue sample may indicate iron is a little bit low, when actually it is very low. Tissue testing often does not provide the entire picture.”
de Jong says he has seen examples in the Eastern U.S. and Eastern Canada where iron levels in the ground are strong, but because of nutrient interactions in the soil – particularly with phosphorus, the iron is inhibited from coming into the crop.
“We had a really high iron level in the soil, so we didn’t connect the dots around the nutrient interactions going on,” de Jong says. “Nitrogen was a bit low, so we applied nitrogen when really we should have been applying iron.”
Novacrop Sap Analysis measures the nutrients actually circulating within the plant and provides an accurate picture of what is available and being used by the plant at that point in time. As sap analysis becomes more widely used, iron deficiencies that were interpreted as nitrogen or magnesium deficiencies are being uncovered. Applying N or Mg may green up the foliage, but it only masks the true issue.
Iron Supports Chlorophyll Production
JP Jacobson, Technical Sales Representative at Agro-K, says adding N or Mg in this situation can compound the problem by creating nutrient imbalances in the soil. Nitrate, in particular, takes more photosynthetic energy because it needs to convert to ammonium first before it becomes amino acids and proteins. Iron is a catalyst that supports photosynthesis, so the demand for iron increases as photosynthesis takes place.
“Nitrogen and magnesium also play a role in chlorophyll formation, so adding them can make the leaves greener,” Jacobson says. “But it’s not addressing the true issue of the iron deficiency. In fact, by applying these other nutrients, we’re not supporting chlorophyll production as well as we should. We’re making the leaves greener, but we’re tapping the iron lower and lower. We’re forcing the plant to do something without providing the food it needs to do it.”
de Jong says he has gone back to growers with a diagnosis of iron deficiency and has been met with surprise. It’s easy to prove, though, by applying foliar iron.
“With the right form of foliar iron, the response is visible within just a few hours,” he says. “All of a sudden, the leaves are getting green. Growers will say, ‘My dad used to put foliar iron on all the time. Why did we stop doing that?’”
A Two-Pronged Solution
Because there are two main causes of iron deficiency, a two-pronged solution is required. Chelated iron products keep it from becoming bound up and unavailable in the soil. Plants take up nutrients only when they are soluble, and chelates are molecules that bind to the iron, holding it in solution. The trick is to balance the strength of the chelate so it keeps the iron in solution while still allowing it to be taken up by the plant.
Several types of chelates exist, and they are effective at different pH levels. EDTA is effective up to pH 6.0, but just 50% of iron remains in solution at pH 6.5. DTPA is excellent up to pH 7.0, but just 40% of iron is still in solution at pH 8.0. EDDHA and its salts, EDDHSA, maintain iron availability to plants past pH 9.01. Using a chelate appropriate for your soil pH range is key.
Agro-K’s Photo-Op 0-0-1 provides liquid chelated iron using EDDHSA to provide maximum iron availability in the soil rhizosphere to correct and prevent iron deficiency. It also contains the essential micronutrients sulfur, potassium, manganese, and cobalt. Jacobson recommends using it in-furrow at planting for best results, and it can also be applied in season through fertigation. Ensuring crops have enough iron during periods of maximum vegetative growth is vital.
Foliar iron in a formulation easily absorbed by plants is the other part of the solution. Agro-K’s Iron Dextro-Lac® 5.5% answers this need by providing iron as a simple sugar (dextrose-lactose) that takes little energy expenditure from the plant to use.
“You get almost 100% uptake with the first 48 to 72 hours,” de Jong says. “Even within a few hours you can start to see a response. It can rapidly address an iron deficiency in a crop and reduce the impact on yield. For a longer-term solution, providing iron in a stabilized form in the soil with Photo-Op early in the season provides support for chlorophyll formation that is so important during periods of vegetative growth.”
1Argo, B., and P. Fisher. Understanding pH Management. Meister Publications, 2002