Pumping iron: Managing an important micronutrient

Iron deficiency is a common greenhouse crop disorder, and it can be especially common during the spring bedding plant season.

Interveinal chlorosis is a tell-tale sign of insufficient iron in the plant tissue and, since it is an immobile nutrient, it is found on the newest growth. Iron deficiencies can show up for a variety of reasons, from higher-than-average iron requirements, reduced iron uptake, diminished iron availability, and/or insufficient iron provided to plants. This article will cover the fundamental causes of iron deficiencies in plants and how to avoid and correct them.

One of the first reasons why iron deficiencies may show up, is simply due to plant requirements. Greenhouse crops are commonly grouped into one of three categories, with respect to their pH requirements: 1) “general” (5.8 to 6.2); “geranium” or “iron-efficient” group (pH 6.2 to 6.5); and 3) “petunia” or “iron-inefficient” group (pH 5.5 to 5.8). There is no real way to change the iron requirements for different plants, and you can’t grow a crop of zonal geraniums in lieu of petunias or calibrachoa. However, there are instances where smarter plant selection can help.

For instance, when selecting the different plants to serve as thrillers, spillers and fillers in mixed containers, consider their different pH requirements and try to minimize the differences in pH groups (which directly relates to iron requirements). By trying to maintain comparable cultural requirements within species in mixed containers, you’ll be more effective in avoiding iron deficiencies resulting from discrepancies among the different plants.

As mentioned, crops can be classified by their pH requirements, which is directly related to their micronutrient requirements. As the root zone pH increases, micronutrient availability decreases; alternatively, as pH decreases, micronutrient availability increases. The three primary factors interacting to affect root zone pH during production are: 1) pH of the growing substrate; 2) alkalinity of the irrigation water; and 3) potential acidity or basicity of the fertilizer. When possible, try to plant crops in the “petunia” or “iron-inefficient” group into substrate with a lower pH than the general mixes. Next, for producers with an irrigation water source that has water alkalinity above 150 ppm or 3.0 meq/L, acid injection can reduce alkalinity which, in turn, makes it easier to maintain the lower root zone pH for iron-loving plants. The specific amount of acid needed will depend on the alkalinity of the water source and which acid (sulfuric, phosphoric or nitric) is used.

The final factor affecting substrate pH is the potential acidity or basicity of fertilizers. A greater proportion of nitrate (NO3-) in fertilizers will cause the root zone pH to rise over time, whereas a greater proportion of urea and ammonium (NH4+) will cause the pH to drop over time. Fertilizers with a higher proportion of NO3- (i.e. 15-5-15) are popular for spring crop production under cooler air temperatures, since the nitrifying bacteria that convert NH₄⁺ to NO₃- for plant uptake are less active. When possible, use potentially acidic fertilizers with a greater proportion of NH4+ to help acidify the root zone and make iron (and other micronutrients) more available to iron-inefficient plants.

Even when iron-loving plants are planted in substrate with the correct pH, watered with acidified water, and provided with an acidic fertilizer, iron deficiencies can still occur simply if insufficient amounts are being provided to plants.

One leading cause of iron deficiency can stem from “growing lean,” or providing low fertilizer concentrations. Greenhouse growers are learning the value of providing less fertilizer to plants, from reduced waste to better growth control.

However, when less nitrogen is supplied to plants, fewer micronutrients are also being provided. Complete fertilizers are the most common fertilizer used in greenhouse production, as they provide all the essential macro- and micronutrients.

When using lower fertilizer concentrations be sure to use fertilizers with a higher proportion of micronutrients relative to macronutrients. For example, comparing “regular” 20-10-20 to 20-10-20 Peat Lite, the Peat Lite formulation has twice the amount of micronutrients in it as the regular formulation at any given nitrogen concentration. There are also fertilizers with even more micronutrients than Peat Lite formulations. Using these fertilizers allow growers to apply less nitrogen, while still providing sufficient iron.

When more iron is needed than can be provided from your fertilizer, supplemental applications of micronutrient blends can be used. Water soluble fertilizers such as Mix of Soluble Traces (MOST) or Soluble Trace Element Mixture (STEM) provide all six essential micronutrients — but only micronutrients — and can be added to fertilizer to enhance micronutrient concentrations or applied as a stand-alone application.

One thing to keep in mind with these micronutrient fertilizers is that the effectiveness of these mixes in solving iron deficiencies will depend on the cause of the iron deficiency and concentrations of other micronutrients. Other micronutrients such as zinc and manganese can compete with iron for uptake, and when the concentrations of these nutrients become too high, they can suppress or antagonize iron uptake.

In an ideal world, substrate, water and fertilizers would all be adjusted so sufficient iron is provided to plants and iron deficiencies can be avoided. Unfortunately, reality is not ideal, and while it is best to manage the root causes of iron deficiencies, sometimes you need a quick remedy.

This targeted approach can be taken by providing iron to crops by using iron chelate drenches. Chelates are a type of molecule that, when combined with micronutrients, keep them soluble for plant uptake in pHs that would normally make them unavailable. There are two common iron chelates for applying in the greenhouse, including: 1) iron-EDDHA (Sprint 138, Sequestrene 138); and 2) iron-DTPA (Sprint 330, Sequestrene 330). Both compounds provide iron when provided to plants.

However, their solubility across pHs and iron concentrations differ, and these two properties are inversely related. Iron-EDDHA has the lowest concentration of all three compounds (6%), however, it is the most widely soluble across pHs and is considered the most effective choice.

Alternatively, Iron-DTPA has a higher concentration (10%), though is not quite as soluble at higher pHs. Always follow label instructions when applying these compounds. After applying chelates to the substrate, be sure to rinse the foliage if applied overhead, as any iron chelate solution left on the foliage can burn.

Do your best to prevent iron deficiencies before they even start.

Thankfully, if interveinal chlorosis does appear, there are opportunities to green crops up using modest interventions, including iron chelate applications.