Iron nutrition in plants is a very complicated subject. It’s difficult to get your head around it. One of the reasons is that there are many chemical forms of iron in soil, and they vary in their solubility and availability to plants. It can comprise up to 5% of the Earth’s crust, the most common form being ferric oxide, Fe2O3, or hematite. You have seen this in the form of red soils in various parts of the world. But this iron form, common as it is, is not available to plants. There are many other forms, including oxides, hydroxides and sulfides. Iron also exists in two common ionic states, the ferrous or +2 ion, and the ferric or +3 ion.
So, as fertilizer people, how do we sort this out? Number one, understand that iron deficiency is almost never a lack of iron in soil. It is a lack of iron AVAILABILITY. What makes iron unavailable? Well, many things. High pH, high manganese, calcium, phosphorus, copper or zinc will reduce availability. Also overly wet or waterlogged soil, especially if they are cool. Hot, dry summers can also aggravate iron deficiency. But why? Aren’t those conditions the opposite of cool and waterlogged?
The answer comes down to roots, and how plants deal with iron and iron availability. First, plants must MINE iron. They expend energy to actively absorb it. When plants are iron deficient, their root structure changes both chemically and physically. They grow more root hairs, and create special cells that spit out hydrogen ions, which are acidity. This is pretty cool. When plants can’t get enough iron, they secrete acids in the root zone to make iron more available. That’s a neat trick. However, this acid secretion only happens right behind the root tip in newly developed roots. Therefore, if your soil conditions are not conducive to new root growth, the plants will have trouble absorbing iron.
Many of us have seen overly wet container plants in nurseries showing iron deficiency. Plants growing in saturated media have difficulty growing new roots, and therefore difficulty in absorbing iron. One of the first things I do is check iron deficient plants for root rot. Root disease and iron deficiency often go hand-in-hand. Sometimes the plants with a little root disease will not suck the pot dry, and will stay wetter than the surrounding plants. The cycle of root disease and iron deficiency will spin until it gets out of hand until you have some plants with good size, good color and good roots, while others are stunted and yellow with few roots. Blocks of plants with iron deficiency from being too wet will also show less iron deficiency along the edge of the block, where plants dry out faster.
Now, let’s talk about the quality of that water in overly wet nursery plants. Many of you deal with customers who have high pH, alkaline water. This water tends to make media pH rise. Overly wet plants will not only often have a higher pH, but the carbonates and bicarbonates in the root zone moisture will reduce iron availability. Some growers have high iron in their irrigation water, but this iron is rarely available to plants. I remember seeing hibiscus plants that were covered in rust. However, when you wipe the rust away, the plants were iron chlorotic underneath. They were covered with iron, but iron they could not use to make chlorophyll.
Grasses, primarily those in the family Poaceae, use an additional trick to absorb iron. They can secrete chelating agents from their roots in order to make iron more available in the immediate root zone. This is oversimplified, but plants basically must either reduce Fe+3 to Fe+2 in order to make it available, or they have to chelate it in the root zone.
A very important consideration in iron nutrition are iron to manganese ratios. Iron and manganese are antagonistic to each other, both in soil and in plant tissue. Too much of one will tend to induce a deficiency of the other. In most plants, I like to see the iron to manganese ratios roughly one to one, both in soil and leaf. If the ratio is more than two to one in either direction, the low element may well show deficiency symptoms. Remember this, it will help you a lot.
However, there is another dirty little secret to consider. (I TOLD you this was complicated, right?) The secret is that you can’t necessarily count on the iron in a tissue analysis being available to the plant. Plants can often take up iron in unavailable forms, especially in monocots. Many times I have compared tissue analysis of green and chlorotic plants. The chlorotic ones often show MORE IRON than the green plants do. Why would this be? The deficient plants “know” they are deficient, and they are working in the root zone to absorb more iron, though it isn’t necessary available for making chlorophyll. I also learned a long time ago that if you look at a tissue analysis of a chlorotic plant, and everything is in the good range, then iron is likely your problem. Dust also contains a lot of iron, and if leaf samples are not properly washed prior to analysis, the dust will affect the iron level in the reading.
So, to sum up, you can take away the following:
- Soil iron it exists in many chemical states, most of which are not available.
- Plants needs to have new root growth in order to properly absorb iron.
- Iron availability is greatly reduced by high soil pH.
- Iron deficiency can also be induced by high manganese, high calcium, high phosphorus, high copper or high zinc.
- Iron in tissue analyses aren’t necessarily available, especially in monocots.
- Your clients can avoid many of these problems with good foliar applications of iron chelates, or with soil applied iron products.