Iron (Fe++)

Functions

Iron is essential for many plant functions. Some of them are:

Factors Affecting Availability

High Response Crops

While this is an essential element for all plants, these crops have been found to be especially responsive: alfalfa, asparagus, barley, beans (white), beets, broccoli, brussel sprouts, cabbage, cauliflower, celery, citrus, grass, oats, peanuts, rye, rice, soybean, sorghum, spinach, strawberry, sudangrass, tomato, and turf.

Some acid-loving crops, such as blueberries, cranberries, some conifers, and some ornamentals such as azaleas and blue hibiscus may exhibit Fe deficiency induced by excessive Mn uptake.

Deficiency Symptoms

Interveinal chlorosis of young leaves. Severe deficiencies may progressively affect the entire plant turning the leaves from yellow to bleached-white.

Toxicity Symptoms

Iron toxicity is primarily pH related and occurs where the soil pH has dropped sufficiently to create an excess of available Iron. As with some other nutrients, the visible symptoms of Fe toxicity are likely to be a deficiency of another nutrient. Fe toxicity can also occur when Zinc is deficient, or the soil is in a "reduced" condition caused by very wet or flooded conditions. Excess Fe can result in Dark green foliage, stunted growth of tops and roots, dark brown to purple leaves on some plants (e.g. bronzing disease of rice).

Using Iron in a Fertility Program

Luckily, most agricultural soils provide an abundant supply of Fe to plants, because iron can be more difficult to use in a fertility program than other nutrients. Most soil application methods are often not very effective, while the needed multiple foliar applications are often too expensive, or labor intensive.

Iron presents some difficulties for plant analysis also because (1) it is often a contaminate on samples that have any dust on them, (2) Fe can exist in a leaf in a non-functional form, and (3) a crop may respond to foliar Fe due to a low Fe:Mn ratio in the tissue, even when Fe is adequate by "critical level" standards.

Recommended Rates

Method

Rate

Broadcast:

Not Recommended

In-row*:

Follow label rates

Foliar**:

1.0 to 2.0 lb./A

*Certain proprietary chelated Fe products appear to be effective when applied in the row, otherwise only foliar Fe is recommended.

**Foliar sprays should include a wetting agent. However, be sure to test the mixture for precipitation before using.

Both chelates and citrate acidified (reduced) sulfates are effective for foliar applications. Cost is the primary factor. Avoid the use of sulfates, or solutions which have a rusty color. This often indicates that the Fe has oxidized and the spray material may not be fully available to the crop.

Be concerned if visible symptoms are evident, since this indicates serious reductions in growth has already occurred. Annual corrective applications to solve Iron deficiencies can be both expensive and difficult. It is best to try and understand, then control the interaction which caused the Iron problem. Often a simple reduction in the application rate of Phosphorus or proper liming more easily solves the problem. Where an excessively high soil pH is the cause, be aware that acidifying the soil is very expensive, and not normally cost-effective for agronomic crops.

Some common fertilizer products containing Iron

Product

Chemical Formula

Typical Iron Content

Ferrous Sulfate

FeSO4·7H2O

20%

Ferrous Ammonium Sulfate

(NH4) 2SO4·FeSO4·6H2O

14%

Iron DPTA Chelate

FeDPTA

10%

Iron HEDTA Chelate

FeHEDTA

5-12%