Iron deficiency is a frequent problem in desert soils.
The underlying problem is not really the lack of iron but the plant's inability to absorb this trace mineral due to high pH. Soils with a high content of calcium carbonate, aka caliche, can make iron unavailable to plants. When calcium carbonate dissolves in water, it forms a solution with a pH slightly above 8; this being the pH level of most desert soils. In this high pH environment, iron solubility is minimized and overwatering makes matters worse.
If an adequate amount of iron is not available to plants, iron deficiency (iron chlorosis) will result. Iron chlorosis is most easily identified as yellowing of the leaves.
However, yellowing leaves could be a symptom of other problems. If the entire leaf is yellow, the problem could be nitrogen. In severe cases of iron deficiency, the entire leaf could be yellow, too.
The way to determine whether it is nitrogen or iron deficiency is by looking at the location of the yellowing leaves. Iron deficiency occurs on the new growth, towards the tip. Nitrogen occurs from the bottom of the plant on old growth.
Another identification of iron chlorosis: the plant leaf reveals that the yellowing, or a very light green coloring, occurs between the veins of the leaf and the veins remain green.
As I have stated, the ultimate cause of iron deficiency is high soil pH. This situation could be remedied directly by lowering soil pH, but this solution is not practical under many circumstances.
Desert soils with caliche are very well buffered, which means the pH is very difficult to change long-term. It may not be practical to change the pH of soils via amendments where perennial plants and trees have already been established. Obliviously you have chosen plants that need very little iron and/or have adjusted to the high pH levels. If it's not broken, don't fix it!
However, for small volumes of soil where plants are replaced frequently (i.e. annuals, bedding plants or vegetables), adding powdered or prilled (pelleted) elemental sulfur at the following rates will increase iron availability:
In sandy or clay soil, two ounces per cubic foot of soil;
In silty soil, one ounce per cubic foot.
Sulfur should be thoroughly mixed into the soil. If you do not get the elemental sulfur to the plant's root zone, it will have little if any impact.
Chelated iron fertilizer is iron that's combined with a chemical called a chelate which makes the iron more available to plants. Chelates are man-made. Natural chelates can be found in soil organics matter.
The best chelated iron fertilizers for alkaline soils are labeled FeEDDHA, and FeEDDHMA.
Even these forms of iron will not remain available to plants indefinitely in high pH soils. When buying chelated iron, read the label carefully to make sure all the iron is in a chelated form. Some fertilizer labels indicate that the fertilizer contains chelated iron; however, many times only a small percent of the iron is chelated.
Apply chelated iron fertilizer at a rate of approximately two to three pounds to 1,000 square feet (three to five ounces to 100 square feet). This treatment will have to be repeated approximately every month during the growing season. Frequency will depend on soil and plant properties, and is best gauged by observing the plant performance.
Perhaps the most effective means of supplying iron to plants is by spraying an iron application directly on the plant leaves. A commonly used, inexpensive material for this purpose is ferrous sulfate.
Mix 1 to 2.5 ounces of ferrous sulfate in one gallon of water. A foliar application of a chelated iron solution can be used at an equivalent rate, but is more expensive and not any more beneficial. Spray this solution on the plant leaves during cool weather. In hot weather, apply in the evening to avoid burning leaf tissue.
Do not use a stronger solution than recommended, as leaf tissue is quite sensitive and easily damaged. It may be prudent to spray a small portion of the plant, wait a day and check for possible foliar damage.
Although foliar iron application is very effective, it is not a permanent solution. Foliar-applied iron moves very little within the plant, and as new leaves appear an additional application may be necessary. Regular visual inspection of all plants is advised.
Iron deficiencies are more easily avoided than corrected. It may be possible to avoid the problem through the use of prilled sulfur thoroughly worked into the soil during soil preparation. Applications could best be made during early spring or late winter, or during soil preparation for a potting medium. Practices that increase levels of soil organic matter, such as adding manure to soil, can help maintain iron in a plant-available state.
Remember, some plants are more susceptible to iron deficiencies than others, and regular visual observation of your plants is always recommended.
For more information, contact The University of Arizona Mohave County Cooperative Extension, 101 E. Beale Street, Suite A, Kingman, (928) 753-3788 or e-mail: firstname.lastname@example.org.