Iron chlorosis is the most common micronutrient problem of ornamentals, shrubs, vines, small fruits, and trees in Utah. Leaves of affected plants are yellow, light green, or white with distinct green veins resulting in interveinal chlorosis. In severe cases, the leaves may be entirely white. The margins of severely chlorotic leaves often scorch and die during hot periods. Some willows, oaks, and other plants express iron deficiency with distinct black spots between the veins. Iron deficiency chlorosis may be persistent or it may vary during the season or year to year depending on environmental conditions. Often times only a portion of a plant such as a branch may be affected while the remainder of the plant looks healthy. If iron chlorosis is persistent for several years, individual limbs or the entire plant may die.
Iron chlorosis is the result of the inability of the plant to extract sufficient iron from the soil. Utah soils are typical of arid and semiarid soils around the world with lime or calcium carbonate in most of the soil profile. These soils are alkaline with pH ranging between 7.2 and 8.3. Iron chlorosis is common in these soils and is exaggerated by excessive soil moisture, soil salinity, high concentrations of phosphorous, and relatively high concentrations of copper, manganese, and zinc in the soil, low or high soil temperatures, large additions of organic matter, or inefficient root function caused by nematodes or fungal pathogens. The most important factor is the presence of lime in the soil as a predisposing factor.
Plants vary in their ability to obtain and utilize iron. This is particularly evident when adjacent plants may show marked differences in chlorosis. Some plant species are capable of obtaining iron from alkaline soils whereas others cannot be grown successfully in native high pH soils.
Preventing and controlling iron chlorosis is difficult and often gives poor results. The following are recommended procedures for controlling iron chlorosis:
- The best control of iron chlorosis is the selection of plant cultivars that are iron efficient. A good way to determine the best adapted plants for soils in your area is to examine neighborhood plants or botanical gardens. Improper selection of plants will result in disappointment and long term efforts to correct the mistake. The correction of iron availability in soils is very difficult and expensive. Therefore, the best solution to the correction of chlorosis may be to replace susceptible plants with iron-efficient plants.
- Some plant species, such as raspberries and strawberries, may be particularly desirable and worth the efforts to correct iron chlorosis. Homeowners may select and transplant raspberry or strawberry plants from existing plantings, but care should be taken to use only those plants with dark green color, vigorous growth, and high quality fruit. Systemic viruses may be present but not yet expressed in new growth.
Soil moisture management
- Water management is probably the most important consideration when growing plants in alkaline soils. In excessively wet or poorly drained soils, the chemistry of the soil changes and iron becomes unavailable. Irrigation applications should wet the plant root zone and should not be repeated until the soil moisture has been reduced by plant use and evaporation. Frequent irrigation in heavy clay soils or cold temperatures often results in a persistent deficiency of iron.
Correction with iron fertilizers or soil amendments
- Numerous iron compounds are available for treating iron chlorosis; however, responses to soil and foliar applied materials varies considerably and no single product has proven to be consistently successful.
- Soil Application: Many compound labels claim to correct iron chlorosis; however, most do not work in our highly buffered, alkaline soils. If your soil pH is below 7.2, it is possible that some products will work, but most of the soils in Utah have a pH greater than 7.2. Under these conditions it is very difficult to correct iron deficiency.
- Inorganic iron sulfate will give good results when applied to turf but their utility in other situations is generally unsuccessful. Iron sulfate will cause unsightly rust-colored spots on pavement when misapplied.
- Chelated iron compounds consist of an organic molecule that binds iron and makes it more available to plants. Chelating compounds can be derived from tree bark or other organic sources or synthetic organic molecules. One of the best compounds is EDDHA, a synthetic chelate that is available as Sequestrene 138 and Millers Ferriplus. For example, it will work on raspberries when the soil pH exceeds 7.5 whereas other chelating compounds are only effective when the pH is between 7.2 to 7.5.
- Chelated compounds must be placed into the root zone to be most effective. Incorporate lightly into the soil or irrigate in. Applications should be made in the spring to coincide with the first flush of growth. In most cases, it is necessary to treat every year.
- Acidified mining residues have a very low pH and work primarily by reducing the soil pH. Soil applications result in small zones where the pH is acidic and, consequently, iron is more available. These compounds are only effective until the surrounding soil neutralizes the acidity. When they work, they usually only last one year and are only effective in some soils. Acidified mining residues should be applied in the root zone by placing in holes 12 to 18 inches apart and 12 inches deep at the dripline of trees.
- Soil incorporation of inorganic compounds such as iron sulfate are not effective in most Utah soils.
- Foliar Application: Iron compounds sprayed on leaves give the most rapid but temporary response. Usually green spots can be seen on the leaves a few days after spraying. Repeated applications are necessary as new foliage appears.
- Chelated iron compounds or 0.1% ferrous sulfate can be applied as foliar sprays. Use a spreader-sticker to obtain better results. Avoid applications when fruit are present because staining may occur.
- Artificial soils or raised beds: An alternative to applying costly iron-soil amendments is to import non-calcareous soils (some are available at higher elevations in Utah mountains) or to prepare artificial soils from peat moss, silica sand and compost. These soil mixes can be used to make raised beds or small areas with improved soil quality. This approach is ideal for growing inefficient iron sensitive plants such as strawberries or raspberries.