The Science of Horticulture
Plants and Iron (Part Three)
In this third and final part, we'll look at prevention and remediation of iron deficiency. You'll find the first part of this series in Issue 9 and part 2 in Issue 11.
Avoiding Fe deficiency through cultural practices
As discussed in part 2, various factors can affect the chemical availability and uptake of iron, even when there's considerable amounts of the element in the soil.
In such situations, it may be possible to alleviate or even prevent iron deficiency by altering the conditions in which the plants grow. This is usually preferable to iron fertilisation, for reasons of cost and long-term effectiveness.
If iron deficiency is chronic in a given location, the choice of suitable planting material is the easiest way to avoid problems. If soils are calcareous, this will mean rejecting "acid loving" plants. Species which are native to the area or which come from regions with similar soils will be better adapted. They may have evolved more efficient iron-uptake mechanisms, for example.
Within a plant group, some species or cultivars may be superior to others. Among citrus, both rootstock and scion cultivars vary in their sensitivity to iron shortage. For example, lemons are more likely to develop chlorosis than oranges .
With the molecular basis becoming better understood, it's likely that many crops will be engineered for more efficient iron uptake in the future.
Alkalinity makes iron unavailable to plants, so excessive application of lime or dolomite should be avoided. Alkaline irrigation water can also be a problem.
Soil amendments that lower soil pH (e.g. elemental sulfur, ammonium forms of nitrogen, aluminum sulfate, iron sulfate) may be useful in mildly alkaline soils, but impractical at more extreme pH.
Phosphorus and various micronutrients can interact with iron, so avoid excess fertiliser application. Other sources of metals such as copper fungicides could be potential problems.
Drainage and Aeration, Plant vigour
Poor aeration can impair root function through oxygen deprivation and fostering root rots, affecting subsequent ability of the roots to take up iron. Ensuring adequate drainage or relieving compaction may correct the problem. Iron uptake being an active biological process, anything affecting the number or activity of roots could potentially cause iron deficiency.
As outlined above, iron deficiency can often be prevented or alleviated if other aspects of plant culture are addressed. Nevertheless, some situations may warrant the use of iron fertilisers.
Soils with inherently low iron levels may benefit from supplementation. Containerised plants in soil-free media will, of course, be largely dependent on external iron sources.
In the case of landscape plantings, both the immediate problem of unattractive chlorotic foliage as well as the long-term health and vigour of the plant overall are considerations.
A deep green colour in turf is especially valued. Iron supplementation of turfgrasses for this purpose is common, the nutrient being necessary for chlorophyll synthesis.
Iron sulfate (ferrous sulfate) is one of the most common forms of iron fertiliser, especially for turf. An additional benefit is its acidifying effect, which may be helpful in some soils (see above). Nevertheless, a lot of the iron applied may be made unavailable if soil conditions are unfavorable.
Chelates hold iron in a plant-available form, avoiding immobilisation by the soil. However, they are relatively expensive and are biodegraded in the soil after a short time. There are many chemical forms. Not all are suitable for high soil pH, becoming unstable.
Note also that iron chelates are sensitive to photo-degradation once dissolved, so growers should aim to protect stock solutions, fertigation tanks, etc from light.
Organic fertilisers may contain some iron, but the quantity will depend on the source of the materials and will require biological action to make them available. Boosting the quantity of natural chelating agents (e.g humates) in the soil is potentially an added benefit of using organic amendments in the long term, although fixation of iron by organic matter may be deleterious in sensitive situations .
A variety of other iron compounds are sometimes used in various parts of the world. These include iron oxides, ferrous ammonium sulfate, ferric sulfate, iron oxysulfate, ferric ammonium citrate (iron citrate), iron sucrate and iron humate (a by-product of wastewater treatment). Besides availability, choice of an iron fertiliser will depend on soil reaction and method of application.
Note that some iron fertilisers can stain, so take care with application near walls, paving etc.
Methods of application
Soil application is feasible if there isn't a strong tendency to immobilise iron and/or the quantity of iron required is economically practical.
If using chelates on alkaline soils, be sure the form purchased is suitable for the pH level.
Sufficient quantities of the micronutrient can be absorbed through foliage to make foliar application worthwhile in some situations.
Unfortunately, the effects are short-lived and largely localised to the treated tissues (recall that iron is not readily remobilised within the plant). Additional applications will be required as new growth appears.
However, it has the benefit that immobilising factors in the soil are by-passed. Also, response is fast. It is especially appropriate for high-value crops at critical phases (e.g tree fruits), important landscapes, or to temporarily alleviate symptoms while more permanent solutions (e.g. improved drainage) are implemented.
Iron sulfate or chelates can be used for foliar feeding. Chelates penetrate better because the organic molecule is not repelled by the leaf's waxy surface as much as a charged ion.
Direct injection of iron compounds directly into tree trunks is possible. It might be an option where valuable specimens are involved, but requires special equipment and expertise.
References and Further Reading
For more information about the general aspects of iron nutrition discussed above, see list of links on www.calyx.com.au/fertiliser_iron.html