The Science of Horticulture
Plants and Iron (Part Two)
In Part One (see www.queenslandgardening.com/horticulture/IH_9.html), the requirement that plants have for iron and symptoms of deficiency were discussed. In this issue, we'll look at the interaction between iron, plants and soil.
Iron in the soil
Iron (chemical symbol: Fe) is generally abundant in soil, but the chemical forms that usually predominate are unusable by plants. Unfortunately, a high total iron content does not necessarily mean that good plant growth can be supported.
Most soil iron exists in the oxidised ferric form, Fe3+. Under certain conditions, it can be converted to the more useful ferrous form, Fe2+. However, the concentration of this plant-available iron in soil is typically far lower and can be growth-limiting.
The formation of insoluble complexes with other soil components such as phosphorus are other impediments to adequate iron availability in soil.
In Australia, there are many soils so high in iron oxide they have a rusty red color. In some parts of coastal Australia, however, iron sulfide is laid down under waterlogged conditions. These are the notorious acid sulfate soils. Sulfuric acid is formed on exposure to air by agriculture or development, with subsequent problems for infrastructure and environment.
Organic residues in soil can also contain iron, and certain molecules such as humic acid can act as natural chelating agents (see below).
Conditions exacerbating iron deficiency
Deficiency in otherwise iron-sufficient soils can be induced by various environmental conditions including certain horticultural practices.
Iron deficiency is a common problem of calcareous soils, when it is sometimes called "lime-induced chlorosis" (referring to the resultant symptom). Bicarbonate (HCO3-) released into the soil solution interferes with iron uptake and transport by plants. Calcium ions can also interfere with uptake.
Excessive application of agricultural lime can likewise induce iron deficiency.
Soil alkalinity in general or irrigation with alkaline water can be a problem because iron becomes increasingly unavailable as pH rises.
Over-fertilisation with phosphate or other nutrients which interact negatively in the soil or within the plant can induce deficiency. Manganese, zinc, copper, molybdenum have been implicated in various cases. Over-use of copper fungicides is a potential danger in this regard.
Anything affecting the number and health of roots, such as temperature extremes, waterlogging, other causes of oxygen deprivation, pests and diseases or other mineral deficiencies/toxicities could contribute to iron deficiency in the plant by restricting uptake.
Waterlogging can also induce deficiency by causing more bicarbonate to become available in calcareous soils.
Given the reactive nature of iron, it's not surprising that plants employ complex processes to ensure uptake and transport. Most iron uptake occurs near the growing root tips. An enzyme (reductase) in the root cell membrane coverts Fe3+ to Fe2+, which is carried into the cell by a transporter protein. It's an active metabolic process that requires energy.
When iron availability becomes limiting, additional mechanisms are activated. Responses fall into two categories:
"Strategy I" is employed by most plants except grasses (but including other monocots). Response includes excretion of hydrogen ions to acidify the soil around the roots, secretion of iron-capturing molecules and increased reductase production. There may also be proliferation of roots and development of specialised cells to enhance uptake.
"Strategy II" is used by the grasses. They exude phytosiderophores from their roots. These are natural chelating agents that can bind Fe3+ and allow it to be transported across the roots' cell membranes.
Chelation is very important in biology because it provides a way for organisms to move elements such as iron and control their reactivity.
A chelating agent is a molecule that is able to "grab" a metal ion. The resultant compound is called a chelate, derived from the Greek word for the claw of a lobster or crab. Among the most significant chelates in nature is chlorophyll, which contains magnesium. The leghemoglobin of plant nitrogen-fixing nodules (see part 1 of this article) is an iron chelate.
Grasses manufacture and secrete their own chelating agents - phytosiderophores - to capture iron from soil and allow it to be transported into the root.
Furthermore, some bacteria and fungi produce siderophores for their own iron uptake and certain components of soil organic matter such as humic acid have natural chelating properties. These can potentially be exploited by plants.
In horticulture, artificial chelates are a way of formulating iron fertilisers to avoid soil immobilisation and improve plant availability (compared to simple compounds like iron sulfate, for example).
EDTA is one commercially-produced synthetic chelator which may be familiar, although better chelates for use in agriculture have since been developed.
While less commonly encountered than deficiency, iron toxicity is also possible. It can be a problem in nursery production of ornamentals, especially with sensitive species.
Iron sulfate is a long-used treatment for moss infestation of turf. The greater sensitivity of the moss provides a certain selectively of action when applied at suitable concentrations, and has the additional benefit of supplementing the surviving grass with iron.
Coming in Part Three...
Prevention and treatment of iron deficiency will be discussed in the final part of this article. In the meantime, refer to the links on www.calyx.com.au/fertiliser_iron.html if you'd like to learn more about iron nutrition in plants.