Iron is the fourth most abundant element found in soil though it is largely present in forms that cannot be taken up by plants. Iron, in small amounts, is essential for healthy plant growth and is classed as a micronutrient. It is important for the development and function of chlorophyll and a range of enzymes and proteins. It also plays a role in respiration, nitrogen fixation, energy transfer and metabolism. As with other nutrients, plants can have too much iron but this primarily affects the uptake of other nutrients rather than producing direct toxicity symptoms.
The amount of iron and its availability in soil is influenced by the following:
- pH ‑ high pH reduces iron availability, low pH increases it
- Organic matter ‑ organic matter provides iron and makes it more readily available
- Moisture ‑ excess water in the soil, particularly in acidic soils, increases iron availability even to the point of toxicity
- Aeration and compaction ‑ compacted and/or poorly aerated soils have an increased iron availability, particularly if the soil is acidic
- Phosphorus ‑ excess phosphorus inhibits the uptake of iron
- Nitrogen ‑ certain forms of nitrogen can reduce iron uptake
- Zinc ‑ zinc deficiency can increase the uptake of iron in some plants, excess zinc decreases iron uptake
- Manganese ‑ excess manganese inhibits the uptake of iron
- Potassium ‑ potassium deficiency may increase iron uptake
- Molybdenum ‑ excess molybdenum can reduce iron uptake especially in alkaline soils
- Nickel ‑ excess nickel can reduce the uptake of iron
- Bicarbonate ‑ bicarbonate in the soil can reduce iron uptake
Ideal Soil Iron Levels
Soil is typically between 1% and 5% iron but because most of this iron is unavailable, it is difficult to set an ideal amount for soil. Some estimates suggest that soil should have at least 0.001 g of iron in every 100 g of soil (or 10 mg/kg).
Iron deficiency is most likely to occur in plants when the soil is alkaline or when the level of phosphorus, nitrogen, zinc, manganese or molybdenum in the soil is high. Heavy metal contamination can also lead to iron deficiency.
Symptoms of Iron Deficiency
Interveinal chlorosis is the most common symptom of iron deficiency. Symptoms are usually seen on young leaves first.
Treating Iron Deficiency
It is particularly important to treat iron deficiencies because some plants respond to a lack of iron by making the soil directly around their roots more acidic and this can cause imbalances in other plant nutrients. Treating iron deficiency is however, quite complicated. Adding phosphorus, nitrogen, zinc, manganese and molybdenum should be avoided unless one of these elements is deficient in the soil. Plenty of organic matter should be added to the soil.
Soil pH should be tested and amended if required. Iron is most available to plants when the pH of the soil is 7 or lower though most plants should be able to uptake sufficient iron if the pH of the soil is 8 or lower provided other essential elements are well balanced. If the soil is too alkaline, it is especially important to lower the soil pH if you're growing acid loving plants.
Apart from these key things, it's also important to ensure the soil doesn't dry out and a thick layer of organic mulch will assist with this as well as adding more organic matter to the soil.
If symptoms persist after taking the above actions, it would be a good idea to have your soil tested to determine the amount iron in the soil (in a form that can be used by plants). You should also test the amount of phosphorus, nitrogen, nickel, zinc, molybdenum and bicarbonate in the soil. If any of the later are present in toxic quantities in the soil you should do what you can to reduce the excess levels. If bicarbonate is in the soil, it would be a good idea to test the water you use for irrigation to ensure that isn't the source of the contamination. If not, it would be a good idea to seek expert advice on how to treat the problem because there are a wide range of factors unique to your soil conditions, which can affect the level of bicarbonate in the soil.
When all of these issues are either excluded or remedied then you may consider adding iron to the soil. Organic sources of iron include some iron chelates (check the label to see if it's certified as suitable to use on an organic property) and synthetic sources include ferrous and ferric sulfate.
Iron toxicity is most commonly associated with highly acidic soil though symptoms of iron toxicity are mostly symptoms of other nutrient deficiencies. For this reason, if you notice such symptoms and can't seem to remedy the issue by ensuring the soil has a neutral pH and then following the recommendations on the relevant page, it's a good idea to order both soil and plant tissue tests to see whether it's actually an excess of iron that's causing the problem.
Symptoms of Iron Toxicity
Symptoms of true iron toxicity usually include bronzing of the leaves and possibly also the formation of brown spots on leaves.
Treating Iron Toxicity
If excess iron is indeed the problem, treatment can be just as complicated as the treatment of iron deficiency. Firstly, the soil pH should be checked an amended if required ‑ aim for a neutral pH unless the plant you're growing requires acidic (or alkaline) soil. Drainage should also be improved if the soil is waterlogged and the soil should be aerated if it is compacted. Also, be careful to only water when necessary.
If a soil test reveals that the soil is deficient in any element, this should be treated next. Pay particular attention to zinc or potassium deficiencies as these can be the cause of iron toxicity or could make it worse.
If nutrient deficiency symptoms and/or iron toxicity symptoms are still visible on plants months after these treatments, you may need to boost the levels of the nutrients in the soil a little to overcome the effects of a severe excess of iron. Always be careful to do this gradually and in consultation with both soil and tissue testing in order to prevent toxicity of any other element.
I would also recommend that you investigate the source of excess available iron in the soil. It may be that the soil is being contaminated by an external source ‑ contaminated bore water or runoff from nearby properties for instance.