Ferrous Sulfate in Ornamental Plant Care

Ferrous sulfate is one of the essential care materials for flower and ornamental plant enthusiasts. Especially for evergreen foliage plants grown in warm regions, proper application of ferrous sulfate can effectively address soil alkalization and iron deficiency, supporting healthy plant growth. So, under what conditions do plants need ferrous sulfate, and how should it be applied correctly?

Ferrous sulfate appears as fine crystalline powder and is readily soluble in water. Its primary functions are to supply iron to plants and to regulate soil pH. Iron is a key micronutrient required for chlorophyll synthesis; deficiency leads to leaf chlorosis. At the same time, the mildly acidic nature of ferrous sulfate helps improve alkaline soils, creating a more suitable growing environment for acid-loving plants.

Not all flowers are suitable for ferrous sulfate application. It is mainly used for plants that prefer acidic or slightly acidic soils, such as cycads, azaleas, camellias, daphne, spider plants, poinsettias, jasmine, gardenias, osmanthus, michelias, and other ornamental flowering or foliage plants. It is particularly effective in preventing and correcting iron-deficiency chlorosis caused by soil alkalization. When any plant shows typical iron-deficiency symptoms, ferrous sulfate can also be used as an emergency remedy.

Typical symptoms of iron deficiency include yellowing of young leaves while the veins remain green, forming a clear “interveinal chlorosis” pattern. As the condition worsens, leaves may turn pale or white and eventually dry out. Older leaves usually do not yellow first—this is a key feature that distinguishes iron deficiency from other nutrient deficiencies.

In routine care, ferrous sulfate is often applied not only after iron-deficiency symptoms appear, but more commonly as a soil pH regulator. Regular application helps maintain soil acidity and prevent chlorosis, especially in northern regions with alkaline soils.

Core Functions of Ferrous Sulfate

1. Correction of Iron-Deficiency Chlorosis

Iron is indispensable for chlorophyll synthesis. When plants lack iron, chlorophyll cannot form properly, leading to yellowing of new leaves with green veins. Ferrous sulfate supplies readily absorbable ferrous iron (Fe²⁺), rapidly alleviating chlorosis and restoring deep green foliage. The effect is especially noticeable in iron-sensitive plants such as azaleas, gardenias, roses, and citrus.

2. Regulation of Soil pH

When dissolved in water, ferrous sulfate exhibits weak acidity, effectively lowering soil pH. This helps release iron that is otherwise fixed in alkaline soils, converting it into a plant-available form. It is particularly suitable for calcareous soils or soils that have become alkaline due to long-term use of alkaline fertilizers, creating an optimal pH range of 5.5–6.5 for acid-loving plants.

3. Enhancement of Photosynthesis and Plant Growth

Adequate iron ensures normal chlorophyll synthesis, improves photosynthetic efficiency, and enables plants to accumulate more nutrients. This promotes lush vegetative growth and abundant flowering. Long-term, proper use can improve weak growth and small, thin leaves.

4. Auxiliary Disease and Moss Control

The mildly acidic environment created by ferrous sulfate solutions can inhibit the growth of certain mosses, lichens, and pathogens that prefer alkaline conditions, indirectly supporting plant health. However, existing diseases still require appropriate fungicides for treatment.

Application Methods and Precautions

1. Prepare Fresh Solutions to Avoid Oxidation

Once dissolved, ferrous iron (Fe²⁺) is easily oxidized by oxygen in the air into ferric iron (Fe³⁺), which is poorly absorbed by plants. Therefore, solutions must be prepared fresh and used immediately. Do not store prepared solutions or use solutions that have been left standing for long periods.

2. Strictly Control Dosage and Frequency

Excessive application can cause iron toxicity, leading to blackened, rotting roots and brown leaf spots, and may interfere with the uptake of calcium and magnesium. For soil mixing, use about 5–7 g per pot. For foliar spraying or root drenching, maintain a concentration of 0.2%–0.5%. Application is generally recommended once per month; during vigorous growth periods it may be increased, but not more than twice per month.

3. Use Suitable Water for Dilution

Ferrous sulfate readily oxidizes and precipitates in alkaline water. Rainwater, snowmelt, or cooled boiled water is preferred. If tap water must be used, slightly acidify it first—add 1–2 g of monopotassium phosphate per 10 L of water, or a few drops of 3% vinegar—before dissolving ferrous sulfate.

4. Combine with Potassium Fertilizers to Improve Efficiency

When applied in alkaline soils, ferrous sulfate can be used together with an appropriate potassium fertilizer (such as potassium sulfate). Potassium promotes iron transport and utilization within plants. Avoid using wood ash at the same time, as its strong alkalinity neutralizes acidity and reduces iron availability.

5. Avoid Light Exposure in Hydroponic Systems

When using ferrous sulfate in hydroponic cultivation, avoid direct sunlight. Light accelerates iron oxidation and precipitation, reducing uptake efficiency. Wrap containers with black cloth or paper, or move them to shaded indoor locations.

6. Mix with Well-Decomposed Organic Fertilizers for Better Results

Applying ferrous sulfate together with decomposed organic fertilizer solutions (such as fermented oil-cake or manure extracts) improves effectiveness. Humic substances formed during organic matter decomposition can chelate iron, preventing oxidation, improving solubility and absorption, while also supplying additional nutrients.

7. Avoid Simultaneous Use with Ammonium Nitrogen and Antagonistic Elements

Ammonium nitrogen fertilizers (such as ammonium sulfate, ammonium bicarbonate, and urea) can disrupt iron–organic complexes and accelerate oxidation of Fe²⁺ to Fe³⁺. Elements such as calcium, magnesium, manganese, and copper antagonize iron uptake. Therefore, avoid applying these fertilizers at the same time; allow an interval of 10–15 days.

8. Choose Between Root Drenching and Foliar Spraying Appropriately

If soil is alkaline but plants show no chlorosis, root drenching is recommended to adjust soil pH. If plants already show clear chlorosis, combine root drenching with foliar spraying. Foliar application allows faster iron uptake and quicker relief. Spray in the early morning or evening, avoiding high temperatures and strong sunlight to prevent leaf burn.