Magnesium deficiency in plants is a widespread nutritional disorder affecting crops, ornamentals, and garden plants across the UK. As the central atom in chlorophyll, magnesium is essential for photosynthesis and numerous metabolic processes. Deficiency typically manifests as interveinal chlorosis—yellowing between leaf veins whilst veins remain green—particularly on older leaves. Sandy soils, high rainfall, excessive potassium fertilisation, and acidic conditions increase deficiency risk. Understanding the causes, recognising characteristic symptoms, and implementing appropriate corrective measures through foliar sprays or soil amendments ensures optimal plant health and productivity. This guide provides practical information for diagnosing and managing magnesium deficiency in various plant types.

Understanding Magnesium Deficiency in Plants

Magnesium deficiency represents one of the most common nutritional disorders affecting plants in both agricultural and horticultural settings. As an essential secondary macronutrient (alongside calcium and sulphur), magnesium plays a critical role in plant physiology, serving as the central atom in the chlorophyll molecule—the pigment responsible for photosynthesis. Without adequate magnesium, plants cannot effectively convert light energy into chemical energy, leading to impaired growth and reduced productivity.

Magnesium functions beyond photosynthesis, acting as a cofactor for numerous enzymatic reactions within plant cells. It facilitates the activation of enzymes involved in carbohydrate metabolism, protein synthesis, and nucleic acid formation. The element also plays a vital role in phosphate metabolism and Mg-ATP production, enabling plants to utilise phosphorus efficiently. Additionally, magnesium contributes to the structural stability of ribosomes and helps regulate cellular pH balance.

Plants absorb magnesium from soil solution primarily as the divalent cation Mg²⁺. The availability of magnesium in soil depends on various factors including soil pH, cation exchange capacity, and competition with other nutrients. Unlike nitrogen or phosphorus, magnesium is mobile within plant tissues, meaning it can be redistributed from older leaves to younger, actively growing tissues when supplies become limited. This mobility characteristic produces distinctive visual symptoms that aid in diagnosis.

Understanding magnesium's fundamental role in plant metabolism helps explain why deficiency can severely compromise crop yields, ornamental plant appearance, and overall plant health. Recognition of this deficiency and appropriate corrective measures are essential for maintaining optimal plant nutrition and productivity.

Recognising Symptoms of Magnesium Deficiency

The visual symptoms of magnesium deficiency follow a characteristic pattern that reflects the nutrient's mobility within plant tissues. Interveinal chlorosis—yellowing between the leaf veins whilst the veins themselves remain green—represents the hallmark sign of magnesium deficiency. This distinctive symptom typically appears first on older, lower leaves because magnesium is preferentially mobilised to support new growth when supplies are inadequate.

As deficiency progresses, the chlorotic areas may develop further discolouration. In some species, affected tissues turn orange, red, or purple due to the accumulation of anthocyanin pigments. Severe deficiency can lead to necrosis, where leaf tissue dies and turns brown, typically beginning at leaf margins and progressing inward. The pattern and severity of symptoms vary considerably among plant species, with some crops showing more pronounced visual indicators than others.

Common symptoms across different plant types include:

• Fruit trees and soft fruit: Interveinal chlorosis on older leaves, premature leaf drop, and reduced fruit quality

• Tomatoes and peppers: Yellowing between veins progressing to brittle, curled leaves

• Brassicas: Distinctive marbling or mottling patterns on older foliage

• Roses and ornamentals: Yellowing lower leaves with green veination, reduced flowering

• Lawns and turf: Pale, yellowish striping between veins, though iron deficiency is more commonly the cause of chlorosis in UK lawns

It is important to note that interveinal chlorosis can also indicate other nutritional disorders, particularly iron or manganese deficiency. However, these typically affect younger leaves first, whereas magnesium deficiency predominantly impacts older foliage. Environmental factors such as cold or waterlogged soils and root damage can also mimic deficiency symptoms. Accurate diagnosis may require soil testing or plant tissue analysis to confirm magnesium levels and rule out other potential causes of similar symptoms.

Causes and Risk Factors for Low Magnesium in Plants

Multiple factors can precipitate magnesium deficiency in plants, with soil chemistry playing a primary role. Acidic soils (pH below 6.0) commonly exhibit magnesium deficiency because low pH increases the solubility and leaching of magnesium, particularly in sandy or light-textured soils with low cation exchange capacity. Conversely, highly alkaline soils can also induce deficiency by reducing magnesium availability, even when total soil magnesium content appears adequate.

Cation competition represents another significant cause of magnesium deficiency. Excessive levels of potassium, calcium, or ammonium in soil can interfere with magnesium uptake through competitive inhibition at root surfaces. This antagonistic relationship is particularly problematic in intensively managed systems where high rates of potassium fertilisation are applied. The imbalance between these nutrients, rather than absolute magnesium deficiency, often triggers symptoms in plants.

Key risk factors include:

• Soil type: Sandy, light soils with poor nutrient retention capacity, common in many parts of the UK

• High rainfall or irrigation: Excessive water movement leaches magnesium from the root zone

• Intensive cropping: Repeated harvests deplete soil magnesium reserves without adequate replacement

• Excessive potassium fertilisation: Creates nutrient imbalance and competitive uptake inhibition

• Liming practices: Over-application of calcium-rich lime can induce magnesium deficiency

• Poor root development: Compacted or waterlogged soils restrict root growth and nutrient uptake

• Low soil Mg index: UK soils with Mg index 0 or 1 are at higher risk of deficiency

Certain crops demonstrate higher magnesium requirements and greater susceptibility to deficiency. Tomatoes, potatoes, apples, grapes, and roses are particularly prone to magnesium deficiency. Container-grown plants face elevated risk due to limited soil volume and increased leaching from frequent watering. Understanding these risk factors enables growers to implement preventative strategies and maintain optimal magnesium nutrition throughout the growing season.

Treating and Preventing Magnesium Deficiency

Correcting magnesium deficiency requires both immediate intervention to address acute symptoms and long-term strategies to prevent recurrence. Foliar application of magnesium sulphate (Epsom salts) provides the fastest response, with improvements often visible within 7–10 days. A solution of 20 grams per litre of water, applied to leaf surfaces until runoff, delivers magnesium directly to deficient tissues. This approach proves particularly valuable for rapidly growing crops or when soil conditions limit root uptake. Applications should be made during cool, overcast conditions to minimise leaf scorch risk, and generally limited to 2-3 applications at 10–14 day intervals unless soil or tissue testing indicates otherwise.

For longer-term correction, soil amendments restore magnesium levels and address underlying deficiencies. Magnesium sulphate can be incorporated into soil at rates of 25–50 grams per square metre for vegetable gardens or across the root zone of established trees or shrubs. Dolomitic limestone, which contains both calcium and magnesium carbonates, serves dual purposes by raising soil pH whilst supplying magnesium—ideal for acidic soils. However, application rates should be based on soil test results rather than generic recommendations, as over-liming can harm plants, especially ericaceous (acid-loving) species.

Preventative strategies include:

• Regular soil testing: Conduct tests every 2–3 years to monitor nutrient levels and pH

• Balanced fertilisation: Use complete fertilisers containing magnesium or supplement with magnesium-specific products

• Organic matter incorporation: Compost and well-rotted manure improve soil structure and nutrient retention

• Appropriate liming: When raising pH, use dolomitic rather than calcitic lime, but only where soil tests indicate need

• Controlled potassium application: Avoid excessive potassium fertilisation that induces magnesium deficiency

For container plants, incorporating controlled-release fertilisers with magnesium or using growing media pre-blended with dolomitic limestone helps maintain adequate magnesium levels. Regular feeding with liquid fertilisers containing magnesium ensures continuous supply throughout the growing season. Optimal soil pH varies by crop—while many plants thrive between pH 6.0–7.0, ericaceous plants require acidic conditions (pH 4.5–5.5), and some brassicas prefer slightly alkaline soils. Always follow product labels and base application rates on soil or tissue test results.

When to Seek Professional Horticultural Advice

Whilst many cases of magnesium deficiency can be successfully managed through home remedies and standard fertilisation practices, certain situations warrant professional horticultural consultation. Persistent symptoms that fail to respond to magnesium supplementation after 3–4 weeks suggest either misdiagnosis or complex underlying issues requiring expert assessment. Professional soil analysis and plant tissue testing can identify multiple nutrient deficiencies, toxic element accumulation, or soil chemistry problems that simple visual diagnosis cannot detect.

Commercial growers, market gardeners, and those managing valuable ornamental collections should consider professional advice when deficiency symptoms appear across multiple plants or entire crops. The economic implications of reduced yields, poor fruit quality, or plant losses justify investment in comprehensive soil and tissue analysis. Professional agronomists can develop tailored nutrient management programmes that address specific soil conditions, crop requirements, and production goals whilst optimising input costs.

Seek professional guidance when:

• Symptoms persist despite appropriate magnesium supplementation

• Multiple nutrient deficiencies appear simultaneously

• Soil or growing media pH falls outside normal ranges and requires correction

• High-value crops or specimen plants are affected

• Deficiency symptoms appear suddenly across previously healthy plantings

• Uncertainty exists regarding diagnosis or appropriate treatment rates

The RHS Advisory Service provides guidance for home gardeners, while commercial growers can access resources through the Agriculture and Horticulture Development Board (AHDB). BASIS/FACTS-registered agronomists offer independent, qualified advice tailored to regional soil conditions and climate. Many organisations offer soil testing services with interpretation and management advice. For organic growers, specialist advisors can recommend approved products and practices that maintain certification standards whilst addressing nutritional deficiencies. Professional consultation proves particularly valuable when managing complex growing systems, protected cropping environments, or intensive production where precision nutrition management directly impacts profitability and sustainability.

Frequently Asked Questions