Manganese Fertilizers in Agriculture

Manganese (Mn) was first identified as an essential nutrient for plants by J.S. McHargue in 1922. Plants generally contain high amounts of manganese, though its levels vary widely. This can be seen in crops such as rice and wheat (grasses) and legumes.

Manganese is one of the essential micronutrients for crops. It acts as a component or activator of many enzymes and plays a catalytic role in chlorophyll synthesis. Applying manganese fertilizers can improve photosynthetic efficiency, promote the transport of photosynthetic products, enhance nitrogen metabolism, and facilitate the reduction of nitrate in plants, which supports protein synthesis and increases nitrogen use efficiency.Manganese deficiency leads to stunted growth, chlorosis and whitening of leaves, malformed or incompletely developed fruits, shortened internodes, and halted root growth.

Table 1. Manganese Content in Several Crops

Crop Type	Grain (mg·kg⁻¹)	Straw (mg·kg⁻¹)
Rice	20 – 250	280 – 900
Wheat	16 – 140	30 – 350
Beans	14 – 80	110 – 130

Causes of Variation in Manganese Content

1. Metabolic control & ion antagonism

   Manganese uptake is regulated by plant metabolism and competes with other cations during absorption. For instance, Mg²⁺ and Mn²⁺ antagonize each other, reducing Mn²⁺ uptake.

   
   

2. Environmental & soil pH influence

   Manganese uptake is highly affected by soil conditions, especially pH. On alkaline soils (pH > 7), plant Mn content is low (usually below 100 mg·kg⁻¹ dry weight). On acidic soils (pH < 7), manganese levels can be high, sometimes exceeding 1600 mg·kg⁻¹, leading to Mn toxicity. As soil pH increases, available Mn decreases, which lowers plant Mn content.

   
   

3. Growth stage & tissue distribution

   Manganese content changes during crop growth and varies among plant organs. For example, in maize, Mn decreases as plants age, concentrating along leaf edges (up to twice the Mn concentration of leaf tissue). In sugar beet, Mn in petioles is only half that of leaves, showing its uneven distribution.

Plants mainly absorb Mn²⁺, which has low mobility inside the plant. Deficiency symptoms usually appear in young to medium-aged leaves, rather than the youngest. In monocots, Mn is more mobile than in dicots, so Mn deficiency in cereals often appears first in older leaves.

Roles of Manganese in Agriculture

1. Participation in Photosynthesis & Prevention of Premature Senescence

Chloroplasts contain high Mn levels, and Mn is essential for maintaining their structure. Without Mn, chloroplast membranes are destroyed, leading to chlorophyll loss. For example, in Mn-deficient spinach, thylakoid layers reduce and merge into vesicle-like structures. Chloroplasts are the most Mn-sensitive organelles.Manganese also regulates Fe³⁺/Fe²⁺ balance in plants by influencing redox potential. Excess Mn uptake can induce Fe-deficiency chlorosis.

2. Regulation of Enzyme Activity

Manganese activates many enzymes, including Mn-superoxide dismutase (Mn-SOD), which protects the photosynthetic system from oxidative stress and stabilizes chlorophyll. Mn influences nitrogen metabolism, photosynthesis, and redox balance by affecting enzyme activities. For example, Mn²⁺ activates several dehydrogenases (citric acid, oxalosuccinate, α-ketoglutarate, malate, oxaloacetate dehydrogenases) in the TCA cycle, critical for respiration.Deficiency symptoms resemble those of Mg deficiency, but differ in location: Mn deficiency appears first in young leaves, while Mg deficiency shows in older leaves.

3. Promotion of Seed Germination and Seedling Growth

Mn stimulates seed germination and early seedling development by enhancing auxin-mediated coleoptile elongation. It accelerates starch and protein hydrolysis in seeds, providing sugars and amino acids for seedlings. Treatments such as seed coating with Mn improve germination, seedling vigor, and fruit set. Adequate Mn supply also enhances vitamin C synthesis and mechanical tissue strength, and supports root growth. Without Mn, lateral root growth nearly ceases.

4. Involvement in Protein, Carbohydrate, and Lipid Metabolism

Mn is closely related to nitrogen metabolism. Its deficiency leads to an increase in nitrate, nitrite, amide, free amino acids, and soluble proteins, while soluble sugars decline (especially in roots). Chloroplast glycolipids and polyunsaturated fatty acids decrease by about 50%.

5. Promotion of Phosphorus Absorption and Utilization

Mn enhances photosynthesis, respiration, and nitrate reduction, indirectly improving phosphorus and calcium availability. Cotton and other crops show higher P uptake with Mn supplementation.Mn-sensitive crops include legumes, wheat, potato, onion, spinach, apple, strawberry; moderately sensitive are barley, beet, clover, celery, radish, tomato; less sensitive are maize, rye, and forage grasses.

6. Improvement of Stress and Disease Resistance

Mn supports vitamin C synthesis and stem strength. Adequate Mn improves resistance to diseases and reduces frost injury susceptibility.

Diagnosis of Manganese Requirement

• Leaf analysis: Normal growth occurs at 20–100 mg·kg⁻¹ Mn in leaves. Below 20 mg·kg⁻¹ indicates potential deficiency.

  
  

• Indicators: Mn deficiency leads to nitrate accumulation in leaves, oxalate crystals in xylem, and high peroxidase activity.

  
  

• Soil available Mn classification:

  • < 50 mg → very low

  • 50–100 mg → low

  • 100–200 mg → medium

  • 200–300 mg → rich

  • 300 mg → very rich

On Mn-deficient soils, fertilizer application generally improves yields significantly.

Why Supplement Manganese Fertilizers?

Mn is crucial as part of enzymatic systems, directly involved in photosynthesis, germination, ripening, and P and Ca uptake. Deficiency is common in soils high in organic matter, neutral to alkaline soils, over-limed acidic soils, coastal saline-alkaline soils, sandy soils low in organic matter, and soils high in Fe and Ca.

Foliar application is the most efficient method since Mn availability in alkaline or sandy soils is low. Foliar spraying matches crop demand periods, is absorbed 8 times more efficiently than soil application, requires small doses, and provides high cost-effectiveness.

Application Methods

1. Basal Fertilizer

   Use slow-release or soluble Mn fertilizers, often mixed with acidic or organic fertilizers. Typical MnSO₄ dosage: 15–60 kg/ha; manganese slag: ~10 kg/ha.

   
   

2. Foliar Spray

   Spray MnSO₄ at 0.05–0.1% concentration (0.3–0.4% for fruit trees, 0.03% for legumes, 0.1% for rice). Apply at critical stages: cotton (flowering to boll formation), winter crops (spring regrowth), fruit trees (flowering).

   
   

3. Seed Dressing

   Coat seeds with 2–4 g MnSO₄ per kg seed (up to 5 g for beet). Dissolve Mn fertilizer in water, spray evenly on seeds, dry, and sow.

   
   

4. Seed Soaking

   Soak in 0.05–0.1% MnSO₄ solution (1:1 ratio with seed) for 12–14 hours. Avoid under drought conditions (may lower emergence rate).

Types of Manganese Fertilizers

Fertilizer	Formula	Mn Content (%)	Properties
Manganese Sulfate	MnSO₄·H₂O	29.3–31.8	Pale pink crystals, water soluble
Manganese Oxide	MnO	66–70	Green/gray powder, sparingly soluble, slow effect
Manganese Chloride	MnCl₂·4H₂O	27.5	Pink crystals, water soluble
Manganese Carbonate	MnCO₃	43–44	Pink or white-brown powder, sparingly soluble
Manganese Dioxide	MnO₂	55–60	Black/gray crystals or powder, sparingly soluble
Manganese Nitrate	Mn(NO₃)₂·4H₂O	21	Light pink crystals, water soluble
Ammonium Manganese Sulfate	3MnSO₄·(NH₄)₂SO₄	26	Light pink powder, water soluble
EDTA-Mn	C₁₀H₁₂N₂O₈MnNa₂·2H₂O	13	White powder, neutral, highly soluble
Manganese Citrate	—	~20	Light orange powder, water soluble
Manganese Slag	—	—	Industrial by-product, effective in Japan for 30+ years

Learn more about Manganese(II) Sulfate Monohydrate:

https://www.kelewell.de/en/post/manganese-ii-sulfate-monohydrate-mnso-h-o-a-key-player-in-agriculture-batteries-industry

Conclusion

Manganese plays multiple physiological roles in crops as a component and activator of various enzymes. It promotes carbohydrate and nitrogen metabolism, and is closely linked with crop growth, development, and yield. Mn is essential in photosynthesis, respiration, and nitrate reduction. This “quiet caretaker of life” delicately regulates energy, nutrition, and resistance within crops. Recognizing deficiency or toxicity symptoms and applying Mn fertilizers correctly ensures healthy growth and high yields.