This is the second part of the Everything You Need to Know About Plant Nutrition, therefore before you read further I would highly suggest you to read the first part of this multi-part series. You can read it by clicking here.
What are Macronutrients in Plant Nutrition
Macronutrients are essential elements for a plant that are required in relatively large quantities. They are essential for the plant’s growth, development, and reproduction. These nutrients play a fundamental role in helping the plant survive by completing physiological, biochemical, and structural processes.
Macronutrients in plants are classified into two parts – primary macronutrients and secondary macronutrients
The primary macronutrients are nitrogen (N), phosphorus (P), and potassium (K)
The secondary macronutrients are calcium (Ca), magnesium (Mg), and sulphur (S).
Primary Macronutrients for Plant Nutrition
Nitrogen (N)
Role of Nitrogen in Plants
Nitrogen is a primary macronutrient that is an essential element for the vegetative growth of the plant. This nutrient is required in large quantities by the plant and is a major component that functions in photosynthesis to make the plant’s food. It is essential for growth and development in the form of amino acids and for cell division in nucleic acids (DNA & RNA). works in the form of amino acids for growth and development, and in nucleic acids (DNA & RNA) for cell division and new growth.
It supports the growth of leaves, stems, and roots, promotes rapid and healthy plant growth, aids in better absorption of nutrients and water by keeping the plant healthy, and helps maintain the green colour of leaves for a longer time. This also helps increase yield and improve the quality of fruits and grains.
Symptoms of Nitrogen Deficiency
Older leaves start to turn yellow from a light green color; the yellowing begins at the tips of the leaves and spreads to the entire leaf, and the leaves become smaller. The stems become weak, fewer buds form, and the leaves start to dry up and fall prematurely. This causes fewer flowers and fruits to be produced, and yield decreases.
Effects of Excess Nitrogen
Excess nitrogen causes excessive vegetative growth, making leaves dense and dark green. It weakens stems, causing them to break easily, while encouraging the plant to produce more leaves than flowers and fruit. Excess nitrogen also increases the plant’s susceptibility to pests and diseases and delays crop maturity, ultimately reducing yield quality.
Remedies for Nitrogen Deficiency
Based on soil testing and crop requirements, apply recommended amounts of nitrogen fertilizers such as urea, ammonium sulphate, or calcium nitrate at the time of sowing/transplanting. Apply nitrogen in 2–4 split doses instead of a single application; this increases nitrogen use efficiency and reduces losses. Also, to quickly correct nitrogen deficiency, spray nitrogen on the leaves. Use well-rotted FYM (farmyard manure), vermicompost, or green manure to increase soil fertility and ensure nitrogen is released gradually. Grow leguminous crops as part of a crop rotation or as intercrops to maintain biological nitrogen fixation. In addition, depending on the crop, use Rhizobium, Azotobacter, or Azospirillum. Use biofertilizers to maintain nitrogen availability. After applying fertilizer, keep the soil adequately moist for better nutrient uptake; avoid excessive irrigation, as this helps preserve soil moisture.
Note– Regularly monitor crop growth and apply fertilizers as recommended by soil testing to avoid both nitrogen deficiency and excess.
Source of Nitrogen Fertilizers
Urea — 46% Nitrogen (N), Ammonium nitrate — 35% Nitrogen (N), Calcium nitrate — 15% Nitrate nitrogen, Magnesium nitrate — 11% Nitrate nitrogen, Nano urea — 4% Total nitrogen (N), Nano urea plus — 20% Nitrogen (N)
Phosphorus
Role of Phosphorus
Phosphorus strengthens the plant’s roots so that the plant can establish itself and properly supply nutrients and water. It also facilitates energy transfer, enabling easy cell division and new tissue formation, and plays a major role in photosynthesis and carbohydrate metabolism. Phosphorus also plays an important role in the reproductive phase, significantly improving pollination, fertilization, seed production, and stress tolerance. This positively impacts flowering, fruiting, and seed formation, helping to maintain seed quality, seed size, crop maturity, and uniformity. As a result, the crop achieves better overall growth and higher yields.
Symptoms of Phosphorus Deficiency
A plant with phosphorus deficiency will have poor root development, reduced new branches and tillering, small and thin leaves, stunted growth, and older leaves turning dark green and reddish-purple. It also delays flowering and maturity, resulting in lower yields and poorer crop quality.
Effect of Excess Phosphorus
Excess phosphorus reduces the plant’s uptake of micronutrients, leading to deficiencies in zinc (Zn), iron (Fe), copper (Cu), and manganese (Mn). The internodes of the plant become shorter. Small leaves, stunted growth, young leaves may turn yellow while the veins remain green, micronutrient imbalance, which leads to poor plant health, and a decrease in the yield and quality of fruits, seeds, and grains.
Corrective Actions for Phosphorus Deficiency
Apply DAP or SSP at the time of planting according to the crop and on the basis of soil testing; also use PSB (Phosphorus Solubilizing Bacteria) culture and maintain the soil pH between 6 and 7 for better phosphorus absorption. Apply well-decomposed organic matter to improve phosphorus availability. If using phosphorus fertilizer near the root zone, the plant will yield better results.
Sources of Phosphorus Fertilizer
- DAP (Diammonium Sulphate) – 18% Nitrogen and 46% Phosphorus
- SSP (Single super phosphate) -16% to 20% Phosphorus ,11%to 12% sulphur and 18% to 21% Calcium
- TSP (Triple super phosphate) – 46% Phosphorus,12% to 15% Calcium and 1% to 2% sulphur
- MAP (Monoammonium Phosphate)- 11% to 12% Nitrogen and 52% to 61% Phosphorus
- APP (Ammonium Polyphosphate) liquid fertilizer- 10% to11% Nitrogen and 34% to 37% Phosphate
- Bone meal (Organic) – 12-16% Phosphorus and 20% to 25% Calcium
Potash (K)
Role of Potash in Plants
Potash controls the opening and closing of the stomata, thereby increasing water-use efficiency, enhancing photosynthesis, aiding the formation and transport of sugars within the plant, and activating enzymes that support protein and carbohydrate synthesis.
Potash strengthens the roots, increasing nutrient and water uptake to maintain disease resistance and prevent the impact of fungi and bacteria. It strengthens plant tissues, which helps in better flower and fruit formation, reduces flower drop, increases fruit set, and improves the quality of the harvest. In addition, it also enhances the plant’s resistance to drought and cold. It helps strengthen stems, prevents crops from falling over, improves grain filling and seed quality, and assists in increasing yield.
Symptoms of Potash Deficiency
Yellowing appears on the edges of older leaves, while the central part of the leaf initially remains green. At the same time, the leaf edges appear brown, dry, and scorched, which is also called Marginal Burn. Some older leaves curl upwards or downwards. The plant’s internodes become short, reducing its overall height and slowing its growth. Flowering and grain filling are reduced. Fruits may be small, pale, and of low quality. Due to this deficiency, disease resistance is reduced, which affects the quality and yield of fruits and grains, leading to lower quality and reduced production.
Effects of Excess Potash
Excess potassium creates competition for magnesium absorption, leading to magnesium deficiency. High potash levels also inhibit calcium absorption, disrupting nutrient balance; this results in impaired cell wall development and weakened plants, often leading to increased incidence of Blossom End Rot in tomatoes and peppers. Excess potash can cause interveinal chlorosis (yellowing between veins) in older leaves and potentially reduce crop yield, while also negatively affecting the quality and size of fruits and grains.
Corrective Action for Potash Deficiency
Apply recommended potash fertilizers based on crop and soil requirements. Conduct a soil test and apply the correct amount of potash fertilizer according to the crop needs to maintain soil fertility over the long term; always use FYM (cow dung manure) to sustain soil fertility. To maintain a nutrient balance, apply appropriate quantities of nitrogen, phosphorus, and potassium, and ensure proper irrigation management to facilitate potash absorption by plant roots. Do not apply the entire dose of potash at once in light or sandy soils, as high leaching rates in such soils reduce the amount of fertilizer available to the plant.
Use Sulphate of Potash (SOP) for chloride-sensitive crops such as potatoes, tobacco, grapes, citrus fruits, and various vegetables. In cases of severe potash deficiency, a foliar spray of 1–2% Potassium Nitrate (KNO₃) or 1–2% Sulphate of Potash (SOP) can be applied for rapid correction.
Sources of Potash Fertilizer
Muriate of Potash (MOP) – 60% K₂O, Sulphate of Potash (SOP) – 50% K₂O, Potassium Nitrate (KNO₃) – 44% K₂O, Wood ash, Compost and organic manure, Potash-rich mineral fertilizers
Secondary Macronutrients for Plant Nutrition
Introduction
In addition to nitrogen, phosphorus, and potassium, plants require calcium (Ca), magnesium (Mg), and sulphur (S) for healthy growth and high-quality fruit and grain production. However, plants need these nutrients in smaller quantities than nitrogen, phosphorus, and potassium. Consequently, they are classified as secondary macronutrients. These nutrients play a vital role in essential biological processes such as photosynthesis, cell formation, enzyme activation, protein synthesis, and nutrient transport.
Calcium (Ca)
Plants require calcium as an essential secondary macronutrient for normal growth and development.
Role of Calcium
Calcium facilitates the formation of calcium pectate, which structures the cell wall and cell membrane; this strengthens the middle lamella and cell wall, preventing the leakage of cellular contents. Beyond cell wall formation, it is essential for cell division and elongation. In addition, it promotes healthy root growth and root branching. It improves nutrient and water absorption and activates various enzymes involved in plant metabolism. It supports pollen germination and pollen tube growth (leading to successful fertilization) and enhances disease resistance.
Symptoms of Calcium Deficiency
Calcium deficiency causes the upper part of the plant to wither, new leaves to curl and turn yellow, and root growth to be stunted and weak. Tomatoes and chillies may suffer from Blossom-End Rot. Tip burn may appear in lettuce and cabbage.
Effects of Excess Calcium
Excess calcium in the soil reduces the absorption of magnesium (Mg) and potassium (K), hinders the uptake of other nutrients, and affects plant growth. The formation of insoluble calcium phosphate reduces phosphorus (P) availability; this leads to deficiencies in iron (Fe), zinc (Zn), and manganese (Mn), particularly in alkaline soils—and causes chlorosis (yellowing) in new leaves due to micronutrient deficiency. Excess calcium raises the soil pH, resulting in micronutrient deficiencies in plants; if this nutrient imbalance persists, it leads to reduced crop quality and lower yields.
Corrective Action for Calcium Deficiency
To increase calcium levels in acidic soil, apply agricultural lime, calcium carbonate, or calcium sulphate; calcium deficiency is often associated with low soil pH. Maintain adequate soil moisture levels so that calcium can reach the roots via water. To specifically address calcium deficiency in fruits and vegetables, calcium chloride or calcium nitrate can be applied as a foliar spray.
Apply compost or well-decomposed farmyard manure to improve soil structure and nutrient availability. Avoid the excessive use of potassium, magnesium, or ammonium fertilizers, as these compete with calcium for uptake. Always test the soil pH before applying any soil amendments.
Source of Calcium Fertilizers
Calcium Nitrate – 19% calcium and 15.5% nitrogen.
Gypsum (Calcium Sulphate) – 22–23% calcium and 18% sulphur.
Agricultural Lime (Calcium Carbonate) – Used to amend acidic soil and supply calcium; it raises soil pH and improves calcium availability.
Dolomitic Lime – Supplies magnesium in addition to calcium; suitable for soils deficient in both calcium and magnesium.
Calcium Chloride – Commonly used for foliar spray or fertigation to rapidly correct calcium deficiency.
Single Super Phosphate (SSP) – Contains phosphorus and sulphur along with approximately 19–21% calcium; it provides calcium as a secondary nutrient.
Magnesium (Mg)
Magnesium is crucial for photosynthesis. Additionally, it activates numerous enzymes and aids in the synthesis of carbohydrates and proteins. Magnesium deficiency typically manifests in older leaves. The areas between the leaf veins turn yellow, while the veins remain green for some time, causing interveinal chlorosis. Severe deficiency can cause premature leaf drop and reduce plant growth and yield.
Role of Magnesium in Plants
Magnesium is a key component in the formation of chlorophyll and is essential for photosynthesis. It activates various enzymes and biochemical reactions, playing a vital role in plant growth and metabolism. In addition, it assists plants in absorbing sunlight and converting light energy into chemical energy, thereby supporting growth, development, and the synthesis and transport of carbohydrates. It helps plants effectively utilize phosphorus and other essential nutrients, facilitating energy provision for cellular processes, maintaining the structural integrity and stability of plant cells, and supporting the healthy development of roots, stems, and leaves. By playing a crucial role in chlorophyll synthesis, it helps maintain the healthy green color of leaves, ensures a proper nutrient balance within the plant, and supports effective seed formation and crop development. Adequate magnesium contributes to enhanced plant growth and increased crop productivity.
Symptoms of Magnesium Deficiency
Magnesium is a mobile nutrient in plants; therefore, signs of its deficiency appear first in older leaves. A common symptom is yellowing of the tissue between the veins while the veins themselves remain green; in cases of severe deficiency, leaves may curl, drop prematurely, and plant growth may be stunted. Maintaining adequate magnesium levels in the soil enables plants to utilize sunlight, nutrients, and water more effectively.
Effects of Excess Magnesium
Excessive magnesium levels in the soil can disrupt the nutrient balance by creating competition for the uptake of other nutrients. High magnesium levels inhibit the absorption of calcium (Ca) and potassium (K), leading to deficiency symptoms that can impair plant growth and development. Visible signs may include leaf yellowing (chlorosis), leaf curling, stunted growth, and poor root development. Impaired root development naturally leads to weakened plants, reduced yields, compromised grain quality, and overall stunted growth. Applying excessive magnesium fertilizer increases unnecessary costs and can disrupt the nutrient balance.
Corrective Actions for Magnesium Deficiency
Conduct a soil test to determine magnesium levels and soil pH; if a deficiency is detected, apply magnesium sulphate (Epsom salt). In cases of severe deficiency, a foliar spray of magnesium sulphate can be used to make magnesium quickly available to the plant. Applying dolomitic lime to acidic soil reduces acidity while supplying both calcium and magnesium. Potassium magnesium sulphate can be used in magnesium-deficient soils where there is also a need for potassium and sulphur. Incorporate sufficient organic matter—such as compost and well-decomposed farmyard manure—into the soil to improve soil structure and nutrient availability. Excessive use of potassium, calcium, and ammonium fertilizers can inhibit magnesium absorption due to competition among these nutrients. Maintain adequate soil moisture and proper drainage to facilitate efficient magnesium uptake by the roots; since magnesium leaching is more likely in sandy or light-textured soils, applying fertilizers in split doses based on requirements is advisable. Determine the fertilizer dosage for correcting magnesium deficiency based on the specific crop and soil test results.
Source of Magnesium Fertilizers
Magnesium Sulphate (Epsom Salt) – Contains 9.5–10% Magnesium (Mg) and approximately 13% Sulphur (S).
Dolomitic Lime – Contains both calcium and magnesium; it is useful for acidic soils as it reduces soil acidity while supplying calcium and magnesium. It should not be used in soils that already have high magnesium levels.
Magnesium Oxide – Has a high magnesium content; it should be used in acidic soils, though its nutrient availability may be lower compared to magnesium sulphate.
Potassium Magnesium Sulphate – Supplies three nutrients: potassium, magnesium, and sulphur. It is useful for soils with a combined deficiency of these nutrients; application should be based on soil testing and crop requirements.
Magnesium Nitrate – Supplies nitrogen along with magnesium. Being water-soluble, it is used for fertigation and, under certain conditions, for foliar spraying.
Sulphur (S)
Sulphur is classified as a secondary macronutrient; for oilseed crops, its importance is on par with that of phosphorus. Plants can also absorb it in the form of SO₂ through their stomata. As an anion, sulphate behaves similarly to nitrate in the soil and can easily leach from sandy soils and areas with high rainfall. Soil microorganisms mineralize more than 90% of the sulphur bound to organic matter in the surface soil, making it available for plant uptake.
Role of Sulphur in Plants
Sulphur plays a primary role in protein synthesis. Plants store approximately 90% of their total sulphur in proteins, while disulphide bonds between cysteine residues help maintain the correct three-dimensional structure of enzymes and structural proteins. Although sulphur is not a component of the chlorophyll molecule itself, it is essential for chlorophyll formation; consequently, a deficiency causes leaves to turn yellow.
Beyond this general metabolic role, sulphur regulates several quality traits that directly influence the crop’s market value. Sulphur is essential for oil synthesis; thus, adequate sulphur levels increase both seed yield and oil content in crops such as mustard, groundnut, soybean, sunflower, and sesame. It facilitates the formation of glucosinolates in cruciferous crops and alliin-allicin compounds in onion and garlic, which are responsible for the pungency, flavor, aroma, and pest resistance of these crops. In wheat, it improves gluten strength and baking quality, while in fodder and pulse crops, it enhances protein quality and palatability. In pulse crops, sulphur promotes nodule formation and nitrogen fixation while improving nitrogen use efficiency. When sulphur levels are inadequate, plants accumulate absorbed nitrogen as non-protein nitrogen and nitrate instead of converting it into protein. For a healthy plant, the N:S ratio is approximately 15:1.
Symptoms of Sulphur Deficiency
Once plants incorporate sulphur into proteins, they cannot remobilize it through the phloem. Consequently, sulphur deficiency first causes a uniform pale yellow-green discoloration on new, upper leaves. This serves as a key distinction from nitrogen deficiency, which typically begins in older, lower leaves—though severe deficiency can cause the entire plant to turn yellow, leading to confusion between the two conditions. Growth is stunted; stems become short, thin, and weak; leaves become small and rigid; maturity is delayed; and flowering, pod formation, and grain filling are reduced. In mustard and *rai* (Indian mustard), new leaves curl inward, forming a cup-like shape, with yellowing and purple discoloration appearing at the edges; flower and pod development is poor, and oil content decreases. In pulses, nodule formation is impaired, and the plant exhibits signs of nitrogen starvation. Tea plants display the characteristic “tea yellows” symptom, where new shoots become stiff, brittle, and yellow. Citrus crops and cotton show interveinal chlorosis on new growth. Sulphur deficiency reduces tillering in cereals, causes yellowing, and lowers grain protein content. In onion and garlic, it reduces pungency. Tissue analysis can confirm the deficiency when total leaf sulphur falls below approximately 0.2% and the N:S ratio exceeds approximately 20:1. Soil testing can also indicate deficiency when available sulphate-sulphur, extracted using 0.15% CaCl₂, falls below the critical limit of approximately 10 mg/kg.
Sulphur deficiency primarily affects sandy soils, soils with low organic matter, eroded and calcareous soils, and intensively cultivated fields. Farmers can increase the risk of deficiency when they use sulphur-free, high-analysis fertilizers such as urea, DAP, and MOP instead of sulphur-containing fertilizers such as ammonium sulphate and single superphosphate.
Effects of Excess Sulphur
Plants can tolerate sulphate levels up to a significant extent; damage from overuse is often indirect. However, excessive sulphate application increases soil electrical conductivity, inducing salt stress that hinders water uptake and leads to symptoms such as leaf-margin scorching, necrosis, reduced leaf size, premature senescence, and stunted growth. Excess sulphur oxidizes in the soil to form sulphuric acid, thereby increasing soil acidity; if the soil is already acidic, this exacerbates manganese and aluminium toxicity and causes the leaching of calcium and magnesium. High sulphur levels can interfere with molybdate absorption, leading to molybdenum deficiency; similarly, it reduces selenium uptake. In waterlogged, anaerobic soils, excess sulphate converts to hydrogen sulphide, which damages paddy crops. This sulphide-induced damage causes blackened roots, root rot, leaf bronzing, and impaired nutrient uptake, a condition known as “akiochi” (autumn decline). High sulphur content in mustard oil cake used for animal feed also raises concerns regarding elevated glucosinolate levels, which act as an anti-nutritional factor.
Corrective Action for Sulphur Deficiency
Conduct a soil test before applying sulphur to ensure it is used only where actually needed. If a requirement is confirmed, apply sulphur-containing fertilizers. Sources include gypsum (13–18% sulphur), which is useful for reclaiming saline-sodic soils, as well as single superphosphate (12%), ammonium sulphate (24%), potassium sulphate (17%), magnesium sulphate, ammonium phosphate sulphate, potassium-magnesium sulphate, and liquid ammonium thiosulphate. Elemental sulphur and bentonite-sulphur granules (85–90%) are concentrated and slow-release sources; since they require oxidation by soil bacteria, they should be applied in finely divided form to warm, moist soil two to four weeks before sowing.
Generally, farmers should apply 20–40 kg of sulphur per hectare, depending on the crop and soil type. Oilseeds and cruciferous crops require about 30–45 kg/ha, while pulses and onion/garlic need approximately 20–30 kg/ha, and cereals require around 15–20 kg/ha. Apply sulphur as a basal dose and incorporate it into the root zone rather than simply broadcasting it on the soil surface. In sandy soils prone to leaching, it is advisable to apply sulphate in two split doses.
If symptoms of sulphur deficiency appear in a standing crop, spray a 0.2–0.5% solution of ammonium sulphate or magnesium sulphate, or a 0.2% solution of wettable sulphur. Repeating the spray after 10–15 days yields rapid partial recovery; however, foliar spraying is an immediate measure and not a substitute for soil-applied sulphur. For long-term management, the regular recycling of farmyard manure, compost, green manure, oil cakes, press mud, and crop residues replenishes the organic sulphur pool, which meets the majority of the crop’s requirements through mineralization.
Source of Sulphur Fertilizers
Elemental Sulphur (S)-80–90%- To correct sulphur deficiency in the soil
Gypsum-17–18%-To supply sulphur and calcium
Single Super Phosphate (SSP) 10–12%-To supply sulphur and phosphorus
Ammonium Sulphate-24%-Sulphur and nitrogen
Potassium Sulphate (SOP)-17–18%-Sulphur and potash
Ammonium Thiosulphate (ATS) 26%-Sulphur and nitrogen
Magnesium Sulphate (13%)- Sulphur and magnesium
Summary of Macronutrients in Plant Nutrition
Plants require macronutrients in relatively large quantities throughout their entire life cycle.
| Nutrient | Primary Function | Fertilizers |
| Nitrogen (N) | Supports vegetative growth, leaf development, and chlorophyll production | Urea, Ammonium nitrate, Calcium Nitrate, Magnesium Nitrate, Nano Urea (Liquid), Nano Urea Plus (Liquid) |
| Phosphorus (P) | Promotes strong root development, flowering, and fruit formation. | Diammonium Sulphate, Single super phosphate, Triple super phosphate, Monoammonium Phosphate |
| Potassium (K) | Enhances disease resistance, regulates water balance, and improves fruit quality. | Muriate of Potash, Sulphate of Potash, Potassium Nitrate |
| Calcium (Ca) | Strengthens cell walls and helps maintain the health of root tips | Calcium Nitrate, Calcium sulphate |
| Magnesium (Mg) | Essential for chlorophyll production and photosynthesis | Magnesium Sulphate, Magnesium Nitrate |
| Sulphur (S) | Aids in the synthesis of proteins, enzymes, and vitamins. | Ammonium Sulphate, Magnesium Sulphate, Potassium Sulphate |
Conclusion
By now, you should have a clear understanding of the essential micronutrients required for healthy plant growth and their role in maintaining crop productivity. However, knowing about nutrients alone is not enough—the real challenge lies in managing them effectively. Proper nutrient management ensures that plants receive the right nutrients in the right amount at the right time, helping farmers achieve better yields, improved crop quality, and long-term soil health.
Hope this blog added some value to your life. I would love to listen to your experiences in the comments.
Update: Here’s the link to the next part, i.e. part 3.
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