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In nature, there are 109 elements, but only 17 are essential for plant growth. The concept of nutrient essentiality was first introduced by Arnon and Stout in 1939 and refined by Arnon in 1954. According to the criteria of essentiality:
1. A mineral element is essential if it is directly involved in plant metabolic functions (e.g., nitrogen in protein synthesis, potassium in stomatal function, and phosphorus in energy transfer).
2. Plants cannot complete their lifecycle without the element.
3. Deficiency symptoms can only be alleviated by supplying the specific element.
Seventeen elements are essential for plant growth. Carbon, hydrogen, and oxygen, though not mineral nutrients, are fundamental as they form the basis of plant structures. These elements, through photosynthesis, are converted into carbohydrates and subsequently into amino acids, sugars, proteins, nucleic acids, and other organic compounds.
The other 14 essential elements, derived from minerals, are classified based on their abundance in plants as macronutrients and micronutrients.
Table
| Essential nutrients | Type |
|---|---|
| C, H, and O | Structural or basic nutrient, but not mineral. |
| N, P, and K | Primary elements, macro nutrients, require larger quantity. |
| Ca, Mg, and S | Secondary elements, macro nutrients, require lesser quantity. |
| Zn, Fe, Cu, Mn | Metallic, micronutrients or trace elements, require lesser quantity. |
| B, Mo, Cl | Non-metallic, micronutrients or trace elements, require lesser quantity. |
| Ni | Metallic, micronutrient or trace element, requires lesser quantity. |
In addition to the 17 essential nutrients, several elements are beneficial to some plants but are not necessary for their life cycle completion. These elements include:
- Cobalt (Co): Essential for the growth of symbiotic microorganisms like Rhizobia, free-living nitrogen-fixing bacteria, and blue-green algae.
- Sodium (Na): Essential for halophytic plants that accumulate salts in vacuoles to maintain turgor and growth.
- Silicon (Si): Important for rice and sugarcane, it strengthens cell walls, prevents lodging, and increases water use efficiency.
- Selenium (Se): Required in moderate amounts by cabbage and mustard, with grasses and grain crops absorbing low to moderate amounts.
- Vanadium (V): Essential for the growth of green algae.
- Aluminium (Al): Not essential for plants, though it can be high in plants grown in soils with large amounts of aluminum.
Classification of Nutrients
a. Based on the Quantity Required:
i. Basic Nutrients:
These nutrients make up 96% of a plant's total dry matter, including Carbon, Hydrogen, and Oxygen. Carbon and Oxygen each constitute 45%, while Hydrogen accounts for 6%.
ii. Macronutrients:
These nutrients are essential in large quantities for plant growth. There are six main macronutrients, including Nitrogen, Phosphorus, Potassium, Calcium, Magnesium, and Sulphur. Macronutrients are further divided into:
- Primary Nutrients: Nitrogen, Phosphorus, and Potassium are crucial for successful crop production.
- Secondary Nutrients: Calcium, Magnesium, and Sulphur, though needed in smaller amounts, are still vital.
iii. Micronutrients:
Required in trace amounts, these nutrients are critical for plant health and include Manganese, Iron, Zinc, Copper, Boron, Molybdenum, Chlorine, and Cobalt.
b. Based on Mobility in Soil:
i. Mobile Nutrients:
These nutrients are highly soluble and do not easily bind to soil particles. Examples include NO3-, SO42-, BO32-, Cl-, and Mn+2.
ii. Less Mobile Nutrients:
Although soluble, these nutrients are partially absorbed by the soil, reducing their mobility. Examples include NH4+, K+, Ca2+, Mg2+, and Cu2+.
iii. Immobile Nutrients:
These nutrients tend to be highly reactive and become fixed in the soil, making them less available to plants. Examples include H2PO4-, HPO42-, and Zn2+.
c. Based on Mobility Within the Plant:
i. Highly Mobile:
Nutrients like Nitrogen, Phosphorus, and Potassium can easily move within the plant.
ii. Moderately Mobile:
Zinc is moderately mobile within plant tissues.
iii. Less Mobile:
Nutrients such as Sulphur, Iron, Manganese, Chlorine, Molybdenum, and Copper have limited mobility within the plant.
iv. Immobile:
Calcium and Boron are immobile and tend to stay in the tissues where they are first deposited.
Functions of Plant Nutrients
- Carbon (C): Fundamental component of carbohydrates, proteins, lipids, and nucleic acids.
- Oxygen (O): Found in almost all organic compounds in living organisms.
- Hydrogen (H): Essential for plant metabolism, ionic balance, and energy relations in cells.
- Nitrogen (N): Crucial for leaf and stem growth, chlorophyll production, proteins, enzymes, and growth regulators. It influences soil microorganisms and has a significant role in the N/K balance for vegetative growth and fruiting.
- Phosphorus (P): Vital for energy transfer, protein metabolism, and early root development. It promotes flowering, seed production, and winter hardiness. Phosphorus is most effective in soil pH between 6 and 7.5 and is immobile in soil but mobile in plants.
- Potassium (K): Enhances vigour, disease resistance, root development, and winter hardiness. It is crucial for protein production, starch, sugar formation, and water regulation. Potassium is mobile in plants but tends to leach from the soil.
- Calcium (Ca): Necessary for cell elongation, protein synthesis, root and leaf development, and plant vigour. It influences nutrient intake and is vital for cell wall structure.
- Magnesium (Mg): Important for sugar, protein, and chlorophyll formation. It regulates nutrient uptake, particularly phosphorus, and is mobile in plants but leaches from acid soils.
- Sulphur (S): Supports dark green coloration, seed production, and overall plant growth. It is part of proteins, amino acids, and vitamins, and is converted to available sulphate by soil bacteria.
- Iron (Fe): Essential for chlorophyll maintenance and involved in key metabolic functions like nitrogen fixation, photosynthesis, and electron transfer. It is immobile in plants and its mobility decreases in soil with increasing pH.
- Zinc (Zn): Important for enzyme function, seed and starch production, and auxin synthesis.
- Manganese (Mn): Involved in photosynthesis and influences auxin levels, increases availability of calcium and magnesium, and can substitute for magnesium in various reactions.
- Copper (Cu): Constituent of enzyme systems, involved in photosynthesis, respiration, lignin formation, and indirectly affects nodule formation.
- Molybdenum (Mo): Needed for enzyme activity and nitrogen fixation in legumes, and is a component of nitrate reductase.
- Boron (B): Related to cell wall formation, reproductive tissue, nodule formation in legumes, and the translocation of sugars, starches, nitrogen, and phosphorus.
- Chloride (Cl): Required for leaf turgor, photosynthesis, and osmoregulation in saline soils.
- Nickel (Ni): Necessary for seed germination and beneficial for nitrogen metabolism in legumes. It is a component of urease, which catalyzes the conversion of urea to ammonium.
Sources of Mineral Nutrient Elements
| Nutrient | Sources |
|---|---|
| C | Carbamate |
| N | Organic matter |
| P | Apatite |
| K | Mica, Feldspar |
| Ca | Dolomite, Apatite, Calcite, Gypsum |
| Mg | Dolomite, Muscovite, Biotite, Olivine |
| S | Pyrites, Gypsum, Organic Matter |
| Fe | Pyrites, Magnetites |
| B | Tourmaline |
| Cu | Chalcopyrite, Olivine, Biotite |
| Mn | Magnetites, Olivine, Pyrolusite |
| Mo | Olivine |
| Zn | Olivine, Biotite |
| Cl | Apatite |
| Ni | Nickeliferous limonite, Pentlandite [(Ni,Fe)9S8] |
Nutrient Use Efficiency (NUE)
Nutrient Use Efficiency (NUE) refers to the effectiveness with which plants utilize available nutrients to produce biomass or yield. It is a critical concept in agriculture, especially in the context of optimizing fertilizer use to enhance crop productivity while minimizing environmental impact.
1. Definition:
- Agronomic Efficiency: The increase in crop yield per unit of nutrient applied. It’s a measure of how effectively the applied nutrient translates into crop production.
- Physiological Efficiency: The amount of biomass produced per unit of nutrient taken up by the plant. This shows how efficiently the plant uses the nutrients absorbed.
- Apparent Recovery Efficiency: The proportion of applied nutrients that are taken up by the crop. It indicates how much of the added nutrient is actually utilized by the crop.
2. Importance:
- Crop Productivity: Higher NUE means that crops are producing more yield with less nutrient input, which is economically beneficial for farmers.
- Environmental Impact: Efficient nutrient use reduces nutrient losses to the environment, such as nitrogen leaching into water bodies, which can cause eutrophication or greenhouse gas emissions like nitrous oxide.
- Sustainability: Improving NUE contributes to sustainable agriculture by ensuring that natural resources (like phosphate rock for phosphorus fertilizers) are used more judiciously.
3. Factors Affecting NUE:
- Soil Health: Soil texture, structure, pH, and organic matter content influence nutrient availability and uptake by plants.
- Crop Variety: Different plant species and varieties have varying abilities to take up and utilize nutrients.
- Weather and Climate: Temperature, rainfall, and humidity can affect nutrient availability and plant metabolism.
- Agronomic Practices: Timing, method, and rate of fertilizer application, crop rotation, and irrigation practices can significantly impact NUE.
4. Improving NUE:
- Precision Agriculture: Using technology like GPS and sensors to apply fertilizers more precisely can improve NUE.
- Balanced Fertilization: Ensuring that crops receive the right balance of nutrients according to their needs can enhance efficiency.
- Use of Enhanced Efficiency Fertilizers: Slow-release fertilizers or those with inhibitors that reduce nutrient losses can lead to better NUE.
- Organic Matter Management: Incorporating organic matter into soil can improve soil structure and nutrient-holding capacity, enhancing NUE.
5. Challenges:
- Measurement: Accurate measurement of NUE is complex and requires understanding the interactions between soil, plant, and environmental factors.
- Trade-offs: Sometimes practices that improve NUE for one nutrient may reduce it for another, requiring careful management.