Introduction to Soil Science: “Plant nutrients and their deficiency symptoms”

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The subject of Agriculture and Rural Development carries 40 marks in the NABARD Grade A/B Exam. Thorough preparation on Topics such as agriculture, crops, types of soils in India, soil science, rural development schemes is needed for success in the NABARD Grade A/B exam. In this article, we are discussing the important topic for NABARD Grade A/B, FCI AGM exam: Plant nutrients and their deficiency symptoms. Let’s begin.

What are Plant Nutrients?

All Plants need nutrients to live and grow. Plants obtain their nutrients from the air, soil, and water. It’s also difficult to understand how nutrients work because they’re invisible — they’re colourless gases or dust dissolved in water or attached to each piece of soil.

Soil is an essential source of nutrients for plant growth. Nitrogen (N), phosphorus (P), and potassium (K) are the three most important nutrients. Calcium, magnesium, and sulfur are also essential nutrients. Plants also require trace elements such as iron, manganese, zinc, copper, boron, and molybdenum, which are only required in trace amounts by the plant. The role of these nutrients in plant growth is vast, and this article only covers the basics.

Three Criteria for Essentiality of Plant Nutrients

According to Arnon and Stout, 1939, the three criteria for the essentiality of nutrients are: 

  1. The element must be necessary for supporting normal growth and reproduction. In the absence of the elements the plants do not complete their life cycle or set the seeds.
  2. The requirement of the element must be specific and not replaceable by another element. In other words, the deficiency of any one element cannot be met by supplying some other element.
  3. The element must be directly involved in the metabolism of the plant

Essential Plant Nutrients for Growth

A plant needs 18 essential elements: 3 structural, 6 macronutrients, and 9 micronutrients. The atmosphere, soil, and water have the structural nutrients carbon, hydrogen, and oxygen. The remaining 15 essential elements [nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), iron (Fe), zinc (Zn), manganese (Mn), copper (Cu), boron (B), cobalt (Co), molybdenum (Mo), nickel (Ni), and chloride (Cl)] are obtained from either soil minerals or organic or inorganic fertilizers.

Macronutrients Micronutrients
Plants need large amounts of macronutrients relative to other essential nutrients.

Three structural are: Carbon (C), Hydrogen (H), Oxygen (O)

Six macronutrients are from soil: Nitrogen (N), Phosphorus (P), Potassium (K), Magnesium (Mg), Calcium (Ca), Sulfur (S)
The amounts of the nine (9) micronutrients in plants can be very low as compared to macronutrients. This isn’t to say they’re unimportant.
Micronutrient deficiencies or excesses, like macronutrient deficiencies or toxicities, may result in yield loss. Micronutrients are sometimes referred to as enzyme nutrients.

All micronutrients are from soil
Six micronutrients are cations: Copper (Cu), Manganese (Mn), Iron (Fe), Zinc (Zn), Nickel (Ni), Cobalt (Co)
Three micronutrients are anions: Chloride (Cl), Boron (B), Molybdenum (Mo).

Macronutrients

ElementNutrient functionsDeficiency symptoms 
Nitrogen
symbol: N; available to plants as nitrate (NO3–), andammonium (NH4+) ions
Amino acids, proteins, nucleic acids (DNA and RNA), chlorophyll, and coenzymes all contain nitrogen.

Nitrogen encourages healthy, succulent growth.

Excess Nitrogen can cause maturation to be delayed.
Since nitrogen is mobile in plants, deficiencies appear first on older plant tissue, then on newer leaves. Chlorosis is a symptom of nitrogen deficiency on leaves.

Chlorosis is a pale green to light yellow colour that appears first on older leaves, typically at the tips.

Chlorosis occurs as an inverted “V” down the midrib of corn.
Other plants have a general chlorosis pattern.
Phosphorus
Symbol: P; available to plants as orthophosphate ions (HPO2^4 –, H2PO^4–).
In plants, Phosphorus’s major role is energy storage and transfer. Plants use ADP and ATP to store and transmit energy, releasing 12,000 calories per mol.
ADP and ATP are needed in almost all metabolic reactions.

Phosphorus is also used in nucleic acids, coenzymes, nucleotides, and other biomolecules.

Phosphorus is needed for seed formation. Seeds and fruits contain large amounts of P.
Phosphorus nutrition helps to avoid lodging by strengthening structural tissues like wheat straw.
Phosphorus is mobile in plants, so deficiency symptoms occur in the older leaves first than the newer leaves.
Overall stunted growth of plants is there.
Purple coloring of leaves and leaf margins
Potassium
Symbol: K; available to plants as the ion K+
It is essential to photosynthesis because involved in:
ATP synthesis
Production and activity of specific photosynthetic enzymes
CO2 absorption through leaf stomata

It is also important in the maintenance of electroneutrality during photophosphorylation in chloroplasts.

Involved in activation of more than 80 enzymes.It provides the osmotic “pull” that takes the water into roots.

Sufficient potassium improves the quality of fruits and vegetables.
Since potassium is mobile in plants, deficiency symptoms appear first in the older leaves. 

Exception: Its deficiency can cause wilting, yellow leaf margins (chlorosis) that often become necrotic in new leaves of fast maturing crops (e.g. cotton and wheat) (dying).
The root system is weak.
Development is slow.
Tillering is reduced.
Plant lodging resulting in the brittle stalks.
Sulfur 
Symbol: S
Sulphur is needed for the formation of S-containing amino acids such as cystine, cysteine, and methionine.
Sulphur is needed for the synthesis of chlorophyll and coenzyme A, as well as fatty acid and amino acid oxidation and synthesis.

Ferredoxins, a FeS protein found in chloroplasts, contain S as a part. Ferredoxin is required for the reduction of NO2 and SO4^2 as well as the assimilation of N2 by root nodule bacteria.

Sulphur is responsible for mustard and onion plants’ distinct flavour and odour.
Sufficient S improves crop quality for ruminants by reducing N/S ratio to 9:1 from 12:1 required for effective use of N by rumen microorganisms.

Plants with low S levels produce less protein and accumulate non-protein nitrogen as NH2 and NO3.

Leaf NO3 accumulates in S-deficient plants, lowering food quality.
CalciumSymbol: CaCalcium is required for cell division and elongation.

It’s crucial for cell membrane structure and permeability.

It increases NO3 uptake and thus affects Nitrogen metabolism.

It controls the absorption of cations. For example, when Ca2+ is low, the uptake of K+ = Na+, but when Ca2+ is adequate, the uptake of K+ >> Na+.
Ca2+ deficiency impairs the production of shoot terminal buds and root apical tips (e.g. no plant growth).

Low Ca2+ weakens cell membranes and increases cell wall permeability, causing cell contents to leak out.

High concentrations of K, Mg, Mn, and Al inhibit Ca absorption.
MagnesiumSymbol: Mg^2+Mg2+ is a major component of chlorophyll, accounting for about 15-20% of total Mg2+.

Photosynthesis would not be possible without chlorophyll.
Mg is an essential structural component of ribosomes, which are involved in protein synthesis.
Protein N decreases and non-protein N increases in plants when Mg2+ is deficient.

Mg is needed for nearly every phosphorylating enzyme in carbohydrate metabolism to work properly.
 
Mg2+ is needed for the majority of ATP-based energy transfer reactions.

Mg2+ is essential in plant metabolism because photosynthesis, glycolysis, the Krebs cycle, and respiration are all fundamental energy transfer processes.
In comparison to grass crops, most forage legumes have a high Mg demand.

The following factors are typically needed:

Soils that are acidic, dusty, and heavily leached, with a low CECLow-magnesium calcareous soils

Acidic soils with low Mg content that receive high lime rates

Fertilization with high levels of potassium or ammonium

Micronutrients

The majority of micronutrients are found in plant enzyme systems. In redox reactions, micronutrients play an important role. Photosynthesis relies heavily on micronutrients. Micronutrients play a role in reactions including nitrogen fixation and protein synthesis.

Identifying all positions is difficult. The following functions have been identified:

  • The main functions of Chloride are osmotic and cation neutralisation
  • Plants use copper, iron, and molybdenum in oxidation-reduction reactions, which are necessary for photosynthesis and other metabolic processes
  • Iron is required for chlorophyll synthesis and can be deficient in turfgrass, especially in alkaline soils. Compaction, for example, may cause iron deficiency because it restricts root development.
  • Plants require Zinc and Manganese for some nitrogen transformations
  • The urease enzyme requires Nickel to act properly
  • The symbiotic fixation of nitrogen requires Molybdenum
  • Cell division and seed creation are also aided by Boron
  • Cobalt is needed for the growth of symbiotic microorganisms like rhizobia, which are free-living nitrogen-fixing bacteria
ElementDeficiency
Iron (Fe)Young leaves develop interveinal chlorosis, which progresses over the entire leafChelate dynamics increases Fe availability
Zinc (Zn)Yellow or white areas between leaf veinsReduced tilleringShortening of stem or stalk internodes, bushy, rosetted leavesSmall, narrow, thickened leaves, malformed by growth of only part of leaf tissuePremature foliage lossMalformation of fruit
Manganese (Mn)Interveinal chlorosis starts from younger leavesReduced root and shoot growth rateIncreases potential for root and leaf diseases
Chloride (Cl)Chlorosis in younger leaves and overall wiltingChlorotic lesions on leaves (visible yellow leaf spot)
Copper (Cu)Chlorosis (leaf spot) in young leavesYoung leaves become yellow and stuntedLodging, wilting
Boron (B)Yield reduction in small grains, poor pollination, grain set and fillThickened, wilted, or curled leavesThickened, cracked, water-soaked petioles and stemsDarkened, cracking, rotting fruits or roots: brown heart, black heart, hollow-heart in peanut
Molybdenum (Mo)Legumes (such as Soybeans, Alfalfa, Clover) need Mo for nodule formation
Nickel (Ni)Legumes are exhibited as a whole leaf chlorosis along with necrotic leaf tips (caused by the accumulation of toxic levels of urea)
Cobalt (Co)It results in reduced seed germination in dry conditionsReduced plant growthIn legumes, cobalt deficiency may result in symptoms of nitrogen deficiency

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