Criteria of essentiality of nutrients, Essential plant nutrients- their function, Nutrient deficiency ,symptoms, Transformation and Dynamics of major plant nutrients.

                                    Rahul Raj Tandon (R.M.D.CARS Ambikapur, C.G)

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Ø                                                                            The criteria of essentiality put forth by Arnon

In the nature there are nearly one hundred and three elements.  Of them nearly ninety elements are taken in by the plants.  In order to distinguish the elements which are essential from those which may be taken in by the plants but are not essential, Arnon (1954) has laid down the following criteria. 

1. The plant must be unable to grow normally or complete its life cycle in the absence of the element.

2. The element is specific and can not be replaced by another. 

3. The element plays a direct role in plant metabolism.

Essential elements

Human beings (19): C, H, O, N, P, K, Ca, Mg, S, Fe, Mn, Zn, Cu, Mo, Cl, Na, I, Co, and F.

 Farm animals (18): C, H, O, N, P, K, Ca, Mg, S, Fe, Mn, Na, Zn, Cu, Mo, Cl, I, and Co.

Plants (16): C, H, O, N, P, K, Ca, Mg, S, Fe, Mn, Zn, Cu, Mo, Cl and B.

Forms of nutrient elements absorbed by plants

i) Absorbed as single nutrient ion

Nutrient element                                              Forms absorbed by plants

Potassium                                                                               K+

Calcium                                                                               Ca2+

Magnesium                                                                          Mg2+

Iron                                                                                       Fe2+

Manganese                                                                           Mn2+

Copper                                                                                  Cu2+

Zinc                                                                                       Zn2+

Chlorine                                                                                 Cl-

Silicon                                                                                    Si4+

Cobalt                                                                                     Co2+

Sodium                                                                                    Na+

ii) Absorbed in a combined form

Nitrogen                                           Ammonium (NH4+) and Nitrate (NO3-)

Phosphorus                                                          H2PO4-,  HPO4

Sulphur                                                                         SO4

Boron                                                      H3BO3,  H2BO3-, HBO32-, BO33-

Molybdenum                                                   MoO4= (Molybdate)

Carbon                                                                               CO2

Hydrogen                                                                            H2O

Every nutrient plays a specific role in nutrition, growth and development of plants. These roles may be described as under:Functions/Role of plant nutrients

Carbon -  It is available in abundance from air. Green plants for photosynthetic activities use CO2. It is also required for cell formation in plants. About 45% or more part, of the plant tissues is made up of carbon.

 Hydrogen -  It is essential for cell and tissue formation in plants. This is obtained from water and is required for energetic reactions. It forms about 6% parts of the plant tissues.

 Oxygen -  Plants take oxygen from air and water. It forms about 43% parts of the plant structure. It is required for photosynthetic and respiratory activities. It helps in formation of tissues and cells.

Mineral elements

Nitrogen -   It plays a vital role in various metabolic activities of plants and is a constituent part of amino acids, proteins, nucleic acids, porphyrins, flavins, purine and pyrimidine nucleotides, enzymes, co-enzymes and alkaloids.

Phosphorus -  It plays a vital role as a structural component of cell constituents and metabolically active compounds.

Potassium - It helps in the maintenance of cellular organization by regulating the permeability of cellular membranes and keeping the protoplasm in a proper degree of hydration by stabilizing the emulsions of colloidal particles.

Calcium - Calcium regulates the permeability of cellular membrane. It is a structural part of the chromosomes in which it binds the DNA with protein. It is required by a number of enzymes for their proper functioning viz. lipase, phosphatase D, £ Amylase and Apyrase.

Magnesium - Being constituent part of polyribosomes, it helps in protein synthesis in the plants. Mg is also a constituent part of chromosomes and chlorophyll

 It plays a catalytic role of numerous enzymes concerning carbohydrate metabolism, phosphate transfer and decarboxilations.

It is involved in photosynthesis and organic acid metabolism. Mg helps in synthesis of fat and increases oil content in oilseed crops when it combines with sulphur.

Sulphur - It helps in synthesis of protein and amino acIron. acids like cystein, methionine, vitamins (thiamine and biotine), lipoic acid, acetyl coenzyme A, ferredoxin and glutathione.

Iron -  It forms cytochromes, heam and metalloproteins like ferredoxin and heamoglobin in plants. These cytochromes play a vital role in oxidative and photophosphorylations during respiratory electron transport and photosynthesis, respectively.

Manganese -  Being a part of nitrite reductase and hydroxylamine reductase, it helps in the nitrogen assimilation. It activates several enzymes related to oxidation-reductions (oxidoreductase), hydrolysis (hydrolases), breakdown of phosphates bonds in ATP or ligases.

It activates photosynthesis and nitrogen metabolism. It also accelerates enzyme participating in calvin cycle, helps in chlorophyll and chloroplast synthesis for boosting photosynthetic rates.

Copper -   It helps in oxidation-reduction process in plants. The compounds containing copper like plastoquinones and plastocyanins help in electron transport from chlorophyll to NADP and from water to chlorophyll during photosynthesis.

Zinc -  It regulates the auxin concentration in plants and helps in synthesis of protein, carotein and chlorophyll etc.

Molybdenum -   It helps in protein and amino acid synthesis. It accelerates nitrogen-fixing efficiency of aerobic (Azotobacter), anaerobic (Clostridium), blue-green algae, Azolla and symbiotic bacteria. It regulates the carbohydrate metabolism in plants.

Boron -   It regulates development and differentiation of vascular tissues formation and lignification of cell-wall. It is associated with reproductive phase in plants and under imbalanced nutrition it causes sterility and malformation in reproductive organs.

Chlorine -  During photosynthesis it helps in evolution of oxygen. It is a part of anthocyanin and affects protein synthesis. It increases turgor pressure.

 Cobalt -  It is required for symbiotic and non-symbiotic nitrogen fixation. It is a part of vitamin B-12.

Sodium - It maintains the osmotic pressure. It also regulates water uptake by plants. Plants take sodium as a substitute for potash under deficient potash supply.

Nutrient deficiency symptoms

Short supply of any nutrient leads to adverse cellular metabolism, growth and development of plants. Such plants bear abnormal symptoms termed as visual deficiency symptoms

  Such plants bear abnormal symptoms termed as visual deficiency symptoms

The deficiency can be corrected or prevented by supplying that nutrient. Visual nutrient deficiency symptoms can be caused by many other plant stress factors, therefore, caution should be exercised when diagnosing deficiency symptoms.

Transformation and dynamics of major plant nutrients in the soil

Transformation

The nutrients present in the soil are subjected to physical, chemical and biological changes or transformations. Several substances are produced during these transformations. Some may be toxic and others are harmless. These transformations may either release or fix nutrients.

Nitrogen Transformation -  On mineralization, organic matter releases ammonium and nitrate and these are available to plants, 'Nitrate may be further transformed into molecular nitrogen and escape into the atmosphere

Phosphorus Transformation -   Phosphorous is present in the soils in organic and inorganic forms. Inorganic phosphorous is more than organic phosphorus. Mineralization of organic matter releases phosphorus in available form.

Potassium Transformation -  Mineralization of organic matter releases potassium from organic matter. Some bacteria and fungi are capable of acting on alumino silicates and release potassium. Iron present in organic matter is released during mineralization by bacteria

Calcium Transformation -   The important sources of calcium are dolamite, calcite, apatite and calcium feldspars. On their disintegration and decomposition, calcium is made available to plants. It may be taken up by plants, lost in drainage, re-adsorbed by clay or a small fraction may be reprecipitated as secondary calcium compound

Magnesium Transformation -   Magnesium is available to plants due to weathering of biotite, dolamite, chlorate, serpentine and olivine. The released magnesium may be absorbed by plants

Principles and Practices of Soil Fertility and Nutrient Management (Agron 502) – 2011-12 43

 and microorganisms, lost in drainage or re-precipitated as secondary minerals. Magnesium in soil solution and exchangeable form are in dynamic equilibrium, similar to calcium. Sulphur

Sulphur Transformation -   Plant residues contain sulphur in the form of proteins, amino acids and vitamins. Mineralization of these compounds releases sulphates.

 The C: S ratio of 50: 1 is critical. Decomposition of plant materials with wider C: S ratio causes immobilization of sulphur.

 In anaerobic soils, sulphates are reduced to hydrogen sulphide by the action of Desulfovibro spp of bacteria. Hydrogen sulphide is toxic to crops.

Elemental sulphur when applied to the soil is converted to sulphate by Thiobacillus spp.

 

                                  Dynamics of Nutrient Availability

The rate of absorption depends on nutrient concentration in soil solution. As the roots absorb nutrients, depletion occurs near the root zone and it is replenished by mass flow or diffusion of nutrients

The constants a, b and c are rate constants. The equilibrium between unavailable and intermediate form is established slowly, perhaps over many decades.

The intermediate forms are the long-term reserves that can be replenished slowly from inert forms or more rapidly by fertilizer reactions with soil minerals. Some examples of intermediate forms are potassium ions in clay inter layers and phosphate ions in dicalcium phosphate crystals

 The equilibrium between intermediate and labile forms' is established over a short period, perhaps a few months to a year. Labile nutrient, which is loosely held, is a fraction of a soil nutrient that comes to equilibrium with soil solution rapidly, within hours or days.

In general, the labile pool represents the major component of the quantity factor while the nutrient concentration of the soil solution is the intensity factor. Nutrient absorption by plant roots is directly dependent on the concentration of the soil solution (intensity factor), which in turn is regulated by the labile pool (quantity factor).