Growth of Plants: 6 Nutrient Elements

This article throws light upon the six essential nutrient elements required for growth of plants. The elements are: 1. Nitrogen 2. Phosphorus 3. Potassium 4. Calcium 5. Magnesium 6. Sulphur.

Nutrient Element # 1. Nitrogen:

Nitrogen is one of the most important primary nutrient non-metal elements which require large quantity for the plant growth and nutrition. It is the most widely distributed elements in nature. It occurs in the atmosphere, lithosphere, and hydrosphere. The soil accounts for only a very small amount of lithospheric nitrogen, and of this soil nitrogen, a very minute amount is directly available to plants.

Nitrogen occurs in soils mainly in the form of nitrate (NO₃^–) and ammonium (NH₄⁺) ions. Nitrogen is known to be a very mobile element circulated between the atmosphere, the soil and living organisms. However, various factors and mechanisms are involved in this N-turnover, some of which are physico-chemical, and others are biological.

Soil nitrogen occurs primarily in organic combination in the soil. The breakdown of such organic forms of nitrogen into inorganic ammoniacal (NH₄⁺) forms depends upon the moisture regime of the soil. If the soil is saturated or submerged, the rate of transformation from organic to NH₄⁺ forms is very slow. Organic nitrogen forms are a potential reserve of nitrogen for rice only after the organic form has been converted into inorganic form.

However, nitrogen as its elemental form is useless to higher plants. Various processes like fixation of nitrogen by Rhizobia, Azospirillum, cyanobacteria and other soil micro-organisms; and fixation as ammonia, nitrate etc. by various industrial processes during preparation of synthetic nitrogenous fertilisers etc. are responsible for the conversion of nitrogen into forms which are readily available to the plant.

Furrow slice soil layer (0-15 cm depth) of most soils contains nitrogen in the range of 0.02 to 0.4 per cent by weight. The amount of nitrogen present in the soil are influences by climate, nature of vegetation, topography etc.

Nitrogen transformation in soils includes various mechanisms namely mineralization, immobilization, fixation, losses of nitrogen including leaching, de-nitrification, ammonia volatilization, run-off losses etc.

The main functions of mineral nutrient, nitrogen that serves as constituents of proteins and nucleic acids are quite evident nitrogenous compounds make up a significant amount of the total weight of plants. Nitrogen occurs in both inorganic and organic forms in plants.

The inorganic form of combined nitrogen generally contributes only a small fraction of the total. Nitrate, the most important inorganic form is mainly taken up by the plants and it is assimilated by reduction to ammonium, followed by incorporation into organic forms.

Most of the organic nitrogen of plants is present in proteins. However, in addition to its function in proteins, nitrogen plays a part in other processes like chlorophyll pigments, hormones, respiration-energy carrier (ATP) etc.

Nutrient Element # 2. Phosphorus:

Phosphorus plays an important role as a structural component of the cell constituents and metabolically active compounds. It is a constituent of the sugar phosphates viz. ADP, ATP etc., nucleic acids, purine, pyrimidine, etc. and various coenzymes. In combination with different organic acids, phosphorus forms esters, phosphatides and phospholipids.

As phosphoric ester of inositol, phosphorus is a major component of phytin. Besides, phosphorus plays an important role in energy transformation and metabolic process of plants. The deficiency of phosphorus disturbs the nitrogen metabolism and also results in an increased accumulation of free reducing sugars, suggesting an involvement of phosphorus in carbohydrate metabolism.

Phosphorus is not reduced in plants but remains in its highest oxidised form. After uptake, at physiological pH mainly as H₂PO₄^– either it remains as inorganic phosphate (Pi) or is esterified through a hydroxyl group to a carbon chain (C—O—P) as a simple phosphate ester (e.g. sugar phosphate) or attached to another phosphate by the energy rich pyrophosphate bond P ~ P (e.g. ATP).

Phosphorus in soils almost exclusively occurs as orthophosphate ions. The total P content is in the range of 0.02 to 0.15%. A large amount of this P is bound with soil organic matter and about 20-80% of the total P in soils is in the form of organic fraction.

The soil P exists in many primary and secondary compounds. For most mineral soils, apatite’s are believed to be the primary phosphate bearing minerals from which the other P containing soil fractions are derived.

Broadly, P fractions in soils can be grouped into two non-occluded inorganic phosphates and occluded inorganic phosphate. The non-occluded fraction includes soil solution phosphate, adsorbed phosphate and some phosphate minerals. The occluded phosphate fraction is held by Fe and A1 minerals, often within a skin of Fe-hydroxy compounds.

However, from the view point of plant nutrition, the following three soil phosphate fractions are important:

Nutrient Element # 3. Potassium:

Potassium is taken up by plants in greater amounts than any other mineral nutrient except nitrogen. Although the amount of total potassium present in the soil has been recorded to be several times more than the amount absorbed by the plant during the growing period.

Potassium (K⁺), a monovalent cation, has a hydrated ionic radius of 0.331 nm and a hydration energy of 314 J mol^-1. Its uptake is highly selective and closely coupled to metabolic activity. It is characterised by high mobility in plants at all levels—within individual cells, tissues, and in long distance transport via xylem and phloem.

Potassium is the most abundant cation in the cytoplasm and K⁺ and its accompanying anions make a major contribution to the osmotic potential of cells and tissues of glycophytic plant species. It is required in reactions in using high energy phosphates (adenosine phosphates, ADP and ATP) and under K deficiency, amino acids and certain toxic substances are produced.

Potassium (K⁺) is not metabolised and it forms only weak complexes in which it is readily exchangeable. In addition, due to its high concentrations in the cytosol and chloroplasts it neutralises the soluble (organic acid anions and inorganic anions) and insoluble macromolecular anions and stabilises the pH between 7 and 8 which is optimum for enzymic activity.

For an example, a decrease in pH from 7.7 to 6.5 almost completely inhibits nitrate reductase.

Nutrient Element # 4. Calcium:

Calcium is a relatively large divalent cation with a hydrated ionic radius of 0.412 nm and a hydration energy of 1577 J mol^-1. Calcium is one of the secondary nutrient elements essential for the growth of plants and therefore, its importance becomes much more evident in acid and alkali soils compared to neutral or alkaline soils.

Most of the functions of calcium as a structural component of macro-molecules are related to its capacity for co-ordination, by which it provides stable but reversible intermolecular linkages, predominantly in the cell walls and at the plasma membrane. Calcium can be supplied at high concentrations and might reach more than 10 per cent of the dry weight.

Calcium is present as calcium pectate, which is a constituent of the middle lamellae of the cell walls. Its deficiency causes a stunting of the root system and imparts characteristic symptoms to the leaves.

In recent years, calcium has attracted much interest in plant physiology and molecular biology because of its function as a second messenger in the signal conduction between environmental factors and plant responses in terms of growth and development.

Nutrient Element # 5. Magnesium:

Magnesium is a small divalent cation with a hydrated ionic radius of 0.428 nm and a very high hydration energy of 1908 J mol^-1. The rate of uptake of Mg can be depressed by K⁺, NH₄⁺, Ca²⁺ and Mn²⁺ as well as by H⁺ ions. The functions of Mg in plants are primarily related to its capacity to interact with nucleophilic ligands through ionic bonding, and to act as a bridging element or form complexes of different stabilities.

Magnesium forms ternary complexes with enzymes in which bridging cations are necessary for making definite geometry between enzyme and substrate, e.g. RuBP carboxylase. In addition, a considerable amount of total Mg is involved in the regulation of cellular pH and the cation-anion balance.

Nutrient Element # 6. Sulphur:

Sulphur, an essential secondary plant nutrient, is required by plants and animals in approximately the same amount as phosphorus and few years back it was a neglected element. However, recently sulphur is gaining importance for crop production in the balanced fertilization programme.

Sulphur, like phosphorus, potassium and calcium is of terrestrial origin resulting from the decomposition of rocks. Because of its volatile nature, a large amount of sulphur has become dispersed in the atmosphere. Such atmospheric fraction contributes significantly to the plant growth and nutrition. On an average, the amount of sulphur content in the earth crust is ranged about 0.06 to 0.10 per cent.

Sulphur occurs in soils as both organic and inorganic forms. In most soils organically bound sulphur are the dominant fractions of sulphur combined with carbon and nitrogen. As for an example, in peat soils this organic S fraction constitutes 100 per cent of the total S.

Organically bound sulphur can be divided into two groups, first one carbon bonded includes S of amino acids and the second is non-carbon bonded includes phenolic and choline sulphates as well as lipids. The inorganic forms of sulphur in soil consist mainly of SO₄—S.

Sulphur is involved in the metabolic and enzymic processes of all living organisms. Sulphur is taken up by plant roots as its sulphate form. Sulphur plays an important role in chlorophyll formation as well as biologically important compounds like thiourea, plant hormones, thiamin, biotin and glutathione.

It also involves in the nitrogen metabolism of plants. Sulphur imparts good flavour to different vegetables and also pungency to oil seeds and onions. Reduction of SO₄—S is necessary for the synthesis of amino acids, proteins and co-enzymes and in green leaves ferredoxin is the reductant for sulphate.

However, sulphate can also be utilised without reduction and incorporated into essential organic structures like sulpholipids in membranes. Besides, reduced sulphur can also be re-oxidised in plants. The oxidation of reduced sulphur compounds also seems to play an important role as negative feedback signal for sulphate reduction.