Essential Plant Macronutrients and Micronutrients: Functions and Key Elements Guide

Plants require mineral elements for proper growth and development, which they obtain from the soil. In small amounts, plants consume micronutrients, while they need macronutrients in larger quantities.

Macronutrients for plants are especially important.

In addition to nitrogen and phosphorus, the macronutrients essential for plant growth include:

• sulfur,
• chlorine,
• silicon,
• potassium,
• magnesium,
• calcium,
• sodium.

Sulfur

Sulfur is absorbed by plants in the form of the sulfate anion SO₄ from sulfuric acid salts. Within plants, sulfur undergoes reduction. This reduction mainly occurs in the leaves and partially in the roots. Carbohydrates are necessary for sulfur reduction.

Sulfur is an organic constituent in the form of sulfhydryl SH or disulfide S-S groups. Sulfur found in certain organic compounds, such as cysteine and glutathione, plays a significant role in redox processes.

Sulfur is part of all proteins and coenzyme A, which is involved in substance transformation processes. Sulfur is also found in vitamin B₁ in garlic and mustard oils. Overall, sulfur content in plants accounts for fractions of a percent of dry matter.

It is most abundant in seeds and leaves, less so in stems and roots. If sulfur is deficient, the veins of the leaves turn yellow, while the pulp remains green. Red spots of dying tissue appear on the leaves. Plant damage begins at the top.

The diagram illustrates the sulfur cycle in nature, where plants absorb sulfur from sulfuric acid salts. In their bodies, sulfur is reduced. Upon the death of plants and animals, proteins and other sulfur-containing organic compounds are mineralized, releasing sulfur as hydrogen sulfide.

This hydrogen sulfide is oxidized by sulfur bacteria to form sulfuric acid, which then combines with soil cations to create salts that plants can absorb. Sometimes in nature, hydrogen sulfide is also produced during the reduction of sulfate salts by desulfurizing bacteria.

Chlorine and Silicon

Chlorine and silicon are sometimes found, in very large amounts, in the ash content of plants, (more on the chemical composition of plants).

Research shows that even negligible amounts of chlorine are essential for all plants. Chlorine is part of the carboxylase enzyme. Chlorine ions influence the uptake of other anions, particularly the PO₄ ion. Chlorine-containing salts are physiologically acidic and thus can facilitate the mobilization of phosphoric acid from phosphorites, as well as participate in the creation of the osmotic potential of cell sap, (more on osmosis).

Despite being classified as a micronutrient, silicon sometimes accumulates in large quantities in plants, particularly in the cell walls of aerial parts, making them tougher, as seen in sedges, grains, and horsetails.

This may impede parasitic fungi from penetrating the cells of these plants. Diatoms have silica shells, requiring high amounts of silicon for formation.

Potassium

Potassium is abundant in plants, particularly in young, active organs rich in protoplasm. Potassium content in these tissues can reach up to 50% of the ash weight.

Potassium significantly influences the structural state of protoplasm, enhancing dispersity and increasing the hydration of colloids. Most potassium in the plant is in ionic form, making it almost fully extractable from plant tissues using water. Studies with radioactive potassium have shown that in young plant tissues, about 30% of potassium is bound.

Potassium binds weakly to protoplasm proteins, occupying a place in the side chains of protein molecules. It is prevalent in sites of carbohydrate formation and accumulation, such as leaves, tubers, rhizomes, and starchy seeds. Potassium participates in photosynthesis and carbohydrate conversion, activating enzymes involved in carbohydrate transformation and facilitating their export from the leaf.

It also accelerates the activity of proteolytic enzymes, catalyzing the synthesis and decomposition of protein substances. Potassium in the cell sap affects the osmotic potential of the cell. Potassium deficiency decreases the plant's drought resistance.

The significant role of potassium might be attributed to its weak radioactivity. Potassium salts contain radioactive potassium isotope with an atomic weight of 40 alongside potassium with an atomic weight of 39. It is known that weak radioactive radiation enhances life processes, whereas strong radiation is harmful.

With potassium deficiency, known as potassium starvation, the plant's potassium can easily move from lower leaves to young growing parts, allowing for reuse. Plants absorb potassium from salts like KCl, KNO₃, KH₂PO₄, K₂SO₄, and others.

Lack of potassium results in yellow or yellow-red coloring of the tips and edges of leaves, followed by the drying of areas between leaf veins.

The dying process starts from the lower leaves. In severe potassium deficiency, protein breakdown begins within plant cells, forming necrotic spots.

Magnesium

Magnesium plays a role similar to potassium in substance transformation processes. Up to 50% of magnesium exists in ionic form, with the remainder in metalorganic compounds. Magnesium is notably central to the chlorophyll molecule, (more on photosynthesis in plant leaves) where it constitutes 10% of the total magnesium in the plant.

Magnesium enhances the reductive action of certain enzymes, playing a crucial role in activating transfer enzymes that cleave phosphoric acid from adenosine triphosphate, transferring it to sugar molecules, amino acids, and other compounds.

However, magnesium's effect on protoplasm differs from potassium, as it decreases colloid hydration and increases protoplasm viscosity. Most magnesium is located in the young parts of the plant. Plants absorb magnesium from salts like MgSO₄, MgCl₂, Mg(NO₃)₂, and others. Magnesium deficiency first appears in the older parts of the plant—the lower leaves—because magnesium can be reused.

Calcium

Calcium is among the most crucial elements for plant nutrition. Its deficiency results in abnormal nuclear division and the death of the growth point. Calcium affects plasma colloids, dehydrating them and increasing protoplasm viscosity.

Calcium's ability to influence the physical and chemical properties of protoplasm, its viscosity, and permeability, is one of its most important attributes. Calcium is a strong antagonist of univalent cations, especially hydrogen. Furthermore, while preventing certain cations from entering the cell, it stimulates the absorption of others. Calcium forms the basis of the middle lamellae, cementing cell walls together.

It neutralizes organic acids formed in plants. Most absorbed calcium is found in older parts of the plant as calcium oxalate crystals. During autumn leaf fall, this calcium is removed from the plant, hence the high calcium requirement is primarily characteristic of green plants.

Calcium is poorly mobile within the plant and cannot be reused. It positively influences soil structure, improving its air and water regimes. Calcium deficiency in soil increases the mobility and physiological activity of aluminum and magnesium, which in excessive quantities, negatively impact plants. This harmful effect is eliminated by applying lime to the soil.

Calcium ions influence the uptake of microelements like boron, manganese, and molybdenum by plants. Calcium neutralizes the harmful effects of hydrogen on acidic soils, eliminating the toxic effects of ammonium salts.

Among cultivated plants, legumes show the greatest need for calcium due to their sensitivity to low soil pH. Plants can absorb calcium from salts like Ca(NO₃)₂, CaSO₄·2H₂O, and CaCl₂. Calcium deficiency leads to mucilage formation and root death in plants grown in aquatic cultures.

With further deficiency, young leaves and shoot tips die off, halting plant growth.

Sodium

Sodium can be found in significant amounts in plant ash. It induces protoplasm hydration and participates with other salts in creating the cell's osmotic potential.

Halophytes, which accumulate large amounts of sodium in their cell sap, have high osmotic potential and can absorb water from saline soils. Certain crops, such as sugar beets, grow better with small amounts of sodium in the soil.

Sodium can displace potassium and other beneficial cations from the soil absorption complex, making them available to the plant. However, increasing sodium amounts in the soil is very harmful, as it disrupts the cation balance in plants due to excessive sodium ion uptake, displacing other cations, including divalent ones like calcium.

Ion Antagonism

The varied and even opposite effects of monovalent and divalent cations are known as cation antagonism.

A solution of any pure salt, even with an anion and cation necessary for the plant, has a sharply toxic effect. Adding another pure salt with the same anion to the solution reduces the harmful effect; toxicity decreases even further with the addition of a third salt with the same anion.

This phenomenon is known as cation antagonism. It is observable in the root development of wheat seeds germinating in solutions of pure salts.

Wheat root growth:

1. in a balanced solution (NaCl+KCl+CaCl₂),
2. in a not fully balanced solution (NaCl+CaCl₂),
3. in a CaCl₂ solution,
4. in a NaCl solution.

The illustration shows that plants have only weak roots in sodium and calcium chloride solutions; where sodium is balanced by calcium, significantly better root development is observed; plants have even better roots in a solution with three cations.

The antagonistic effect depends on the valency of the introduced cation: the higher the cation's valency, the lower the concentration at which its antagonistic effect appears. A solution where the toxic effects of cations are not evident is called balanced.

Balanced solutions include seawater, tissue fluids, soil solution in soils where plants thrive, and solutions recommended for hydroponic cultures.