Classification and physiological role of mineral elements in plant nutrition

Classification and physiological role of nutrients

For full development and completion of the life cycle, plants require 17 chemical elements. Carbon, hydrogen, and oxygen are absorbed mainly from the air and water, while the remaining 14 elements are obtained from the soil in the form of mineral compounds. Although the demand for these elements varies significantly, a deficiency of any one of them blocks crop development.

Based on their content in dry matter, all elements are divided into macro-, meso-, micro-, and ultramicroelements. There is also a group of conditionally essential elements — aluminum, sodium, chlorine, fluorine, tungsten, iodine, and lithium, whose benefits have been proven only for certain crops. At the same time, the share of carbon, hydrogen, and nitrogen in the Earth's crust is only about 1%, whereas these organogens account for about 99% of the mass in plant dry matter.

• Share of carbon in dry matter — 50–60%
• Share of oxygen in dry matter — 25–30%
• Share of nitrogen in dry matter — 9–10%
• Share of hydrogen in dry matter — 3–4%
• Average ash content — 0.5–5%

To clearly illustrate the difference in consumption volumes, one can calculate the number of atoms of each element per 1 billion atoms of plant dry biomass:

• Nitrogen (N) — 10 million
• Potassium (K) — 3.8 million
• Calcium (Ca) — 1.8 million
• Magnesium (Mg) — 1.7 million
• Phosphorus (P) — 1 million
• Sulfur (S) — 580 thousand
• Iron (Fe) — 130 thousand
• Boron (B) — 3 thousand
• Manganese (Mn) — 1 thousand
• Zinc (Zn) — 300
• Copper (Cu) — 100
• Molybdenum (Mo) — 5
• Cobalt (Co) — 1

Below is a complete classification of elements that enter plant tissues from the soil, water, air, or as a result of anthropogenic impact. It divides all substances by their accumulation level in dry matter into groups ranging from macro- to ultramicroelements.

Ash composition of plants and nutrient removal

Mineral nutrition determines the ash content of produce — the percentage of non-combustible residue after burning dry biomass. On average, this indicator ranges from 0.5% to 5%, but it is distributed extremely unevenly across plant organs. Understanding this distribution helps to estimate the actual nutrient removal from the field.

The accumulation of mineral substances in various organs and environments differs significantly:

• seeds contain about 3% ash by dry mass;
• herbaceous stems and roots — 4–5%;
• leaves — 10–15%;
• living cells always contain more ash than dead tissues;
• aquatic plants accumulate significantly more ash elements (in algae — 50% or more).

Chemical composition of ash reflects the specific nutrient requirements of particular families. Cereals, grain legumes, row crops, and industrial crops accumulate metal and non-metal oxides differently.

Analysis of ash composition reveals important patterns in the distribution of elements across plant parts. In the ash of cereal and grain legume seeds, the sum of phosphorus, potassium, and magnesium oxides is about 9%. In vegetative organs, the share of phosphorus is significantly lower, while potassium predominates. For example, the ash of potato tubers and sugar beet root crops consists mainly of potassium oxide, the share of which reaches 40–60%. Grain legumes are notable for their high sulfur content.

Cell water balance and cell sap composition

The composition and concentration of cell sap directly determine how efficiently a plant absorbs moisture from the soil. Mineral salts in vacuoles create osmotic pressure, which, combined with the turgor pressure of the cell wall, forms the "suction force" of the cell. The magnitude of this pressure fluctuates significantly depending on growing conditions and the specific characteristics of the crop. In addition to mineral salts, free sugars and amino acids regulate the level of osmotic pressure.

Depending on the plant's age and environmental conditions, salts are distributed unevenly within tissues. Young, growing organs actively concentrate phosphates, which are necessary for cell division. Vegetable crops, potatoes, sunflowers, and beans are prone to accumulating nitrates. Chlorides, meanwhile, predominate in plants on saline soils, helping them adapt to an unfavorable environment.

Phosphate is the predominant anion in the cell. Most of its ions are bound in the form of phospholipids, nucleotides, phosphoproteins, and phosphorylated carbohydrates. Free ions in the form of dihydrogen phosphate (H₂PO₄⁻) and hydrogen phosphate (HPO₄²⁻) act as a buffer, maintaining a stable pH level of the cell sap.

Influence of mineral elements on enzymes and ash composition of seeds

Mineral elements regulate metabolism by influencing the spatial structure of proteins and nucleic acids. Magnesium ions are essential for the stable function of ribosomes, and calcium participates in the hormonal regulation of the cell. Ions affect enzymes directly by becoming part of their composition or changing their conformation, and indirectly by influencing the properties of the cytoplasm and biomembranes. Microelements (iron, zinc, manganese, copper, and others) act as enzymatic cofactors, triggering key catalytic reactions.

The indirect influence of ions on enzymatic activity is manifested through changes in several factors:

• physicochemical properties of the cytoplasm and the structure of intracellular water;
• the state of biomembranes, with which many enzymes are associated;
• the availability of substrates for specific enzymes;
• the intensity of the synthesis of the enzymatic proteins themselves.

The pattern of element accumulation in the harvest is also specific to each species. For example, sugar beet accumulates sodium more actively than others. Silicon is concentrated primarily in the fruit hulls of hulled crops such as oats, rice, and barley. At the same time, the ash of the vegetative mass of rice consists of 60–65% silicon oxide, although the chemical composition of rice seeds does not differ fundamentally from other cereals.

• Osmotic pressure of field crops — 5–10 atm
• Pressure in halophytic plants — 80–100 atm
• Silicon oxide in rice leaf ash — 60–65 %
• Ash content of rice grain — 5.9 %

Organic sources of ash nutrition elements

Ash analysis shows not just the amount of free mineral salts in a plant, but the result of the decomposition of complex organic compounds. Most of the elements found in the ash residue were, during the life of the plant organism, part of proteins and storage substances. Understanding this mechanism helps the agronomist more accurately assess the actual nutritional value of the harvest and the quality of the resulting seeds.

For example, sulfur trioxide (SO₃) is formed in the ash during the combustion of plant proteins. Its direct source is sulfur, which is part of the most important sulfur-containing amino acids:

• cystine;
• cysteine;
• methionine.

Phosphoric anhydride, which makes up a significant part of the ash, is released during the combustion of phospholipids, nucleoproteins, and phytin. Phytin is a calcium-magnesium salt of phytic (or inositol phosphoric) acid. When grain is burned, it is phytin that becomes the main source of phosphorus. In some crops, it also contains potassium and other elements in addition to calcium and magnesium.