Influence of the molecular structure of water on agrochemical processes in the soil

Water is the primary solvent in agrochemistry. Its unique properties, which determine the mobility of nutrients in the soil and their uptake by plants, are due to the structure of the H₂O molecule. Total water reserves on Earth amount to 1385984610 km³, and this entire mass possesses anomalous physicochemical characteristics thanks to the specific spatial arrangement of its atoms.

In a water molecule, bonds are formed by the overlap of s-orbitals of hydrogen atoms and p-orbitals of the oxygen atom. Due to the mutual repulsion of the positively charged hydrogen atoms, the bond angle is not 90°, but 104.5°. The molecule is an isosceles triangle in which electrons are shifted toward the electronegative oxygen. This displacement creates four charge poles: two negative ones due to lone electron pairs on the hybrid sp²-orbitals of oxygen, and two positive ones at the locations of the hydrogen protons.

• Total water reserves on Earth — 1385984610 km³
• Bond angle — 104.5°
• Molecular radius — 0.138 nm
• O—H distance — 0.099 nm
• H—H distance — 0.152 nm
• O—H bond energy — 110 kcal/mol

Hydrogen bonds and the structure of liquid water in soil

Water molecules are capable of combining into structural aggregates (associates) with the general formula (H₂O)n, where n takes values of 2, 3, 4, or 5. This occurs due to the formation of hydrogen bonds — a partially donor-acceptor and partially electrostatic interaction of hydrogen with the more electronegative oxygen of a neighboring molecule. Such bonds constantly form and break, and their half-life does not exceed 1·10⁻⁹ s. Despite the instability of individual bonds, their vast quantity ensures the stability of liquid water, which is extremely important for plant metabolism.

The comparative strength of different types of chemical bonds determines the physicochemical behavior of water in the soil and the plant organism. The hydrogen bond occupies an intermediate position in terms of its energy, being weaker than a covalent bond but significantly stronger than van der Waals interactions. This allows water to remain liquid over a wide temperature range while easily participating in exchange processes within the soil solution.