Understand Wetting Agents

James Grundy

Understand Wetting Agents

Skilful management of water is a fundamental aspect of producing high quality and consistent turf surfaces. Water is a master variable which directly impacts the function of grass plants and soil organisms. It is also a master solvent responsible for a whole host of essential chemical reactions underpinning the soil ecosystem.

Wetting agents are types of chemical compounds classified as surfactants, they are defined as substances which reduce the surface tension of water allowing it to spread onto a surface as a film.

Intermolecular forces (IMF) are the forces which govern the interaction between different molecules at the macro scale (water droplets) rather the micro (water mist) or atomic (water molecule) scale, broadly they are defined as attractive or repulsive.

  • Cohesive forces are forces exerted between molecules, water has a strong cohesive force tightly pulling its molecules together into a sphere, which is why rain falls as droplets rather than mist.
  • Adhesive forces are attractive forces between unrelated molecules. When the adhesive forces between water and another molecule are strong enough they overcome the cohesive force of the water molecules them out of their spherical shape. When the water is pulled out of its spherical shape it spreads across and clings to the surface it is adhered to.

In the context of turf management cohesive and adhesive forces govern how water penetrates the ground and interacts with the surface of soil particles.

Optimal Conditions - Hydrophilic

In optimal conditions soils have high adhesive forces able to overcome the cohesive force of a water droplet. In this instance the surface tension of the water droplet is broken, it then spreads out over the surface of the soil particle coating it in a liquid film. This liquid film acts as storage reservoir with the soil and plants are then able to access it via their roots. Soils which act in this way are said to be hydrophilic. The definition of "hydrophilic" can easily be gleaned from its Greek roots; hydro (meaning water) and philic (meaning love or affinity for).

Non-Optimal Soil Conditions - Hydrophobic

Over time certain conditions can lead to waxy organic material building up and coating soil particles with strong cohesive forces. In turn these forces encourage water to bead and repel away from the surface of a soil particle. When these conditions are present the soil is said to by hydrophobic, or water repellent. Again, the Greek roots reveals meaning; hydro (meaning water) and phobic (meaning fear or hate). It is this water repellency which causes drought stress and localised dry patch because water, either from rain or irrigation, is unable to adhere and spread across soil particles where roots can access it. Instead it either fails to penetrate the surface; either running away or evaporating or; it simply passes straight through the profile into drainage systems or subsoil.

Understanding Types of Wetting Agent Surfactants

Soil Surfactants (wetting agents) can bridge the gap between philic and phobic soils. It is important however to understand that all wetting agents are not the same. With the increase of new technologies and formulations; understanding the mechanisms which govern how water behaves in a soil, aids understanding of the action and intended effect of different classification of wetting agent surfactants.

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