Agriculture throughout large parts of Europe is still recovering from summer 2018’s heatwaves and drought. Many farmers are now facing challenges with irrigation restrictions and trying to re-wet dry soils. Once soils become water repellent, it can be a laborious task to get moisture levels back to optimal.
Soil repels water due to its hydrophobe content. Hydrophobes are organic molecules that repel water. They can be released in soils as a result of micro-organism activity, organic matter and decomposing plant tissue. Hydrophobes create a thin layer of waxy coating around each soil particle thus making the soil hydrophobic (water-repellent). The higher the soil hydrophobicity level, the lower the infiltration rate of water.
Water molecules are bi-polar and have strong cohesion forces: they attract molecules of the same kind. Their strong attraction to each other and their weak ability to bond with the waxy soil particles lead to the formation of droplets with a high contact angle. This so-called high surface tension stops the water droplets from spreading over a large surface area.
The likelihood of a soil being or becoming water repellent is determined not only by the presence of hydrophobic material, but also soil texture (Hunt and Gilkes, 1992). Coarsely textured sandy soils that contain less than 5 % clay are very susceptible to becoming water repellent.
Graphic 1: Water in hydrophobic soils
In these situations, it is likely you’ll apply extra irrigation, but that will only lead to increased costs for water, labor and pumping, and have little or no effect on improving the soil’s moisture level. Even minimal levels of water repellency can negatively impact water movement in soils, which in turn affects plant growth and development, that will lead to a reduction of the crop yield and of the quality of the final produce.
How do soil wetting agents work?
Soil wetting agents are products based on surfactants, designed to improve water penetration and to allow better water distribution into the soil profile, both horizontally and vertically.
When mixed with water, the active ingredients reduce the cohesion force and increase the adhesion force (the force attracting water molecules to other substances – i.e. soils) so the soil no longer repels water.
Soil wetting agents also prevent water pooling on the soil surface by reducing their surface tension. Put simply, surfactants are formed by a hydrophilic head and a hydrophobic tail. When mixed with water, the hydrophobic tail protrudes the water surface, reducing its surface tension. A lower surface tension decreases the contact angle allowing water to spread across a larger surface area. This reduces water losses via runoff, especially on modeled soils (e.g. vegetable beds, ridges formed after potato planting, carrot sowing etc.) and improves water infiltration rate.
Graphic 2: Surfactant molecules break the surface tension of water when the water-repellent ‘tail’ protrudes the water surface
Graphic 3: Water with wetting agent in hydrophobic soil
When coping with hydrophobic soils, combining agro-technical practices, like claying and furrow-sowing, with the latest soil wetting agents is an effective way to reduce soil hydrophobicity. Giving water the chance to penetrate more rapidly into the soil will reduce water wastage and increase healthy root development. A healthy root system results in improved nutrient uptake and better plant establishment. Maximize the use of your irrigation water to save costs and obtain yields at the optimum potential of the crop.