Hydrophobic Soils

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Courtesy of Soil Doctors

Water is the driving force of all nature.

“Hydrophobicity” ( Hydro  meaning “water”  phobic  meaning “fear”), is the physical property of a molecule that is repelled from water - a molecule known as a Hydrophobe . A classic example of a hydrophobe is oil - it will not mix with water. Hydrophobic soils contain hydrophobes and resist moisture penetration. Hydrophobes are organic molecules and are very common in our environment.
They are found in soils as the result of naturally decomposed plant tissues, microbial by-products, naturally occurring waxes and resins, and organic matter such as those found in mulches and potting mixes.

Hydrophobicity develops in soils when hydrophobes surround the soil particle forming a waxy coating, or intermix with minerals currently in the soil. Moisture migration through hydrophobic soil is irregular and non-uniform. The soil does not wet and water runs off or moves through fissures and cracks.

Water is the key to life on earth. Liquid H2O is at the heart of all ecosystems and access to water can determine the survival of an organism. Ensuring water reaches the roots of plants is essential and the primary process from which all others follow.

 

Movement of water is affected by three forces;

 

1. GRAVITY  - Constant downward pull

2. COHESION  - The attraction of water molecules to each other   (the force that holds a water droplet together)

3. ADHESION  - The force attracting water molecules to other   substances. (i.e. soil)

In hydrophobic soil, the hydrophobes disrupt adhesion and the cohesive force is stronger. Water molecules 'bead' together, resisting the soil and running off with the force of gravity - down gradients or through cracks. When soil is hydrophobic, action is required reduce the cohesive forces and allow water to 'stick' to the soil again .

Even minimal levels of water repellency can negatively impact water movement in soils, resulting in preferential flow/channeling patterns that encourage nutrient leaching and non-uniform distribution of applied water and input chemicals. This reduced effectiveness of water movement and poor distribution of fertilizer and chemical inputs can result in reductions in plant growth and vigour, as well as crop yield and quality.

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