101 Questions answered
Understanding Water Repellent Soils: The Role of Organic Matter and Dry Conditions
Soils become water repellent (AKA non-wetting) when the particles become coated in organic compounds that repel water. Think of it as soil particles putting on a raincoat.
In broadacre farming, animal manure, decomposing crop residues and soil microbes are the main sources of these repellent compounds. As they break down, some repellent compounds are released and coat soil particles, while some repellent compounds are degraded. The lack of rain in drier areas limits the breakdown of these substances by soil microbes.
Certain crops such as clovers and blue lupins can increase the severity of water repellence (covered more in Question 58. ‘Can certain crop rotations help prevent water repellent soils?’). Stubble retention has meant organic matter accumulates (one of the main points of the practice) but also means repellent compounds can accumulate too.
Sheep camps are another culprit as the waxy compounds in the plants aren’t broken down as they pass through the sheep’s digestive system and end up concentrated in their dung.
On balance, keeping organic matter in the system is better than reducing it to try and stop repellence from forming. The other benefits of organic matter such as being a source of nutrients, improving water holding capacity, erosion protection, etc, far outweigh the repellence risk. Microbes and crop root exudates are also sources of repellent organic materials, and we can’t stop these.
