101 Questions answered
Why Dry Conditions Intensify Water Repellence in Soil
Drought does not make a soil water repellent, but it does make water repellent soil more repellent. For example, if you have a mildly repellent soil where water usually takes 5–10 seconds to filter in, during drought it might take 30 seconds or more to filter in. This happens for two key reasons:
- Water repellence is caused when soil particles become coated with repellent organic compounds. See Question 50. ‘Why do soils become water repellent?’ These compounds have a hydrophilic (water-loving) and hydrophobic (water-fearing) end. When dry, the hydrophilic ends of the compounds bond strongly, leaving the hydrophobic ends exposed, resulting in the soil becoming water repellent. As the soil becomes more dry, these ends bond more strongly, making it even harder for water to infiltrate.
- The organic compounds can be broken down by soil microbes. Without rain, the microbes are not very active and can’t break down the organic compounds.
When it rains, the water repellence is less severe.
Drought (or more specifically loss of soil surface cover) can reduce the amount of rain entering the soil.
Fine-textured surface soils (e.g. clays) have very small pores and, therefore, slow infiltration rates. Heavy rain falls faster than the soil can absorb it, so puddles form on the soil surface (and can evaporate before infiltrating) or water runs off. Soil surface cover (crop and pasture residues) will hold the rain in situ, giving it more time to soak in and reduce evaporation losses.
