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Why Wet Years Temporarily Improve Water Repellent Soils
Water repellence is transient, appearing worse in dry years and better in wetter years. This is because the organic compounds that cause water repellence have a hydrophilic (water loving) and hydrophobic (water fearing) end.
At a certain soil moisture point, which is different for every soil, the bonds between the repellent compounds are weak and it is easy for water to infiltrate. As the soil dries out, the water-loving ends bond more strongly, leaving the hydrophobic ends exposed. And since they don’t like water—they don’t want to let it infiltrate.
Water then runs off and filters into the soil where it can, such as old root channels and into less repellent areas. This is why repellent soil doesn’t wet up evenly.
If enough rain falls eventually the weight of the water (head pressure) is stronger than the water entry pressure of the repellent soil, and the water filters in. Once wet, the hydrophobic compounds are only weakly bonded and it is easier for water to filter in. The repellent compounds are still there so when the soil dries out again the repellence returns.
