Eli5 How can dirt become hydrophobic?

Question

I saw a video about some plants that had become dehydrated because the soil had turned hydrophobic, and was wondering how and why it did that?

In: 11

Answer 1

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The topmost layer of soil is where most of the organic detritus is such as fallen leaves. When this stuff decomposes, the soil can get waxy – especially if the temperature is high enough to turn the wax to gas, such as from a wildfire.

Wax buildup is mitigated by most forms of life that till dirt, which is why compost doesn’t have a strong risk of making soil waxy, even though compost can get hot enough to spontaneously combust.

Sand is more susceptible than silt or clay, and there are detergent-like products that claim to help break up the wax to try to restore absorption.