Hydrophobic soils after fire? All soils are more or less hydrophobic...

As explained by "Geomorphology: Themes and Trends".  The book is worth it just for the classic essay on erosion and runoff.  A combination, it turns out, of pore size and particle characteristics.

These images are from Chapter 5, "Geomorphological processes, soil structure, and ecology" written by A.C Imeson.  Yes, the x-axis is scaled as square-rooted minutes.

Plant roots can change both the permeability and wettability of soil (as well as many other qualities).  For more information, read Reid, TB and Goss, MJ. 1981.  Effect of living roots of different plant species on the aggregate stability of two arable soils.  Journal of Soil Science.

The book also has a chapter on the Geomorphology of stream channels and craters on Mars!