Water repellency in soil is caused by organic acids being produced through the decomposition of plant materials.
The organic acids form wax like coatings on soil particles causing repellency. It is thought that sandy soils are more likely to have this condition as the particles have a relatively lower surface area than other soil materials. Water repellent (or hydrophobic) soils is often a more common problem in sandy soils and potting mixes. In weather conditions that are dry and windy, soil can dehydrate.