A new study available without charge in the journal PLOS ONE explains what until know how been considered a hydrological mystery −biochar's "seemingly contradictory ability to make clay soils drain faster and sandy soils drain slower." (1).
Looking for the condensed version? Agrinews has published an excellent synopsis.
(1) "Study: Biochar alters water flow to improve sand, clay" http://agrinews-pubs.com/Content/News/MoneyNews/Article/Study-Biochar-alters-water-flow-to-improve-sand-clay-/8/27/12083#sthash.fx6HbMBj.uxfs&st_refDomain=t.co&st_refQuery=/TVX5CEcefE