The Required Characteristics of a fragipan according to the "Keys to
USDA soil taxonomy, tenth edition, 2006" are • The layer is 15 cm or more thick;
and • The layer shows evidence of
pedogenesis within the
horizon or, at a minimum, on the faces of structural units;
and • The layer has very coarse prismatic, columnar, or blocky structure of any grade, has weak structure of any size, or is massive. Separations between structural units that allow roots to enter have an average spacing of 10 cm or more on the horizontal dimensions;
and • Air-dry fragments of the natural soil fabric, 5 to 10 cm in diameter, from more than 50 percent of the layer slake when they are submerged in water;
and • The layer has, in 60 percent or more of the volume, a firm or firmer rupture-resistance class, a brittle manner of failure at or near field capacity, and virtually no roots;
and • The layer is not effervescent in dilute HCl. What this means in plain English is • The fragipan has to meet a certain thickness requirement. • There needs to be evidence that the fragipan formed from altered
parent material, as opposed to being simply undeveloped dense subsoil. • The layer cannot have been altered by plant roots, which would result in granular
soil structure, and plant roots are unable to penetrate the fragipan except along intermittent cracks. • The requirement that a fragipan can slake in water separates it from a
petrocalcic horizon and a
duripan. • Fragipans are very hard, even when thoroughly wet, to the extent that roots cannot penetrate. • A fragipan does not contain
calcium carbonate or
magnesium carbonate, which would make it a
petrocalcic horizon. ==Genesis==