A histic epipedon is almost always composed of fibric, hemic, or sapric organic soil materials, or any combination of these. However, in plowed fields, a histic epipedon may be composed of minral material very high in OC.
A histic epeidon:...
Is an Ap horizon that, when mixed to a depth of 25 cm, has
an organic-carbon content (by weight) of:
a. 16 percent or more if the mineral fraction contains 60
percent or more clay; or
b. 8 percent or more if the mineral fraction contains no
clay; or
c. 8 + (clay percentage divided by 7.5) percent or more if
the mineral fraction contains less than 60 percent clay.
Please note that the OC content required for organic soil materials may be lowered. If so, then almost all hisitc epiepdons would be composed of organic soil materials.