表土 is read as ひょうど (hyoudo) and means top soil.
ひょうど
hyoudo
あらわす
to express
ひょうげん
representation (of a group)
ひょうめん
appearances
おもて
Omotesenke school of tea ceremony
ひょうじょう
facial expression
ひょうき
writing on the surface (e.g. an address on an envelope)
ひょうし
to appear on the cover of a magazine
おもてどおり
main street (as opposed to a side street)
ひょうめい
declaration
ひょうじ
(copyright) attribution
ひょう
memorial to an emperor
おもてむき
outward appearance