See definition 2 of “earth” in the EID : the tillable substance (the “soil” meaning) is talamh or ithir (or úir ). Cré is used only in the chemistry context of “earth”, e.g. cré alúmanach for “aluminous earth”, créanna alcaileacha for “alkaline earths”, cré úcaire for “fuller’s earth”. “Earth” has a wider meaning than cré does.
I really don't want to spend a lot of time kicking this to death but ...
" 3 noun AGRI, HORT soil cré fem4 C M U créafóg fem2 C M U ithir fem5 úir fem2 there's good earth in that field tá ithir mhaith sa gharraí sin he dug into the earth thochail sé sa chré, chart sé an chré "
cré is given, in first place, under soil although not used in the example.
if in fact cré is only used in the chemistry context of 'earth' then the Duolingo people must be spanked firstly for not specifying whether the earth is Fuller's or otherwise and secondly for putting something so esoteric into a basic learners' tool.
(My reply is here since the comment that I’m replying to doesn’t have a Reply link.)
Look at the cré entry in the FGB — as the word suggests, the primary translation of cré is “clay”, with “earth” and “dust” also possibilities. Its closest non-chemistry examples of “earth” (i.e. not specifically a type of clay) are closer to English “ground” or “dust”, as a burial place — Bheith, dul, i gcré, to be laid in earth, buried; Go dté mé i gcré, to my dying day; Ag déanamh cré, turning to clay, to dust; Ó chré go cré, dust to dust.
The choice of vocabulary here often has little to do with everyday topics of conversation — consider the animal choices earlier in the course.