Prithvi, earth, is traditionally described as the last and densest of the five mahabhutas to unfold, with its associated sense being smell. Ghrana is the organ side of that same correspondence, the perceptual faculty whose one object, gandha, is also the newest quality added at the final stage of Samkhya's cumulative grossening, after sound, touch, form and taste have already been layered in by the elements preceding earth in the sequence.
This closes the full five-fold correspondence the tattva system builds between organs, tanmatras and elements: shrotra to shabda to akasha, tvak to sparsha to vayu, chakshu to rupa to tejas, rasana to rasa to apas, and finally ghrana to gandha to prithvi. Each pairing was fixed by the classical texts as a single ordered chain rather than as five separate lists that happen to share a length, which is why Samkhya literature treats the correspondence as structural rather than as a mnemonic convenience.
Ghrana's position also explains something about how the system treats materiality generally: earth, the grossest element, is described as possessing all five tanmatra-qualities at once, sound, touch, form, taste and smell, while akasha, the subtlest, possesses only sound. Ghrana, correspondingly, is the one organ whose sole tanmatra is also the newest and most exclusively physical of the five, present only in genuinely solid matter and absent from air, fire, water or empty space, which is exactly what makes smell, in this account, the sense most tied to substance as such.