Tafsir of Ad-Dhariyat 51:20

And in the earth are signs: proofs derived from the various types of minerals, plants, and animals; or [the word "signs" refers to] aspects of evidence resulting from its expansion, the elevation of some of its parts above the water, and the variation of its constituent parts in their qualities and properties.

If the proof refers to the former—that is, the existents found within the earth—then the prepositional phrase "in the earth" (fi al-ard) is literal, and the plural [of "signs"] is taken at its apparent meaning. If it refers to the latter, then it signifies the earth itself, and the pluralization is with respect to the various facets and conditions of its evidence; the prepositional phrase here is that of a quality [residing] within the thing described.

These signs point to the existence of the Maker—glorified be His majesty—His knowledge, His power, His will, His oneness, and the extremity of His mercy—mighty and majestic is He—for those who are certain: the monotheists who have traversed the straight, demonstrative path that leads to true knowledge. They are those who observe with discerning eyes and penetrating understandings. Qatadah read [the word] "sign" (ayah) in the singular.