humus

Humus, humi is second declension but feminine, which is the first thing to fix in your head: it declines like dominus and agrees like a feminine noun, humus sicca, not humus siccus. A small group of second declension nouns behaves this way, mostly trees and a handful of words for the earth.

It also keeps a locative, humi, "on the ground", used with no preposition after verbs of sitting, lying and falling. That form is not a genitive, whatever it looks like. Terra is the earth as a whole and as a country; humus is the dirt itself, the surface you stand on and are buried in. The adjective humilis, "low to the ground", gives English humble and humility, and homo, "human being", is very probably the earth-creature from the same root.