Some of the Estonian words and phrases that use põhi and põhja with the meaning of "bottom, base, floor" or "fundamental, base" include:
põhi (põhja) = north, bottom, basis
merepõhi = seafloor
orupõhi = valley floor
põhjavesi = ground water
topeltpõhi = false bottom
põhjani = completely, to the end
põhjast põhjani = thoroughly, from top to bottom
põhjani tungima = to get to the bottom of
põhja jõudma = to bottom out
põhimik = substrate
põhikool = primary school, elementary school
põhiliselt = fundamentally, basically
põhiline = essential, primordial
The world lõuna means "south," and also refers to "midday" or "noon". (Although an argument could be made that this is the time when the sun is highest in the sky, and therefore the opposite of põhi "base, bottom," it's also the time when sun is farthest south in the Northern Hemisphere: the opposite of põhi "north".)
Interestingly, "lounas" in Finnish also means "noon," but in terms of directions it refers to "southwest," not "south"; and in Livonian the cognate "lȭnag" (lõõnag) means "southeast." (More on the eight directions of the Finno-Ugrian world in another post.)
"Noon" or "midday" in modern Estonian is more commonly "keskpäev," but the word lõuna is still commonly found in other words and phrases referring to noontime:
lõuna = south, noon, midday
pärastlõuna = afternoon
pealelõuna = afternoon
enne lõunat = before noon
lõunasöök = lunch, midday meal
lõunat sööma = to eat lunch
lõunastama = to dine, to eat lunch
lõunavaheaeg = lunch break