Why are labor, capital, and land traditionally denoted by the letters L, K, and T? AFAIK, at least in the English-speaking world, labor, capital, and land are (in economics) usually denoted by the letters L, K, and T.

Who first popularized the use of these letters and why?

My uneducated guesses are that:

L would certainly make sense in English.
K I think is from the German Kapital.
T I think is from the French or Latin Terre or Terra.
I'm only guessing (sorry) that $T$ for terre comes from the Physiocrats' work. K for capital is easiest than $c$ (for consumption). 

Btw, labor is not always $L$ but sometimes $N$ depending of if you mean labor force vs working population (or hours). And I've no ideas why $N$!