Screenshot of part of Merriam-Webster web page addressing pronunciation of 'loess'

The short answer is that you can take your pick between pronouncing it like "LERSS," or "LUSS," or like "LOW-ess." However, there's more to it than that, so here's the longer answer:

The word "loess" is taken directly from German, because loess was first scientifically studied in Germany. In German the word, at least in the old days, was written Löß, but nowadays usually they opt for Loess, written with a capital L because it's a noun.

You can hear how Löß is pronounced in German if you copy the word Loess, paste it into the German box (left side) at the Google Translator Page, and click on the audio icon . There the German speaker pronounces it more or less like the English word like "luss." On that same page you can go to the right side, into the English box, click on the audio icon and hear that the English speaker opts for "LOW-ess.

Both of those pronunciations surprise me because back in the 1970s my university teachers pronounced it "lerss," approximating the German lip-rounded "Löß I heard during my summers i Germany during the 1970s and 80s. However, German is influenced by many local dialect, and mostly I heard south-German German. With the English pronunciation given as "LOW-ess," I'm guessing that with time so many English speakers began pronouncing it the way it looks that now that's considered "standard."

Other pronunciations have been heard coming from the mouths of reputable people, and probably their sounds are as good an any.