I have a 2003 Nissan Xterra, and in the last year, it has been randomly filled with a very heavy silica/dirt-type smell (no matter the ambient weather). It just starts up one day, usually lasts for a day or two, and then goes away. I live in the Southeast, in a very non-dusty area, so I don’t think it’s dirt getting in from the outside. I lived in Arizona for two years after I bought it, but never had this problem then. At first I thought it must just be an old in-cabin air microfilter, so I went to check it, and lo and behold, Nissan had never installed one in the first place! Where it was supposed to be was just an empty slot. So I bought a new one, installed it, and this seemed to have no effect on the smell, which went away a few days later, only to return intermittently thereafter. I regularly change my other air filter, and do all the other scheduled maintentance, so I don’t know what this could be…Has anyone ever heard of this problem???
Maybe a leaky heater core? Any drop in the coolant reservoir? Do any of these newfangled long life coolents smell like dirt?
Hmmm…well, you can smell it in the car even when I haven’t been running the air (cool or hot) at all…If it was a heater core problem, would the heater not be working right (seems to be working fine, as is the AC)?
It smells like soil/earth? As an Aggie, you should know the difference between soil and dirt. Pure silica won’t have a smell. Are you sure this is coming from the HV system or is it coming from the cabin?
Different noses smell differently. Have someone else take a sniff and describe the odor. Don’t tell them before hand what it smells like to you-----Believe it or not, what one expects to smell can have a (biasing) effect on what one smells (the interpretation of).
Is the smell like the smell, when you are out in the open, that you smell before a thunderstorm, or an electric motor? (You see, I compared the two …oops, bias!)?
Ha ha! 
I was in Community Health, unfortunately never set foot in the Ag area! Don’t laugh but the reason I’m saying it smelled like silica was I once had a makeup loose powder that had “silica” something in it, and that was EXACTLY what this smell is like…But I guess you could say it smells like soil too. I really can’t tell where it’s coming from - it’s just there. It’s happening again today, and it does seem to be a heavier smell when you have the air on, but it happens when you have the air & vents off too. Can’t imagine where it’d be coming from in the cabin - it’s my first new car (well, not so new anymore), and I keep it very clean on the inside. Think I’d notice if someone snuck a pile of dirt in there.
It is very important to determine where it is coming from, the cabin or the HVAC system.