I have a pole barn with a dirt floor, and I want to improve the cleanliness of the space and reduce humidity. Concrete would be fantastic, but pretty pricy, too. What are some halfway measures?
I’ve been thinking of gravel, maybe with stall mats on top of it. Or perhaps a plastic tarp to block moisture, then plywood right on top of it.
Gravel with stall mats makes a good floor if the drainage is good. You’ll need fines on the top (granite dust, road base rock) and a tamper. It isn’t permanent, things settle and shift, animals burrow etc. but it’s easy to pull up a mat and fix it.
The main thing that is going to reduce humidity is really good drainage.
I’d think the plastic would trap moisture, not block it.
It does. Traps it in the ground.
How about Soil Cement, also know as Dirtcrete.
And at the bottom of the Wiki, check out Rhino Snot.
Check with local construction companies and/or their suppliers. There are a variety of mixtures you can use that compact well and form a hard and durable but imperfect surface. But to stop the moisture you’ll need something solid or a vapor barrier like plastic.
I’m not sure you can beat the cost of concrete over time, although compacted mixtures will make a good base for pouring concrete in the future.
Note that concrete isn’t waterproof either: if you want to exclude moisture, you have to put a waterproof layer under the concrete.
I don’t know if it’s any cheaper, but a properly compacted surface made with gravel, sand and water (fines) wears better than dirt. Another possibility would be would be to lay wood directly on the floor: it doesn’t last forever, but it’s nicer than dirt. Or just wood chips or straw, and muck out as necessary. I wouldn’t go with a fibre-dirt floor – it’s a lot of work and difficult to get right.
I wonder about the vapor barrier. Sheet polyethylene is very cheap but not so strong. Pond liner is tougher and more nearly designed to block moisture in dirt. Are there special barriers? Do I have to spread sand or geotextile or something above and below it to avoid puncturing it with gravel?
I think I like gravel with a barrier under it. I don’t need the surface to be very fine – in fact it would be nice if I could use a leaf blower on it without picking up the gravel.
You can always apply cow dung, as they do in India and parts of Africa.
Really. Apparently it has many advantages, including being a disinfectant. (Reallly, again.)
The wood 1 1/2"X16" floor in the drivein section next to the milkhouse in my 100yo barn still exists. It’s been open to the elements (no garage door) at least the past 25 years. Good place to do my car oil changes – no pan needed. Edges have curled a little but really not much.
I was going to enquire if the OP had access to any bullshit… 
It really is a good flooring. But it takes experience to apply it, I gather.
Well, you can’t just slap just any old shit around and expect to get good results.
YMMV according to wood species and local climate.
What are you using the pole barn for? Since you mentioned stalls, is it safe to assume horses are involved? Once upon a time (pre-divorce) I had a barn full of horses and had uneven, poorly draining footing in the stalls. To fix that problem, I put in enough finely crushed limestone to level everything up and then put down rubber stall mats. We would then bed the stalls with coarse sawdust from a local lumber mill or bagged shavings. The hay stall had the floor covered with a tarp then the hay bales stacked on wooden pallets. The feed/tack room had a wooden floor. To keep the dust down, the center alleyway had a layer of the coarse sawdust spread out and raked periodically. That worked pretty well for us for the 15 or so years we lived there.
Depending on cost where you live, rather than normal gravel go with red-stone/red-dog
or reclaimed crushed brick gravel
(not the best images but my Google-fu is weak today)
We always had much better results with it than the usual gravel but we’re also close to cheap surface mines of the natural stuff and the cost is dirt (sorry) cheap.
Depends on what you’re using the pole barn for, but my dad lined the crawl space of his house with discarded billboard plastic. He had a friend who gave them to him for free, but you might call some advertising companies to see if they’ll sell you some cheap. It’s a really thick plastic and is nicer than dirt, at least. Might be an option if you’re looking for a very cheap option.