humidity in earth berm type root cellar

I'm having problems with out of control humidity in my root cellar, its a dirt floor cinder block walls that are backfilled around it almost to the top, plywood roof which is the floor to canning shed built above with tin roof. Any suggestions.
would lots of insulation help? do I need moisture barrier on floor and then some concrete or something. it does not freeze in winter and is cool in summer but jar lids rust out in no time it seems. mold is a big problem too

Spraying borax can suppress mold quite a bit.
Waxing the lids can prevent rusting.

What provisions do you have for airflow?
A solar chimney up the south side of the canning shed,combined with a perforated black drain pipe earth tube for incoming air,  might provide temp and humidity moderated airflow.



Any water coming from above that point might pool on top of the plastic,  causing other problems.
An easily removed walking surface could allow easier changes, so if you try the vapor barrier on the floor,  maybe cover it with pavers and sand  at first.

Sealing the walls with a waterproofing paint might help as well.

Does water condense on the plywood ceiling?
If so,  I would insulate there.

A root cellar is supposed to be very high humidity to allow the roots/tubers to last as long as possible without drying out.  I'm guessing you may not have enough ventilation, normally root cellars want a vent high and low to get a bit of air exchange.  Limewashing can help with keeping the mold down.  Plywood ceiling will be an issue since it's likely colder than the cellar in the dead of winter and will be a condensation point.  Insulating may help but I'm not sure.

Canning jars are not recommended to be in a root cellar because... they'll rust. They just need a dark, dry and cool location.

Root Cellars are actually historically used to keep 'root crops' not frozen(warm) in the winter.
Heated winter air can also be very dry. So the high humidity of root cellar are actually a feature.

If you want to preserve food, you can also dehydrate, ferment, salt/brine,  and smoke, vs just cool in a root cellar.