Well, here is my opinionated response yo your questions.
Compaired to others options, how much maintenance goes along with a dirt tank. As far as nutrition and cleaning go?
Less as far as nutrients go. You will not need to use root tabs to feed your plants, for a long long time. Water column dosing may still be required based on the type of plants you have (think mosses), but even that is less required if you ask me. I have done very little dosing in my main dirt tank...I think maybe a few ounces of Leaf Zone until I ran out a couple months ago.
Cleaning is really no different. Plants will need trimming occasionally, glass will need cleaning...etc, same as any other tank. Vacuuming is totally optional is you ask me, I never do.
I think most dirt folks, and the majority of non-dirt folks will agree, dirt tanks require more effort in the setup, but are lower maintenance in the long run.
How long do the natural nutrients stay in the dirt?
You will get some varied responses here, most likely due to circumstance. For instance a tank full of root style plants (think swords) will eat up nutrients faster than say a bunch of stem plants (think bacopa). The consensus I have seen is anywhere from a year to three years and then some. At that point I think most might be ready for a tank overhaul, but if not you can start doing root tabs at that point...whenever that point arrives. The coolest thing for me about dirt tanks has been the initial growth. Under low light I have plants that have quadrupled in size in under two months. It's astounding, and it's a nice way to get a tank to "fill in" quickly.
Do you ever have to add more dirt/cap?
Adding dirt after the initial setup would be darn near impossible. That said I have seen the idea of making dirt ice cubes and stuffing them under the cap...though I haven't seen the end result of this method, lol.
Cap, you might have to add. I have about half a bag of Black Diamond left from my setup, and once in a while after I rearrange (*see note) something I will add a cup or two back on the surface to smooth things out.
*Note: Rearranging is the only true downfall of the dirt tank in my opinion. It can get messy if you are in a hurry, clumsy, or depending on what kind of cap you use. I have moved a large root heavy plant, it took me a few minutes to work it out of the soil, and even then there was a little plume of dirt. It quickly cleared up, but you do want to avoid excessive replanting with a dirt tank.
I am finding most guys plant all there plants right when they add the dirt. Is it hard to add plants later on or just a PIA?
No, I continue to add plants to this day. The reason you see most adding a bunch of plants is; one, they just set up the tank and have a lot of plants, lol; two, many believe (myself included) that heavily planting a tank right from day one prevents potentially harmful spikes of bad stuff and gets the nitrogen cycle going quickly. Meaning, your tank is safe for fish faster with lots and lots of plants.
What about soil straight out of a river? This is already mineralized right? so if I dry and sift and then add some dolomite and maybe some clay would that be the same thing?
Again, highly debated topic. My thoughts are why go rooting around in a nasty river and bring God knows what back to your new tank, when there are brand spanking new, clean, dry bags of Miracle Grow Organic at the store.
That isn't to say it couldn't be done. My other thoughts are, if the soil is in the river...are there plants in it? If no, why not? If yes, have those plants already sucked most the good stuff out of the soil?
These are questions I cannot answer. Again, I'm sure it would work...there is a fellow on the forum who made some MTS using dog poo and coffee grounds.
I have more thoughts, but I figure this will get the party started, lol.