Having clean floors is what having a clean home is all about. You just can't have one without the other they are inseparable! Floorboards tend to get dirty rather easily; so before you even start cleaning keep in mind, that if you had cleaned your floorboard on regular basis, you won't have to deal with this right now.
I am only saying this because I have personal experience with cleaning floorboards, and one thing I have learned during this time is that wood, any kind of wood, is best treated regularly or else it might be too difficult later on. But hindsight aside, there are some ways to treat wooden floors, which don't involve using dangerous chemicals; such as solvents, strong acids/bases, commercial detergents etc.
We will start buy manually cleaning our floors to the best of our abilities. That includes a complete and thorough sweeping of the premises. You have to make sure that any large contaminants are taken care of before proceeding any further. Do a couple of rounds with the vacuum cleaner as well taking special care to get the nozzle between the floorboards themselves. Don't mop or use any kind of liquid just yet this is step 2.
Prepare a cleaning solution
Your cleaning solution can be made, using nothing more than a couple of common household products; such as baking soda and white vinegar. It is best to dilute any acid in water before applying it to wooden elements. Not only can you ruin the finish, but you will be wasting a whole lot of vinegar too. Use one cup of vinegar (always pure, white, distilled vinegar) in combination with one cup of water to make a cleaning formula, which we will then load into a pre-washed spray bottle. Use the spray bottle to apply the vinegar directly ONE SMALL PORTION AT A TIME!
Too much moisture in one place, and you risk ruining your floorboards for good. Spray, scrub (using a brush) wipe and repeat. Sure it might take you a while, but at least you know you are doing a good job.
This wonderful confection additive is in fact one of the best organic cleaners you can find! It is good because soda bicarbonate is by no means poisonous to the human body, and it is good because it's cheap and it actually gets the job done. You have to add a little bit of water just to turn the powder into fine paste (again, use as little liquids as possible). This time we are going to cover just a little bit of the floorboard just to see if the paste will ruin the finish. Leave it like that for half an hour, and rinse. If the wood seems okay (and you see that the baking soda is doing it's job), apply all of the paste, wash and repeat if necessary.
Some people like mixing bleach in with their ammonia, but because we are about to clean "dark" wood, a 'whitening' agent such as bleach, might not be the best solution (you can do more damage than good this way). So plain old ammonia will have to do the trick. Make a 50/50 water-ammonia mixture, making sure that you are wearing protective gear.
Concentrated ammonia is not something you want to be around without protection, trust me. Again, clean one small portion at a time before all your floors look as new.