It’s really good to have some form of absorbent material in the bottom of your soil factory.
The absolute best is charcoal, or biochar as it is often known. You can use the same charcoal you’d use on the barbecue as long as there is absolutely no chemical additives in it, and that is made from some form of responsibly harvested wood.
Crush it into smaller bits if you can. No briquettes, and no ashes – they’re not good with microbes.
The reason that charcoal is so good in this context is that it absorbs a lot of moisture. It tends to take care of any odor issues. It retains a lot of the bokashi nutrients and microbes that it absorbs, and microbes really enjoy hanging out in charcoal.
When charcoal is charged up with nitrogen (which is what happens when it absorbs bokashi nutrients from your soil factory), it becomes known as biochar.
This is the same as terra preta, something that has become very trendy in recent years for good reason, as the Amazon farmers that invented it centuries ago knew what they were doing. Adding biochar to soil makes it really rich and dark in a long-term way that few other methods can match.
So long story short.
A layer of charcoal in the bottom of your soil factory will take care of any potential drainage issues and become a great additive to your soil in the long term. Easy.
There are alternatives though, all quite ok.
Newspaper is an easy fix. A couple of newspapers in the bottom of your soil factory will absorb a lot of excess moisture. Shred them if you wish. Egg cartons, toilet paper rolls, corrugated cardboard etc all do the same job.
If you end up with felt-like wads of newspaper when you empty your soil factory you can either bury them in your garden or use them as mulch under berry bushes or around other perennials. The newspaper is soaked in nutrients and microbes and will attract worms from the entire district, it will also prevent weeds from growing up and retain moisture in the soil.
Cover the paper with something like wood chips or bark, leaves or grass clippings to improve its appearance and keep it in place.
If you use sawdust pellets for heating in your home, you can use a few handfuls of these as a base in your soil factory. They absorb a lot of moisture and expand accordingly. No problem having them in your soil later, assuming they are chemical-free.
Absorption pellets used in cat litter products are also a good base, assuming they are made of natural materials and chemical-free.
Clay drainage balls, called leca in Europe, are also good, although they don’t actually absorb moisture.
You don’t always need this absorbent layer in your soil factory, it depends largely on how dry the soil is that you are using. If you are using really dry, depleted soil that you’ve emptied from pot plants etc, you won’t need any further absorption material. The soil will take up any excess moisture from the bokashi.
And actually, a soil factory shouldn’t be allowed to get too dry either.
Microbes work best in a reasonable balanced humidity, not too wet not too dry. More or less the same as you’d have in good growing soil. So if your soil factory seems really dry you may have to water it a bit to get things going.
Another way to maintain the humidity balance in the soil factory is to lay a newspaper or old towel on top. This will stop it drying out too much, or absorb surface moisture if needed. If your soil is a bit damp, the towel will prevent any odor from spreading out into the room — especially important if your soil factory is in the bathroom!