You can give the cat litter a try, just soak it before. I put them in and feed them soaked quality dog kibble, just a few 2 or 3 large pieces, soaked in water for 1-2 hours so they are very soft, put them on top of the substrate and flatten them with a fork, lay an old credit card on top, and use a wet brush to collect the worms, rinse the brush in the water. I use a few boxes of these so i can rotate. When the box gets to wet i put it on an incline so the water drains to one side and i can pour it of. The soaked dogfood adds water each time, and the metabolism of the worms adds some moisture too. If the culture gets foul, it does after a few weeks, i rinse the total setup with water in the plastic container it sits in, stir a bit so all gets moved around, let it sit a bit so the worms sink to the substrate ,and drain the foul smelly water, repeat a few times. After that i ad a bit of food, it takes the culture a few days longer to be harvested again. In general i feed the cultures every 5-6 days. if the culture starts fouling you will see all the worms escaping the substrate (hence the clear containers are nice). I usually remove the worms escaping ( wash them of the lid in a container and use a brush to remove them of the sides) and start a fresh container or take them to my 400 gallon.
I do still have and used in the past soil based cultures, i always seem to get infestations, flour mites, springtails, flies, and when the soil gets soggy you have to restart. I think these work better outside and in wooden boxes so the moist gets out easier and it ventilates more.
My best wormculture in soil is in an old whine box with a wooden lid.
I believe my worm cultures are grindal mixed with some enchytraea (white worms), though some have more white then grindal. White worms need different cultures, cooler i think.
