I've heard worms can help. They tunnel through the soil and help your roots breath and grow. Before I started growing I looked into that and found multiple sources that said worms can help your root structure and overall growth of the plant. "Worm Water" is the secretion from a worm and is benefitial to your plants.
If you need any more proof here's a quote from one site;
"Marijuana soil should compact when you squeeze it, but should also break apart with a small pressure and absorb water well. A nice test for either indoor or outdoor growing is to add a bunch of worms to the soil, if they live and hang aroung, it is good soil, but if they don't, well, change it. Worms also help keep the soil loose enough for the plants to grow well."
How to Grow Marijuana