Can You Use All Worm Castings for Plants?

Features

• Add a maximum of 15 to 20 percent of worm castings to existing soil, is the consensus of Ohio State University soil ecologist Clive Edwards and other researchers. You can, for example, mix worm castings with potting soil in a ratio of between 1:7 and 1:5, or add an inch of worm castings to the top of the soil and work it into the top 6 inches of soil.

Warning

• If more than 20 percent worm castings are added, the soil reaches a saturation point where the vermicompost no longer increases plant growth and can potentially damage the plant, states technician Lisa Wickland of the Washington State University extension service. She found that a mix of 20 percent worm castings worked better than either 50 percent or none at all.

Expert Insight

• Worm composting pioneer Mary Appelhof writes in "Worms Eat My Garbage" that pure vermicompost may hold potentially harmful salt concentrations. So avoid using pure worm castings when for example growing plants in pots or raising seeds in flats; mix the worm compost with homemade or commercial potting mix.