Castings contain 5 times the available nitrogen, 7 times the available potash and 1 1/2 times more calcium than that found in 12" of topsoil. Therefore, castings are supplied with available nutrients. The nutrients are also water-soluble and immediately available to the plant. You will find that most potting soils have a nutrient life for 2 to 5 days, where worm castings will last up to 6 times as long. You will need 5 times as much potting soil to do the same job as worm castings. So in the long run, worm castings are much cheaper and do a much better job. Also, castings hold 2 to 3 times their weight in water. That means you water less and the pot will stay damper for a longer period. Worm castings will not burn your plants; unlike using any fresh raw manure (cow, horse, etc.) which can burn root systems if not applied properly. . . . The manure passes through the worms' digestive system producing rich organic plant food and a slow releasing fertilizer which allows for better growth.