Worm castings are only as good as what was used to produce them. Worms may reduce paper, cardboard, and wood to what appears as lovely castings but they contain no more nutrients than the paper, cardboard, and wood. Therefore you have to consider the nutritional value of your mulch. If it were devoid of nutrients as mulch, there won't be any more nutrients in it after it has been broken down by the worms.
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