Answer: A four inch layer of mulch should be more than enough to keep on a planting bed. You don't need to add more at this point. The nutrients in the mulch will leach down into the soil as it decomposes. In a few years you can add another inch or two right over the top of the old mulch, but not to exceed 4 inches total. The only wat to really enrich your soil is to dig that organic mulch into the dirt. This will help change the structure by loosening it and helping it drain well yet hold just the right amount of moisture. If you are unwilling to dig or rototill that mulch material into the soil, it would be best to leave it as it is and only add new mulch as the current layer gets down to about 2 inches.
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