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Lots of questions! Relative Lots of questions! Relative to causing a nitrogen deficit in the root zone, just keep fertilizing your roses as you always do. Your mulch will break down slowly and won't deplete nitrogen at the root zone. But check out this column that contains information specific to roses. Compost chopped leaves as a carbon-rich addition to an ordinary compost pile. Or make leaf-mold compost—just pile leaves (chopped or whole)and leave them alone for a couple of years. Give your local extension office a call to see if they accept soil samples for testing. If they do, they'll give you instructions for collecting your sample. If they don't, they should have information on where you can send it.