If you put about a 6"+ of mulch on the a food forest floor, do you need to worry about nitrogen fixing?
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What is the mulch made out of? – Brōtsyorfuzthrāx Jul 01 '16 at 07:43
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1I hope you didn't pile the mulch around any tree stems? – Stephie Jul 01 '16 at 07:58
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Not sure what you are asking. For nitrogen fixing you need nitrogen fixers (fabaceae). If you give some nitrogen rich fertilizer (mulch?), nitrogen is already (nearly) in a state which can be absorbed by roots. – Giacomo Catenazzi Jul 01 '16 at 09:58
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You appear to have confused nitrogen fixing (the process where bacteria and legumes work together to fix nitrogen from the air into the soil in a plant-usable form) with nitrogen depletion, a temporary condition caused by the breakdown of carbonaceous materials tying up nitrogen reserves in the soil and keeping plants from using them.
MY observation and methodology is that mulch, on top of the soil, not mixed in, has a limited ability to pull nitrogen from the soil, since it is only contacting the surface of the soil. So, I don't get too concerned about it. When or if it gets mixed/tilled in, you'll need to wait 3 weeks or so for the stuff to decompose, or things planted there will be nitrogen-starved.
Depending what you are mulching with, 6" is probably way over the top, which seems to be a recurring theme with you. Straw, which is hollow, does not pack easily, and has lots of air-space might be OK - your coffee grounds that we keep telling you to put in the compost pile would be a moldy disaster 6" deep. You certainly need to maintain a clear area around the stems of plants and especially trees/shrubs or you will kill them with mulch that deep.
Ecnerwal
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We have sandy soil that you need to mow every month, or every other month unless you get a heavy rain, like this year. – black thumb Jul 01 '16 at 15:09
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1Mulch at a reasonable depth, maintaing a clear area around trunks, and put any extra material on the compost pile, to be spread after it composts - or till it into a fallow area to decompose in place (sheet-composting - no hope of killing weed seeds that way, though) stopping at least 3-4 weeks before you intend to plant that area. If you mean "mulching with grass clippings" my personal experience is that 2-3 inches is OK, and 6" will turn into a nasty anaerobic slime, wasting most of the nutritive value to the air. – Ecnerwal Jul 01 '16 at 15:15