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Using sawdust (including from N-fixing trees) to build soil  RSS feed

 
Rob Fetter
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I have a lot of sawdust from some recent stump grinding. Probably on the order of 10-15 cubic feet of sawdust. I've gained a LOT of information from this forum (and some extension pubs) about C:N ratios, how they work, why I don't want to just mulch with fresh sawdust because the microorganisms will tie up soil nitrogen and plants won't be able to access it until the bacteria die off.

I still have a few questions that don't seem to be answered by the posts I've found so far:

1. Some of the sawdust is from a mimosa tree (Albizia julibrissin), a nitrogen-fixer. The tree was probably 15-20 years old (unfortunately, it was half dead). Can I trust that the C:N ratio in this sawdust is lower? Any idea what it is?

2a. If I use a very thin layer (say, 1/4 to 1/2 inch) of sawdust sprinkled on cover crop seed as a top mulch to retain moisture for germination, will this have the effect of binding nitrogen and inhibiting germination or plant health?

2b. If the answer to #2a is yes, what about if I add some slow-release nitrogen fertilizer at the same time, so that now I'm top-mulching cover crop seed with a mixture of sawdust and fertilizer that has a C:N ratio of (say) 30:1?

3. Can I plant nitrogen-fixing herbaceous plants directly into fresh sawdust? Say, ladino clover? (my theory is no... but I thought I'd make sure.)


Thanks!
Rob
 
Dale Hodgins
gardener
Posts: 6155
Location: Victoria British Columbia-Canada
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I would let the cover crop get a few inches tall and then lay the sawdust between the plants so there's no danger of matting and hurting germination. A nitrogen hungry product like sawdust is a good spot to sink nutrients from clover, vetch etc. I would mix it with coffee grounds and some dolomite lime to correct ph. The coffee gives up N slowly and is neutral acidity with no weed seeds.
 
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