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Composting/Do I need to compost sawdust from N-fixing trees?

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Hi,

I have a lot of sawdust from some recent stump grinding. Probably on the order of 15-20 cubic feet of sawdust. I've learned a LOT from forums and extension publications about C:N ratios, how they work, why I don't want to just plant into fresh sawdust because the microorganisms will tie up soil nitrogen and plants won't be able to access it until the bacteria die off.

But I still have two questions about C:N ratios in sawdust that don't seem to be answered:

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 than for wood chips in general? Any idea what it is?

2. I'm putting in a fall cover crop and would ideally top-dress it with compost. But the material I have in abundance is sawdust. If I use a very thin layer (say, 1/4 to 1/2 inch) of sawdust, combined with a slow-release nitrogen fertilizer to bring the C:N ratio of the mixture to 30:1, will this (basically uncomposted fine-grained material) be an effective seed cover?

Thanks so much!
Rob

Answer
Hello Rob

I see you have been busy looking things up. Well played sir


A tree or a plant that can take Nitrogen from the air, and make it available to others that surround it, is a Legume. Or as you call it a Fixer. With that in mind, I think you are asking me, if this means that since it's a Nitrogen fixer, does that mean it has a lower Carbon to Nitrogen ratio than average for other trees? Since it's a fixer of Nitrogen to the root zone, I think, but will not confirm, that, it doesn't change anything in that regard. Treat it as wood chips, with also the idea that, what did that tree die from, and could you be transferring a disease? Changing the size of chips to sawdust, doesn't change whatever the C/N Ratio is, it is what it is, it's just that sawdust is wood chips, composting quicker. I hop I helped you with that answer, if I got the question right.


The problem with your next question, and I know you have done your homework on this, is that, as carbon decomposes, it will take nitrogen where ever it can get it from, including the plants. I usually just deal in composting, where you have an area, where it decomposes, and there is nothing that needs to grow there. I guess if it was clover, which is also a legume, this would work, but I have no idea, what your cover crop is. If I guess right, you are trying to get a cover crop that you can till in next year as a "green manure?"


I may be gone this weekend, but if you could answer a few things here, and I can get a better picture of what you are trying to do, I could give a better answer



Greg

Composting

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As a Master Gardener: taken classes in All Phases of Gardening...I Specialize in Lawns... Pererennials... Compost... Organic Gardening.

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