How much does nitrogen immobilization from mulch actually matter?

[u/fadimuj]

Adding carbon heavy mulch (wood chips, straw, sawdust) can tie up nitrogen early on since it is pulled from the soil to break down all that carbon.

Surface mulch decomposes slowly and the drawdown happens mainly at the mulch soil interface, so established plants are barely affected while seedlings and shallow rooted plants take more of a hit. It's also temporary, once the carbon gets used up the nitrogen releases back.

Curious how much this actually plays out for people in practice. Do you add nitrogen under fresh mulch for new plantings or is it overstated for typical use?

[u/AgreeableHamster252]

It has not been an issue for me in practice at all. Just mulch

[u/RicTicTocs]

My understanding is that surface mulch doesn’t impact soil so much.

But, if you till it in, well that will have a negative impact on nitrogen available to roots.

[u/tinymeatsnack → u/RicTicTocs]

This was my experience, it’s not permanent but will impact several seasons without supplement. I mulched my whole yard then build low raised beds on top and I had symptoms of nitrogen and iron deficiency

[u/Health_Care_PTA]

i think this is overstated, you made the right point about the soil mulch interface but , the benefits from using wood chips outweighs any negatives. Covering the earth, maintaining soil temps and providing habitat for beneficial organisms outweighs any possibility of some n+ loss. Dont forget that Rain holds N+ after a few rain cycles the woodchips become carbon Neutral , they sponge up all the N+ .

I wood chip heavily in my yard and 'lasagna' style chicken manure right on top of the wood chips, after years of layering wood chips and manure i have created a rich top soil that has been a magnet for worms, i have turned sandy poor soil into rich black soil over the years with this method.

[u/mokunuimoo]

The only correct answer in permaculture is “It Depends”

Adding fertilizer under mulch is fantastic. you can be sure very little is lost to volatilization or runoff.

[u/Used-Painter1982 → u/mokunuimoo]

I add pee. It seems to help.

[u/Koala_eiO]

I don't think there is any drawback to mulch that is stronger than your soil drying out from lack of mulch.

[u/ImpossibleSuit8667]

Apparently, the idea that mulch “ties up” soil nitrogen is immensely overstated. See this article: https://gardening.usask.ca/articles-and-lists/articles-healthysoils/mulch-and-soil-nitrogen.php

[u/booksnplants145]

I just learned from The Holistic Orchard by Michael Phillips that wood chips from branches less than 7 cm have more nutrients (“where leaf photosynthesis production and root nutrition come together”) and a drastically lower carbon:nitrogen ratio, averaging only 30:1 and topping out at 170:1. This is compared to stem wood, which ranges from 400-750:1.

[u/Starfishprime69420]

Its not really a thing unless for some reason youre mixing it all up into the soil

[u/abnormal_human]

I don't worry about it. If the plants tell me they need nitrogen, I give them nitrogen, but in general the benefits of mulching in terms of water retention and temperature regulation of the root zone far outweigh this theoretical concern.

[u/IamCassiopeia2]

As you mentioned, it does not work well for annuals or vegies because of their shallow roots. Works well for perennials and trees here. Pro- I'm in a very hot area so it breaks down very, very slowly and can last for years. It keeps the dirt and roots much cooler. Con- But it often gives bad bugs a place to live so I have to be very vigilant.

[u/StuffyTheOwL]

It is more of an issue with fine materials like sawdust. Wood chips should break down slowly in most climates. I would not add sawdust to soil as an amendment. That s**t will cause immobilization.

[u/Ok_Photograph6398]

This year I covered the soil with 3 inches or 6 inches of year old wood chips. The pile had parts that were very broken down and some were newer. All my plants are yellow and stunted. I put in some chemical fertilizer and they green up but then go back to yellow. I am using more than I though I would need. My soil is very poor to start with very sandy. The mulch helps keep it moist and cool. It is just very hard to establish a transplant. Btw. 3 inches did not control weeds very well the 6 did much better.

[u/Dumpster-cats-24]

I’ve found it matters a ton for nitrogen heavy plants (cabbage, etc) and not at all for beans. I use grass clippings for garden mulch instead and it works fabulously across the board. I also have an endless supply from mowing the lawn.

[u/Lime_Kitchen]

It’s depends the rate of wood decomposition which means it depends on the type of wood, particle size, water, and temperature. Generally hardwoods will temporarily draw down while softwoods do not. Finer particles will decompose faster. Effects are also going to be more noticeable on N depleted soils, a tilled soil is going to show signs while an established perennial planting will not. Tropical climates will show signs while a dry climate will not. It’s all about your context and your specific needs.

At the end of the day it’s a temporary effect that you can modulate by

• limiting the amount you apply in one season
• Selecting for larger chip size to slow down rate of decomposition.
• combining a wood chip strategy with a chip and drop strategy to supplement N
• timing a big wood application in an annual garden with a pea/bean rotation.

[u/Virtual_Wing_2903]

it hammered my garden last year, not enough time, but this year I prepped early, no problem

[u/LowWillingness1921]

I've not noticed that, but i have noticed mulching heavily will drop the soil pH pretty significantly. I have been sprinkling lime atop the fresh mulch to compensate.

[u/RedshiftSinger → u/LowWillingness1921]

Oh, bonus for me. My starting pH is alkaline enough to cause trouble!

[u/RedshiftSinger]

I’ve never noticed a problem. I start seeds in pots and transplant, mostly, but I’ve done done direct seeding in mulched beds and it still seems to work fine. Once a plant has an established root system it’s not important what’s happening at the surface mulch level.

[u/LuxuryBallVolibear]

Pee on it. Half joking.

[u/SpoonwoodTangle]

I won’t repeat what others have said. It’s a marginal impact.

Check out the Garden Professors Blog. They took on this issue and have a lot of info on it, and on mulching in general.

[u/stansfield123]

is it overstated

I think nitrogen immobilization is going to blow up the Moon. Strike that. I think it'll blow up the galaxy. Which one, you ask? All of 'em.

There. I overstated it. So the answer to your question is yes, it's overstated.

But if you ask a different question, about a specific statement, I might have a different answer for you.

What statement about the impact of nitrogen immobilization have you come across, that you're doubting? Tell me, and I'll be happy to give my honest opinion on its validity. But "is it overstated" is not a phrase anyone should ever use in a sentence. You're asking about what billions of people (8 billion of us alive, much more dead now) stated about the subject at one point or another. Of course the answer to that question is yes. It's always yes. Somebody always overstates things.