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I have a few football-sized holes in a yard where I removed some rocks. I have access to a large amount of finely chopped wood mulch.

How can I encourage the mulch to decompose into fill dirt?

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    What's the longest you're willing to wait? – OrganicLawnDIY Jun 3 '16 at 2:19
  • You really want coffee grounds as it will be much finer to work with, or you could do a mix of the two to have a better dirt mixture for things already in the soil. – black thumb Jun 3 '16 at 17:32
  • @OrganicLawnDIY I'd expect this normally to take between a few months and a year. It's not really a critical issue, it's just a bit nicer if I can get grass to grow sooner. – Joel Harmon Jun 4 '16 at 15:30
  • As Ecnerwal indicated it won't break down into dirt. If you want grass to grow I think you're better off just getting some good top soil to fill it in for now so you have grass there so weeds don't grow if it's not to hot there yet. You can compost the wood at your own pace and use it as an amendment in other parts of your yard. at least a 3'x3'x3' pile, add sources of nitrogen, keep it moist but not too wet and the more air it can get into the center of the pile (the more you turn it) the faster it will decompose as others have suggested. You can search for "hot composting" . – OrganicLawnDIY Jun 4 '16 at 15:49
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There are several steps you can take to speed up decomposition of organic material. Decomposition of organic debris is both a physical and chemical process. Some of the steps listed below are needed to counteract some of the side effects of mulching that slow the mulching process.

The amount of heat generated by your mulch pile is an indicator of how fast breakdown is occurring. More steam from the pile interior when you mix it indicates a faster chemical breakdown but this higher heat also means you will need to perform regular physical mixing of the pile and keep adding water to keep the pile from getting too hot and killing the fungus and bacteria inside (slowing breakdown). It is entirely possible if you don't keep the pile wet enough it may get hot enough to come close to ignition, so keep a close eye on the pile during early mulching.

Some of the other answers here mention adding worms to the pile. The problem with this is that if you are trying for a fast breakdown, the heat inside will tend either cook any worms inside or drive them out of the pile. Worms are great as natural aerators in slow decomposition mulch piles or in normal soil, since neither of these create the large amounts of heat that "fast" decomposition does.

1) First, use mechanical force to break the contents of your mulch pile into as many small pieces as possible. Set your wood chipper to "liquefy". The smaller the starting mulch pieces, the faster the mulching breakdown process. Thin organic matter like tree leaves and grass break down faster than wood chips and twigs which break down faster than large wood pieces like branches.

2) Add water as needed to keep your pile wet. Moisture encourages faster breakdown by encouraging bacterial and fungal growth. Usually keeping the pile covered by a plastic cover sheet keeps too much water from escaping the pile and keeps the surface of the pile moist.

3) Mix your mulch pile on a daily basis using a garden fork. If your pile is breaking down quickly, you will notice steam coming from inside of the pile due to the heat from chemical reactions during breakdown. Mixing allows heat to escape from the pile as well as spreading bacteria and fungus around and aerates (allows air to mix into) the pile. Add more water as needed to help keep things moist as well as to cool the mulch.

4) Mix some crushed lime into the pile using a garden fork a few days after starting the mulching process. The acid level of your pile will increase over time as your mulch pile breaks down, and this slows bacterial growth. Adding lime will decrease acidity, encouraging bacterial growth and speeding breakdown.

The steps listed above apply to any mulch pile, but if you added a lot of wood to your pile when you created it, there are two further steps you might want to use. Wood in a mulch pile changes the chemical breakdown process.

5) Mix nitrogen fertilizer (such as ammonium nitrate) into the mulch pile. Nitrogen is used up during chemical breakdown of wood fiber, and insufficient nitrogen slows this breakdown.

6) Sprinkle/mix fungi into the pile. Fungus (such as mushrooms) breaks down the physical structure of wood pieces so the bacteria can get into the interior of the wood chips.

Eventually your mulch pile will no longer be warm and you will no longer see steam escaping when you mix it. This means that the breakdown of the mulch is slowing and is mostly complete. There may still be some wood chips mixed into the black organic crumbly mass of the decomposed mulch. Once the mulching breakdown slows like this, you can choose to age the pile for a few months longer to allow the mulch to completely break down, or if you are in a hurry you can use the mulch as is.

If you choose to age the mulch further, you won't need to mix the pile as often as you did during the early mulching process. Continue to mix the pile once a week or so and add water when needed to keep it moist. Continued aging will allow any remaining un-decomposed matter such as wood chips to completely decompose.

If you decide to use the mulch without aging it completely and wood still remains in the mulch, you should add nitrogen fertilizer to the mulch before using it on plants. If you don't add nitrogen, you risk starving the plants if when the wood decomposes it uses up all the nitrogen out of the soil.

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The highest nitrogen fertilizer you can easily find, and water to keep it damp/humid, but not so much as to wash the fertilizer out of it. If you have sufficient privacy, or a dog, urine will also help.

Of course, it does not become "dirt" it becomes humus.

If you want dirt you'll need some sand and clay as well. If you need fill dirt, buy fill dirt, or scrape out a shallow layer of dirt and replace with a shallow layer of humus, after you use the actual dirt to fill the holes. If they are only football sized a $2 bag of bagged dirt at a hardware store/garden center will do at least a couple of holes, probably for less than the cost of fertilizer to help break down the wood, and without the wait. Once the wait is over you can spread a thin layer of the resulting humus over the entire lawn (might as well make a lot if you have access to lots of raw material) and somewhat more on any garden areas.

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Basically it is a simple matter of keeping the mulch damp to get it to decompose.

Small bark chips/shreds actually make an excellent growing medium because it admits lots of oxygen (air) and holds fertilizer well. It is fair at water retention, but I would mix in a bit of the soil dug from the hole you are trying to fill, just to make the fill more similar to the soil around the hole and hence will be less noticeable in its effect on the grass growth. It likely will matter little if you mow frequently.

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In a nut shell you need several things to help out with decomposition: firstly you need moisture, secondly bacteria and fungus's, thirdly a source of nitrogen- organic is best, animal manure or human pee will do?... I didn't say that out loud LOL. And finally oxygen- a compacted layer of anything will have a hard time breaking down, so fluff it up with a good forking over and not walking on it afterward- next is time- depending on how deep it is, deeper the better and you will notice a significant rise in temperature anything up to 60 degrees on a good summers day- just don't let it dry out. If one is talking about composting then things get a bit more complicated and that's a whole new thing to talk about.

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I saw a video on the life of worms, and you can get them easily by:

  1. put a metal circle on the ground with a few metal probes in it about a foot down
  2. take a car battery to force them to surface (a mild pain for them will force them to surface)
  3. grab the worms
  4. put them in the compost
  5. leave the worms eat the compost
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    what nonsense - in the main , it's not earthworms that break down composting materials, its brandlings and other types of worm. Leave the worms in the ground alone, this is a pointless exercise. Bacteria do most of the work in a compost pile, specially aerobic bacteria, and cellulose rich materials like your wood shavings need fungal activity. Not all guidance on the internet is useful or accurate. – Bamboo Jun 3 '16 at 11:15
  • they eat the green material, which is in compost. the more they eat, and produce the more the compost will convert to dirt. the worms can always leave, but they're more likely to just stay where the food is, and reproduce. if you're talking about bacteria, no they're not bacteria, but they may be quicker. – black thumb Jun 3 '16 at 16:08
  • @Bamboo why do you want worms in the garden if they don't do composting/soil enriching? – black thumb Jun 3 '16 at 16:10
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    Oh dear, black thumb, I have to say, you do make me laugh, genuinely (not in a bad way), so thanks for that. Of course you want worms in the soil - its just the worms usually found in compost heaps are not the same type of worms found in ordinary open ground. I think red wigglers are the USA version of what we call brandlings, and they're most commonly in composting heaps. But with a high wood content, you're relying on fungal activity to do most of the work of composting, hence Jim Young's answer re keeping it damp - it encourages fungal activity. – Bamboo Jun 3 '16 at 16:39
  • Some of the nightcrawlers in the soil might be one of the species used for composting. I've heard rumors of people using earthworms, but I'm guessing they were talking about nightcrawlers or such, since they might seem to be just giant earthworms if you don't realize nightcrawlers and earthworms are different species. There are pros and cons to different kinds of worms, I've read. – Brōtsyorfuzthrāx Jul 27 '17 at 0:10

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