Sawdust - compost or mulch?

burra_malucaAugust 20, 2008

I have access to an enormous amount of untreated sawdust and I'd like to experiment with it to make compost and/or mulch.

If I just stacked a load and kept it damp, would it be usable as a mulch in a few months?

If I layered it with greens, would it compost properly or would it still only be usable as mulch?

Would the result be usable everywhere in the garden or just around the fruit trees?

My soil is really thin and poor, and it's very hot and dry here in the summer, so it would be nice if I could convert the stuff into something that would improve the texture and water-holding ability of the soil even if the nutrient content isn't great. I'm also making a 'normal' compost heap to improve the soil generally, but there's two acres of poor soil so it's going to be a long job unless I can find a way of using what's readily available. I know the carbon/nitrogen ratio isn't good, but there's only so much 'green' available to add and it seems a shame not to use the stuff it it's possible.

Should I just experiment anyway to see how it goes?

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You can use it as mulch immediately, but I think you need to be careful not to pile it too thickly. I think it's one of those things that can form a crust that will repel water.

It will compost fine if mixed with some greens. It's very high in carbon, so you'll need a fair amount of nitrogen to offset that. In addition to garden waste and so forth, if there are any Starbucks near you, you can get coffee grounds from them. Other restaurants may make them available if asked, but it's standard policy for Starbucks.

    Bookmark   August 20, 2008 at 11:28AM
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Sawdust can be used as a mulch, although it is better to compost it with many other types of material and then put it on the soil. I have not seen any problem with Nitrogen deprivation where I have used sawdust as a mulch.

    Bookmark   August 20, 2008 at 1:47PM
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joepyeweed(5b IL)

I would do both, use it as mulch and compost it.

You did say that you have access to an "enormous" amount.

    Bookmark   August 20, 2008 at 2:01PM
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If you're brave word...humanure

    Bookmark   August 20, 2008 at 4:29PM
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Ive been planning the same thing, here are my ideas,

First use it for compost for whatever amount of greens I have available then create garden paths with some of it, little at a time to stomp it in good and mulch some of it over beds.

Keep a bag(or two or twelve) of it handy for when the irresistable rotten punkins start appearing on curbs after halloween.

    Bookmark   August 20, 2008 at 11:21PM
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Thanks guys!

I'm going to try all those suggestions (yes, I *am* that brave, thanks takadi ;-) ) except the Starbucks as there aren't any around here. In fact, there's hardly anything except pine and eucalyptus forest here... I'm in a very rural part of Portugal where the old farms have no sanitation whatsoever and discovered the sawdust when I started the humanure heap. I felt silly turning up to collect sawdust with just a bucket or two when there was such an enormous pile the guy wanted to get rid of. He has chipped bark, too, which looks useful.

Currently all the greens are going on the humanure heap as the sooner I stop adding to it, the sooner it will be ready to use, and I have to wait a year for it to mature before using it. In the meantime I want some 'normal' heaps that I might be able to use a bit sooner.

Kimmsr - I'd heard about the nitrogen robbery not being a problem with mulch. I read that it will only effect the very surface layer so any plants already rooted will be fine but it will help stop weed seeds germinating. Nice to hear from a 'real person' rather than from a book though, so thanks!

I think I'm going to build a couple more compounds and do a few experiments, with some sawdust (and bark!) going straight as mulch, some composted straight for mulch, and some composted with whatever greens I can find and use it however seems appropriate.

    Bookmark   August 21, 2008 at 2:47AM
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paulns(NS zone 6a)

Scroll down to the chart at the bottom. Softwood sawdust degrades more slowly than hardwood, due to its tannin/lignin/resin content. Something to keep in mind. Softwood sawdust is great in paths, or as a mulch around berry bushes (making sure the soil is wet before applying it), because of these same qualities. I'll use it in compost if I have no other choice, and can leave the compost to work for a year or so.
Not to be negative or anything. :) We just got a pickup-truck load of mixed hard/softwood sawdust from a guy with a sawmill who was happy to get rid of it. We're thrilled - mulch for paths is worth its weight in gold.

Here is a link that might be useful: humanure - lignin content of wood

    Bookmark   August 21, 2008 at 10:03AM
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spiced_ham(z5 OH)

If handled right, sawdust can be one of the best composts for your soil because of the high lignin content. Lignin, and leaf polyphenols [tanins] are the only part of vegetation that chemically break down into stable humus, everything else goes into feeding bacteria and fungi.

Lignin forms stable complexes with proteins even before it is broken down (so it binds nitrogen during hot composting without the aid of microbes). Humus itself binds to soil mineral nutrients, as well as proteins, so wood chips and sawdust with C:N ratios of 100:1-600:1 (30-40% of which is lignin) bind alot of nitrogen which is then a long term organic fertilizer.

Unfortunately it is difficult to get these levels of nitrogen through the addition of greens, because the small amount of nitrogen has to be slowly recycled as the carbon is burned off. It is much simpler to feed the microbes mineral nitrogen (either nitrogen only fertilizers or high nitrogen-urea lawn fertilizers). The microbes feed on the cellulose and mineral nitrogen, die, have their proteins bound to lignin,... repeat process. Because the lignin binds nitrogen before it is fully composted if you add enough you can add immature compost (6 mos) to the soil without robbing it of nitrogen.

A ten pound bag of high nitrogen lawn fertilizer will be sufficient nitrogen for a 1-2 cubic yard pile of sawdust or wood chips. Add about 1/3 bag at each turning. The wood bits [and the fungal threads] will turn chocolate brown due to the lignin-protein chemical reaction, and humic acid accumulation on the fungi. Soil fungi will slowly break down the lignin-protein complex after you mix it into the garden, releasing the nutrients and leaving humus behind.

Its an inorganic way to quickly make organic fertilizer.

NDSU hot composting

    Bookmark   August 23, 2008 at 10:26AM
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Thanks for that spiced ham!

I'd been thinking of making up a pile of neat sawdust and adding urine daily (I'm training the family with little lidded buckets!) Any idea if four people's output would match a ten pound bag of high nitrogen fertiliser over a couple of months??

Stable humus is definitely what I'm after, and it seems that sawdust has plenty to offer on that score. Can I assume that if my experimental heap turns chocolatey brown then I'm providing enough nitrogen?

I can see myself with a whole row of heaps adding urine to one, layering one with donkey poop, one with sulphate of ammonia...

    Bookmark   August 24, 2008 at 2:46AM
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paulns(NS zone 6a)

No, unfortunately, sawdust + petrochemical fertilizer isn't an option for the organic grower. The best we can do, as far as I know, is to use Time in the recipe, eg. sawdust+fish scraps+time, sawdust+urine+time, etc.

    Bookmark   August 24, 2008 at 8:49AM
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spiced_ham(z5 OH)

Organic gardeners could add a ton of blood meal, or any animal matter you have available (road kill etc). Wood chips are really good at absorbing odors for the same reason as activated charcoal. I have a skunk, racoon, and groundhog providing nitrogen to my chips. If I still lived on the coast I would go out with a castnet and get a pile of little fish. As a biologist who knows the numbers I am against the use of fish emulsion and kelp extract because of the overfishing, kelp ecosystem destruction, and water pollution (trash and deisel from the harvesting-fishing boats). All of the world's fisheries providing fishmeal are overexploited, which is having drastic effects on marine mammal and seabird populations in those areas.

There are two reasons to go organic, 1) to better improve the soil by using techniques that improve the soil organic matter content (which making high quality chemically-organic compost with mineral fertiizers accomplishes), and 2) to help the environment by not avoiding toxic pesticides and using nonrenewable resources. The latter point is a very gray area in that organic pesticides can be just as nasty as synthetics, and it takes nonrenewable resources to get renewable resources unless you do it yourself with hand labor and using recyclables.

Plus, if you use blood meal you will really tick off the VEGANS.

A 10 lb bag of fertilizer costs $8, which is equal to 2 gallons of gas, which is what I would spend driving around trying to get non-petro organic fertilizer equivalents, so in this case doing it the organic way would cost more petrochemicals than it would save. I do my part to save the environment by not eating cows (all that greenhouse gas methane that comes out of them), and not having any children or warm blooded pets. It's all in how you look at it, and the end result of the overall lifestyle cost benefit analysis.

Now back to the toilet humor.

I don't think a family can produce enough urine to keep up with the nitrogen use rate of a decent sized hot compost pile. Doubling time of those microbial populations is about 1 hour when they get going, but it wouldn't hurt to add urine to the pile as long as you are free of the few parasites that release eggs into the human urinary tract - rare in developed areas outside of the tropics.

The color development from the nitrogen reaction begins fairly quickly, and at the first turning you will see the difference between in wood that didn't get any nitrogen. It will still be the original color, or the fungal mass around it will be green-gray-white. The color gradually deepens with time, and penetrates the wood as the fungal threads work their way in. If you add nitrogen to the raw wood you may smell a little ammonia for a few days until the microbes use up the simple sugars and amino acids from the sap and have to start in on digesting the lignocellulose. You don't want to add any lime, because you want fungi to do the work, not bacteria. If you see mushrooms sprouting up it is a good sign.

    Bookmark   August 24, 2008 at 10:21AM
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Spiced ham - you are a godsend!! That is exactly the kind of information I was looking for.

I agree it's not 'Organic', but the time factor is exactly the problem. We've been struggling for three years to build the soil up following organic principles, but in this part of the world there just isn't much we can use to help - except the sawdust. Even with the donkey's help for the last year it's so slow that at this rate we'll be driving backwards and forwards (20 mile round trip) to buy fruit and veg forever. It makes much more sense to me to sacrifice the organic principles for the greater good of getting the soil healthy, water-holding and productive. In the end less fossil fuels will be used as we won't be driving around so much or running the generator to pump water so often as the soil should be holding more water. When the soil has improved sufficiently, I'm sure we can manage to keep it that way without 'cheating', but in the meantime...

I wouldn't use fish meal for the same reasons as you, and the thought of using blood is pretty repugnant. I might be persuaded to put some in the centre of the hot humanure heap, but not in the sawdust heaps which I want a faster turnover on.

If you think one family can't pee enough, I'll try the 'organic' heap with alternating layers of sawdust and donkey poop and add the urine to that.

Do you have any idea of the timescale for composting sawdust with fertiliser? How do I know when it's 'safe' to put on the soil, ie when all the nitrogen is safely tucked away and won't harm the soil organisms?

Also, can you recommend a really good book on understanding the soil?

    Bookmark   August 24, 2008 at 11:00AM
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paulns(NS zone 6a)

In some parts of the world things are done differently. Our local shellfish - crab and lobster - are overexploited, but that doesn't make using the unprocessed waste from the fish plant for compost a bad thing. For years, tons of this waste - the shells and bodies - have been trucked to faraway landfill sites and buried. The plant has been happy to have us divert as much as we can to our compost bins, and last year the local landfill site also, at last, started composting shellfish waste with wood chips.

But it takes a long time to cure, at that. A fellow a couple of hours from here used to sell compost made from sawdust and fish waste. When the fish was composted the sawdust still had a long ways to go, and I talked with farmers who used it and had bad experiences with nitrogen tie-up.

    Bookmark   August 24, 2008 at 11:40AM
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spiced_ham(z5 OH)

If you are in the tropics you might want to do a google search on the research done on Ramial Chipped Wood, especially the study on tomatoes in Senegal [there was also a study done in NY in the 1960s on growing vegetables with wood chip ammendment that you can find described on the web]. RCW technology is the use of chipped wood from branches (hardwood trees only, no conifers), which have higher nutrients and lower C:N than trunk chips or lumber sawdust. It can be applied directly to the soil without composting [about 2 inches mixed into the top 4-5 inches of soil every three years]. Generally there is little or no nitrogen robbing the first year, and benefits are seen if not the first year, definitely the second through the tenth year. The uncomposted RWC feed the soil microbial community and nutrient cycles, unlike finished fine grain compost, which acts like a slow release fertilizer.

How long for a nitrogen ammended pile before it is ready? I had some wood chipped in December pre snow, and used it to make a brand new garden out of poor clay soil in the spring- not hoping for much, but with the attitude that you can't make poor soil any worse- I did use 10-10-10 fertilizer as well though. Tomatoes grew exceptionally well but there was some nitrogen deficiency in the bell peppers and onions. Not bad for plants trying to grow in a rough mix of barely aged wood chips and clay clods. I also had brush chipped at the beginning of the summer, and now (3 mos) it looks like it could be used (chips are brown inside and can be broken with my fingers, but it is still hot [hot agian] because I just mixed in the third batch of nitrogen 7-10 days ago. It should be perfect for next spring, big chunks and all. Sawdust has a larger surface area so it should compost faster than chipped wood, nitrogen availability would be the limiting factor to the speed. You would also have to worry about aeration and compaction. I would say that if the sawdust is broken down enough to be crumbly, and looks like compost, it is ready to be used as compost.

    Bookmark   August 25, 2008 at 9:24AM
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I've spent the morning starting to build three more compounds for composting, and I've managed to source a bit of spoiled hay so as soon as possible I shall start experimenting. One is going to be layered with hay, sawdust and donkey poop with added urine, and one is going to be sawdust and added nitrogen. I need to reserve one for the next humanure heap. When the nitrogen one looks ready, I'm going to inoculate it with some aerated compost tea to try to get the microbe level more like 'normal' compost and maybe figure out a way to compare the numbers of critters in it with the hay/sawdust/donkey poop/urine pile. Might have to dig the microscope out... I'll take photos as I go along and let you know how I get on.

I wonder how many compounds my other half will be willing to let me build... ;-)

    Bookmark   August 26, 2008 at 9:16AM
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spiced_ham(z5 OH)

The best innoculant for sawdust and wood chips is forest floor litter and surface soil. It contains the microbial community needed for digesting wood (white rot fungi). You won't find those microbes in herb and grass based compost, or in agricultural soil.

    Bookmark   August 26, 2008 at 6:25PM
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Good idea! I'll try that when I start the heap off, and maybe use the compost tea at the end to introduce microbes more normally present in the soil.

I also found this 'recipe'...

1 cubic meter = 35.2 cubic feet - sawdust.
Urea = 1 kg.
Triple phosphate = 1 kg.
Potassium sulfate = 1 kg.
CuSO4 = 200 grams.
ZnSO4 = 250 grams.
FeSO4 = 250 grams.
MnSO4 = 150 grams.
Boron = 150 grams.

How does that sound? Should I worry about adding all that stuff?

I also found the following (I saved the page and now can't find the link...)

Consider using sawdust when it is easily and cheaply obtainable. Prepared properly and sawdust is a great organic waste for the garden. Sawdust is low in nutrients such as nitrogen when compared against products like straw or lucerne. This factor detracts many gardeners from using sawdust. In their raw state, hardwood and softwood contain poisonous substances, which may be harmful to your plants, but they can be easily leached.

Hardwood sawdust, the coarser of the two, stays longer in the soil before decomposing. Both aid in making the soil more friable for increased water and air holding capacity. The material also holds nutrients well in the garden and potting mixes.

Composting Sawdust

Sawdust, while void of weed seeds, pests, and diseases, in its raw state contains the poison phenol. When used in the raw state, nitrogen is drawn from the plants causing a nitrogen deficiency. Bacteria need increased nitrogen as they digest the high carbon content of the sawdust. Instead of the nitrogen going to the plant, it goes to the bacteria. Prevent this by adding more nitrogen.

Simply remove the phenol by positioning the sawdust heap in a location where runoff can leach away. Place on a slope or a mound of soil. Water it well and turn weekly for a month and then leave until you require it in the garden. The longer the heap is left, the better the product. The leaching will be dark and staining on timber and concrete. A potting mix containing wood materials will leach away the stained water after first potted. Consider this when placing newly potted plants on balconies.

How-to make sawdust safe by composting. Consider the following three methods, adapted from Queensland Gardener, May 1986:

  1. Use sawdust in the garden compost pile in the ratio of 5 parts sawdust, 1 part fowl manure, 3 parts grass clippings and weeds, 1 part leaves, 1 part cow or other manure and add blood and bone in the pile building process. Construct the pile in the manner mentioned in my compost article.

  2. Compost by adding a cubic metre of sawdust to 1/3 to 1/2 of chicken litter or 1/3 fowl manure. The finished product looks similar in texture to the original sawdust and suitable for gardens.

  3. Dilute 2.5 kg urea in water and spray onto or mix in about 150 to 170 litres of fowl manure for every cubic metre of sawdust. (A 2 gallon buckets holds about 10 litres.) Water the pile well and turn it a few times over the initial two weeks. Following this, the toxic phenols should be removed. There will be little nutrition in the composted sawdust, so add manure or natural fertiliser in the usual manner.

    Bookmark   August 27, 2008 at 4:01AM
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spiced_ham(z5 OH)

Your first recipe looks like a way to make nutrient rich soil-less potting mix. It also looks like a basic recipe for 10:10:10 fertilizer, maybe even lower N than that [I didn't do the calculations). Not what you want. There is no where near enough nitrogen to deal with composting the carbon quickly, cut down on the phoshorus and potassium by at least half, double the nitrogen and forget the trace elements, they are already in the soil and manure based ammendments, and generally in high enough levels in "dirty" non lab grade chemicals for plants.

The white stuff in bird droppings is uric acid-solid nitrogen source. Reptiles and birds do not produce liquid urine as an adaptation to conserve body water. Urea = liquid, uric acid= solid.

I don't know where they got the Phenol idea, Australia?. I would worry about it more with soft wood, conifer sawdust, which is chemically different lignin than hardwood, also maybe a big problem from Eucalyptus, which produce nasty alelopathic chemicals to prevent other plants from growing around them. Lignin and tannins are polyphenolic chemicals so composting will break down or bind single phenols as well. The white rot fungi are even used to break down nasty phenolic synthetic chemicals such as PCPs.

    Bookmark   August 27, 2008 at 9:31AM
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I've been playing with a few figures and think we may have been overly pessimistic about the pee option!

I've been collecting about 5 litres/kilos of urine per day from the family, and my compost book gives a figure of 0.6% nitrogen by weight, making 30g nitrogen per day.

A ten pound bag of fertiliser, or lets say 5kg of ammonium sulphate (which wikki says contains 21% nitrogen), will supply 1050g nitrogen.

So at that rate, it will take 35 days (35 x 30g = 1050g) for a family of four to supply the same nitrogen to a heap of sawdust as a 10lb bag of high nitrogen fertiliser would.

I think my maths is ok, but can't vouch for the figures...

    Bookmark   August 27, 2008 at 4:26PM
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spiced_ham(z5 OH)

Depends on how much meat/protein is in your diet and if you are growing or not (children vs adults). Vegetarian children might be a bad choice for fertilizer production. LOL

35 days isn't bad considering it would take about that long to incorporate the fertilizer within three turnings, but I would think that it wouldn't allow the pile to stay as hot, but I may be wrong about that. In any case a combined unine + some nitrogen sounds like a winner.

    Bookmark   August 28, 2008 at 8:20AM
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Well there's only one way to find out!

As soon as the compounds are finished and we get the sawdust in, I'll get two new heaps started - one nitro, one pee and donkey poop, and monitor what happens with the assistance of my trustee thermometer and we'll see what happens.

Watch this space! Well, maybe have a look in a few weeks...

    Bookmark   August 28, 2008 at 9:58AM
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I've been playing with numbers again, and discovered that if you collect four people's pee for 35 days, it will give you, on average, the same nitrogen as:

5 kg sulphate of ammonia (about 11lb)
7 kg blood meal
13 kg fish meal
15 kg cottonseed meal
45 kg alfalfa/lucerne meal
52 kg coffee grounds
100 kg olive pommace
240 kg fresh horse manure

And, most important for me, one day's pee from the family will treat one big black tub (35 litres or 7 gallons) of sawdust. Which means I can now forget the figures and just chuck one tub of sawdust on the heap for every pee-pot I empty!!

    Bookmark   August 30, 2008 at 11:55AM
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