Everything you ever wanted to know about Compost

Compost is THE most asked about topic on local gardening listservs, so we've compiled what we think is the very best thinking on the subject.  We're also putting it on our website for reference any time, so help us keep it accurate and up to date.  Just leave a comment below.

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Organic gardening experts are constantly telling us to improve our soils with organic matter, but what kind?  Fully decayed organic matter - compost - is, in the words of *one of my favorite gardening books, "the creme de la creme, the piece de resistance, the best in show, the big rock candy mountain of organic matter." Not convinced yet?

Why it's SO Great  

• Compost improves soil structure, no matter what kind of soil it is.  Got clay? It'll loosen it, letting water drawn from it and oxygen get down to the roots.  And in sandy soils, the ability to hold moisture is increased by compost.   
• Compost contains nutrients.
• Compost also feeds micro-organisms, thus increasing  plants' abiltiy to   USE the nutrients.
• Compost attracts earthworms, which further enrich the soil and improve its   structure.      
• Using homemade compost reduces the need for products you have to buy, especially the synthetic stuff. 
• Unlike those fast-acting synthetics that end up polluting our waters, compost releases its nutrients slowly.

Buying it vs. Making It Yourself    

Of course you can buy Leafgro, an excellent leaf-based product available everywhere, or one of the generic bags of compost available in stores.  But why not make it yourself? 

• It's free.
• It requires no trips to the store.
• It avoids adding all that nutritious waste to the local landfill.  (I've seen estimates that 10 to 30 percent of our landfills are filled with yard waste!)

What to Compost - and Not To    

• Leaves (chopped for faster results) and other green yard waste.
• Kitchen scraps, except for animal parts, dairy or oil, which can attract rodents.
• Shredded newspaper and cardboard.    
• Don't use possible contaminants like pet waste, diseased   plant parts, or anything sprayed with pesticides or herbicides.   
• To avoid having your plants seed all over the garden, remove the seedheads before composting the rest of the plant.
• Don't use woody plant matter over 1/2 inch thick without   chopping it up first.
• The more diversity of composting materials, the better.  So get creative.

Composting by Piling it On    

• The cheapest technique (actually free) is to simply put organic materials in a pile, turn and water them occasionally to keep them aerated and moist, and wait while the invisible microorganisms do their job and compost happens.  The only drawback I know of to this lazy gardener's method is that the heat generated is ususally not enough to kill weed seeds.   

To speed the process, chop or shred the contents first running over them with a bagged lawnmower or putting them through a chipper-shredder of some kind.  I once tried a cheap shredder that used a Weedwacker-type plastic filiment to shred my leaves but found that every tiny stick and acorn broke the filiment, so the process was extremely slow-going.  Serious chipper-shredders that work quickly cost over $1,000. 

• Also cheap and even easier is what I do with the huge amount of fallen leaves on my property.  I dump them in a pile and do nothing but add my green garden waste to the pile.  Then by fall the pile has settled and I use my handy pitch fork to pile it higher, making room for the next batch of fallen leaves.  Now this works but because the ingredients aren't chopped or turned and watered regularly, the result is an uneven product - not uniform in size and consistency - but it still makes a fine soil amendment.  I even use it as a mulch (on top of the soil) in the woody areas of my garden, where its imperfect, natural appearance looks just right. 

Composting with  Homemade Containers  

• If you don't have huge quantities of leaves to compost, just put them in black plastic garbage bags to slowly break down.  Spray enough water in the bag to moisten the leaves, then punch holes in the sides, tie the top, and check back in 12 to 18 months. 

• Another cheap and easy container is a plastic or metal trash can with the bottom cut out and holes punched in the sides and lid for air circulation.   To ensure good drainage, place it on top of wood chips or bricks. 

• Create a homemade container using concrete blocks, bricks, or wood.  It should be 3-sided, a 3 to 5-feet square, and no more than 5 feet high.   Also, wooden pallets can be nailed or wired together at the corners to create a compost container.   A round container can be easily constructed using wire fencing.  Whichever method, it's best to have two or three side-by-side containers so you can move the contents from one to the other to aerate.

• Even people who have very limited space could 4-foot-tall wire fencing, to any size they like and fill it with leaves. Whether the compost has completely decomposed or not, I dig it in into my garden soil in the spring.   

One commenter to this article told me plants seedlings right in the top of the leaf bin.  He makes a slight depression and adds a quart of so of soil, then plants vegetable seedlings. "The thing to remember is that these leaf beds do need to be kept watered. Then at the end of the season, after the harvest, Voila! You have more nearly finished compost!"  And "Leaf compost is amazing." See?

Containers to Buy 

• Buy a plastic compost bin, either stationery or made to be tumbled using a lever.  The tumbling type is meant to be filled all at once, so you need some space to save the materails until they're ready to be added, then turned.   And remember that shredding first makes it faster.

Hot vs. Cold Composting 

Unlike the cold composting methods described above, which often takes a year or more, some gardeners prefer speeding up the process in a method called hot composting.   It produces results much faster - as fast as three months is possible! - and kills weed seeds, too.  Temperatures inside the pile can be as high as 160 F. 

On the negative side of the equation, hot composting emits into the atmosphere carbon dioxide, a primary greenhouse gas, so that may be a factor in your choice of techniques.  Also, a great deal of nitrogen is lost during the curing process in hot composting, as much as two-thirds as the amount produced using cold techniques.  

How to Maximize Results from Cold Composting 

Experienced cold composters often aim for the perfect combination of 8 parts brown mateirals, 2 parts green, and 1 part soil.  The finished compost from such a cold pile will release nutrients over a 3-month to 5,000-year period and is preferred for long-term fertility of the soil.  (Compost from the hot process might better be used on one-season plants.) 

How to do Hot Composting 

• Use approximately 3 parts brown (carbon-containing) materials to 1 part green (nitrogen-containing).  Brown ingredients are dry leaves, newspaper, and straw.  Green are yard waste like grass clippings, and most kitchen scraps.  If there's not enough green, the decomposition takes longer.  If there's too much green, your compost bin will smell bad.  Layer the ingredients brown, green, brown, and so on.
• Hot piles are best created all at once, not gradually.  Place materials in 1-2-inch layers or green or brown, watering between layers.
• Add to the mix a little soil, by itself or with sod attached, or manure.
• Chop or shred the ingredients first.
• Turn the ingredients monthly by moving them to a   different bin or pile.  This ensures good aeration.
• Keep the contents moist by hosing them down during the turning process.  Compost with the right consistency feels like a wrung-out sponge.
• A  compost thermometer  tells you what's going on and is a big help.

What to do with it 

When your compost product is finally crumbly and black or dark brown, it's time to use it in the garden. Possible uses include: 

• For new planting areas spread 1-2 inches on the surface, then mix it into   the soil before planting.
• For vegetables, work a 1-inch layer into the soil at the beginning of the season, and follow by adding a bit more (no more than a half-inch layer) before each new planting throughout the season.   

• Apply 1-2 inches on top of the soil and just leave it there to work itself into the soil gradually (letting earthworms do most of the work).   To make the garden look more finished and to help prevent weeds, apply a thin layer of actual mulch on top.  (Wind-borne weed   seeds will germinate when they land on compost but not on mulches.)
• Add to the planting holes of new plants, up to one-third, and mix well   with the garden soil.
• Apply 2-3 inches over the root system of troubled plants

How much do you need? 

To cover an area with anything, including compost, here's the formula:   

Take the square footage of area to be covered by the desired thickness of the application, then divide by 324 to yield the amount in cubic yards. For example, a 200-square-foot area covered with 2 inches is 200 x 2 = 400 divided by 324 = 1.23 cubic yards. 

From the Experts

• Here's Part 1 of a debate by two experts over compost tea.  Here's Part 2, the counterargument, and here's   the rebuttal and lots of great comments.

More Good Info in Print 

• Brooklyn Botanic Garden's Easy Compost      

• Let it Rot! by Campbell
• Mike McGrath's Book of Compost
• The Rodale Book of Composting
• *Burpee's Flower Gardening by Ellis and Cutler

Posted by Susan Harris

Comments

[Matt]

Great post, I know I'm posting this a little late but I was searching for some basic info on composting and this answered exactly what I was looking for.

[Jon Herriott]

1) I believe that plastic bags can work to compost leaves but not if left alone for long periods. Water is used up in the decomposition process and must be replaced periodically or the process grinds to a halt.
2) I was so impressed by the clarity of Ed Bruske's videos at Monkee see, that I wanted to add some comments on "WHY?" to go with the "HOW?" revealed in the videos. It now appears that this is a better forum to discuss theories about "WHY?" than that site. Anybody disagree?
3) I am a retired faculty member from the Dept. of Biochemistry,
University of Washington in Seattle and believe that I have some insights into the physics and chemistry ( but not microbiology) of
the composting process but am a newbie to blogging. Any tips on how to make this a useful discussion would be appreciated.

[Ed Bruske]

Regarding tumblers, I think there is definitely a prejudice against them among people who prefer to deal in large volumes of compost. Myself, I've made fine compost using the tumbler at my daughter's charter school. Tumblers are probably the best option for urbanites who want to avoid rats.

I also am skeptical of trying to kill weed seeds in home composting. Those high temperatures always drop off, then the pile has to be turned to get the outside material into the middle. It's a high-maintenance affair, and not what most gardeners are looking for, I'd say.

[Ed Bruske]

Susan, the info I provided on generation of carbon dioxide and loss of carbon in hot vs. cold composting comes from Jon Jeavons and his Ecology Action organization. Absent specific citations from other scientific research, I'm sticking with that. Likewise, the loss of carbon as carbon dioxide is not the same as shrinkage in the compost pile. All compost piles shrink as the material is broken down.

Most composting texts advise a 30:1 carbon to nitrogen ratio for a hot pile or 60:1 for a cold pile. But those are absolute carbon and nitrogen values, not the volumes of "brown" versus "green" materials used in the pile. This is the source of much confusion. I doubt most people have any idea what 30:1 means, but they surely know how to mix together equal volumes of "browns" and "greens," which results in a 30:1 carbon-to-nitrogen ratio.

[Paul Crumrine]

Hello Susan, Lot's of good info.
I've been a MontCo Recycling/Composting Volunteer for 13 years. You might mention for locals the "bins" the county sells for $5 at many community events and CoAgFair and at regional centers, I expect - rolled up sheets of heavy poly/plastic something with big air holes, about 3' high and 4' across when opened full and secured, Hold over a cubic yard.
I've never seen success with the tumblers - when they're full the tumbling doesn't mix things up to aerate which is the whole idea.
Carbon dioxide it released by the respiration of the microoganism doing the work - eating the carbon. (They use nitrogen for other life needs - amino acids, I believe. If it's not provided by greens, they get it from rain and the soil.)
So I'd say both methods release the same amout of CO2 to get the same amount of compost. Really "done" fine compost is only about 20-25% of the original mass by either method.
Tell people they don't need to add microbes with "Compost Powder" - there are plenty in the materials. A shovel full of soil will add millions.
Forget about killing weed seeds with home composting - can't sustain the high temps needed. The 160 F you mentioned is only of short duration. 13 years ago I achieved that in a large bin with last of year grass clippings added to leaves. I got a thermometer. It hit 170 F within 7-10 days but fell back in a week to 120 or so for a while (in Novembre). I stirred it up and he temp went up not as high, then down faster. It's all related to those microoganisms you mention - population booms under ideal conditions, then crashes - heat generated by those little bodies. Compost was ready by spring.
Haven't done it since. Cold method can be speeded by adding commercial (yech, I've done it because the microbes need it) nitrogen - even watering with liquid fert. with high N content. By the next Fall, stuff in the middle is ready - scrape away and add the undecomposed from the top to start the new pile. Any pile gets more water in it by having a depression on the top of the pile so some rain will flow to middle and not all down the sides.
My father and I layered in the '50's-60's. We added a thin layer of soil, too. I haven't seen it recommended in these 13 years. Total mixing is best - and 1:1 nitrogen to carbon is much more than necessary. 24:1 is ideal for microorg. metabolism. (The Nature and Properties of Soils, (Textbook), Ray Weil et al., UMD

[susan harris]

Thanks, Ed and others I heard from directly. The amended version is here: http://www.sustainable-gardening.com/Compost2.php
and will soon be on our site, too. Susan

[Ed Bruske]

My thoughts on this text:

I have a leaf shredder from Flowtron that has never failed me and cost less than $100. It's basically a weed wacker in a can and fits over a garbage can to catch the shredded leaves. I've never had a problem with the filament breaking--and you can always get heavier filaments at the hardware store that are almost guaranteed not to break.

It will take years for leaves alone to break down in a plastic bag. I just used the last of the bagged leaves I collected last fall. They'd been in the bags for more than a year and there was no sign of decomposition.

Hot composting not only emits lots of carbon dioxide into the atmostphere, but loses much of the carbon and organic matter in the process (not nitrogen).

A formula for hot compost is equal parts brown to green materials and a small portion of soil or previously made compost to introduce microorganisms to the pile.

I think I have read almost every text ever written on compost. The best I have seen is in John Jevons' "How To Grow More Vegetables..."