A good compost how to guide shares the science behind the process.

A simple compost how-to guide where science reigns supreme

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Millions of gardeners compost. They save their kitchen scraps, pile up their leaves, collect their grass clippings, and hoard their coffee grounds. Then, they put all this “stuff” into a pile or a bin, and they wait. They wait for the process of decomposition to turn it into “black gold.” Maybe they turn the pile from time to time. Or maybe they don’t, because they know that eventually, they’ll get compost. But, do all of those gardeners really know what they’re doing? Do they understand the science behind composting? Do YOU? Many gardeners are surprised to discover how excitingly complex composting really is. To help demystify the composting process, I’d like to present this compost how to guide based on the science behind creating the “black gold” all gardeners covet.

Understanding the basics of the nutrient cycles

Most of us learned about nutrient cycles in middle school. We learned how ecosystems naturally recycle nutrients through the processes of life and decay. Plants are major players in both the carbon and nitrogen cycles as they photosynthesize, grow, transpire, decompose, or become part of the food chain. In an undisturbed ecosystem, plants are self-feeding, so to speak. In a nutshell, carbon, nitrogen, and many other essential plant nutrients are released back into the soil upon a plant’s death (or upon the digested plant being excreted by whatever organism ate it). As plant matter decomposes, the nutrients it contains go on to nourish another generation of plants.

Composting creates a sort of semi-artificial nutrient cycle. Yes, the nutrients are eventually recycled back into the soil, but instead of allowing plant and animal waste to sit around and naturally decompose wherever it falls, composting makes all the decomposition take place in one spot. The “waste” is condensed into a small area to break down, and then, once it’s fully decomposed, it’s spread back onto the garden where it can help nourish further plant growth.

These basics of nutrient cycling are important to understand because in order to maximize the speed and quality of your homemade compost, both the carbon and nitrogen cycles play an important role. Let me explain.

compost how to guide with nutrient cycling

In a forest, nutrients are recycled via the processes of life and decay.

Compost how to guide: Begin by selecting the right materials

Any good compost how to guide will tell you that the first step in building a quality compost pile is choosing the proper ingredients. Different materials bring different things to the process of decomposition. There are two basic classes of ingredients constituting a proper compost blend: the carbon suppliers and the nitrogen suppliers.

  • Carbon suppliers are materials added to the compost pile in a non-living state. They’re usually brown in color and have low moisture content. Carbon suppliers are generally high in lignin and other slow-to-decompose plant components, so they take longer to fully break down. Carbon suppliers include fall leaves, straw, hay, shredded newspaper, small amounts of sawdust, chopped corn stalks and cobs, and shredded cardboard.
  • Nitrogen suppliers are those ingredients used in a fresh state. Nitrogen suppliers are often green in color (except in the case of manures) and have high moisture content. Because they contain many sugars and starches, they’re quick to decompose. Good nitrogen suppliers include untreated grass clippings, plant trimmings, farm animal manures (but not dog or cat waste), kitchen scraps, coffee grounds, rinsed seaweed, and other plant material.

    How to compost is easy with the right materials

    Properly made compost piles have the right ratio of ingredients.

The relative proportion of carbon suppliers to nitrogen suppliers is an incredibly important factor in determining both how well your compost pile breaks down and the quality of the finished compost. This ratio is called the C:N ratio, and it’s far more important than most gardeners realize. The target C:N ratio for a compost pile is 30:1 (meaning it contains thirty times more carbon than nitrogen). You can get this ideal ratio by building a compost pile that contains about three times more carbon-based ingredients than nitrogen-based ingredients by volume. So, for every five-gallon bucket of grass clippings you put in your pile or bin, you need to add three five-gallon buckets of straw or leaves to make the most of the science discussed in this compost how to guide.

Related post: 6 things every new vegetable gardener needs to know

Here’s why having the right C:N ratio in a compost pile is so important:  

  1. The microbes love it. First and foremost, the microbes and other organisms that process whatever ingredients you add to your compost pile, use these carbon materials as a source of energy, and they need A LOT of it to work efficiently and quickly (more on these composting microbes in the next section). If the ideal C:N ratio is created, the days to finished compost are reduced because these organisms will be working at the fastest pace possible. In addition, piles with a C:N ratio of 30:1 reach up to 160 degrees F, while those with a C:N ratio of 60:1 will seldom get above 110 degrees F. Decomposition occurs faster at the ideal temperature of 160 degrees F, and perhaps most importantly, more pathogens and weed seeds are killed, an important item that should always be mentioned in a compost how to guide.
  1. There won’t be any nitrogen “borrowing”. If your compost pile doesn’t have the right C:N ratio, the finished compost will not have it either, and this can lead to some pretty unfavorable situations. For example, if finished compost with a C:N ratio much above 45:1 is spread on the garden, the microbes will actually “borrow” nitrogen from the soil as they continue to break down the organic matter in the compost. Microbes need nitrogen too, and if it isn’t in the compost, they’ll take it from the surrounding soil which may negatively affect plant growth. On the other hand, if the C:N ratio is too low (below 20:1) the microbes use all the available carbon in the compost and release the extra, unused nitrogen into the atmosphere, depleting the finished compost of this essential nutrient.

    A compost how to guide should be based on science

    Whether you compost in a bin or a pile, microbes are hard at work breaking down the ingredients.

  2. You’ll get quicker – and better – compost. Finished compost used on certified organic farms must have a measured C:N ratio between 25:1 and 40:1, but home gardeners don’t necessarily need their C:N ratio to fall exactly within this range. However, if your compost does, you’ll discover the pile finishes faster and the resulting compost is of exceptional quality.
  1. You won’t need to “water” your compost pile. The appropriate C:N ratio also prevents the need for additional supplies of water. However, if your compost pile ever appears dry, don’t hesitate to add extra water. Your pile should consistently feel like a wrung out sponge.

This compost how to guide cannot stress enough the importance of having three times more carbon suppliers than nitrogen suppliers in your compost pile. But, to get the best compost, it’s also important to understand and encourage the microbes who are doing all that work on your behalf.

Meet the composting microbes

Once the right ingredients have been used to create your compost pile, it’s the job of billions of microbes and other soil-dwelling organisms to break it down into compost. The organisms needed for this decomposition process are already present in most compost ingredients. However, throwing some finished compost into your pile as it’s being built may increase populations more quickly.

There are literally thousands of different decomposers at work in the typical compost pile, and they number in the tens of billions. They all do their part, and they do it year-round. Some species of bacteria continue to work even in freezing temperatures. Fortunately, in a properly constructed compost pile, these bacteria usually generate enough heat to support other species of bacteria that prefer warmer temperatures. The most rapidly decomposing bacteria work between 100 and 160 degrees F. At 160 degrees F these rapid decomposers are happiest and the decomposition process is at its fastest. These microbes require very little from you. In fact, they only ask for two things: food and oxygen.

Related post: How to build a worm bin

Aerating your compost pile

The ingredients you add to your compost pile provide plenty of food for these microbes, but they also need oxygen. The decomposition of a compost pile is an aerobic process, meaning the microbes inhale oxygen and exhale carbon dioxide while decomposing. To maintain aerobic conditions, oxygen must be provided by turning or otherwise aerating the pile on a regular basis (ideally, at least once a week).

If the pile is not turned and no oxygen is present, your compost pile’s decomposition switches to fermentation. Different organisms are at work during fermentation, and they release both methane and ammonia, and as a result, your pile will stink. In addition, fermenting piles don’t generate enough heat to kill pathogens or weed seeds, creating more than one potential problem. Decomposition does not smell bad when adequate oxygen is present. A good, science-based compost how to guide will always tell you to turn your pile.

how to turn a compost pile

Turning your compost pile regularly is an essential step in supporting the decomposition process.

Good compost is hot… until it’s not

The process of decomposition naturally creates heat, so properly constructed compost piles are hot to the touch and should reach up to 160 degrees F. Sustaining this temperature for 10-15 days is enough to kill most human and plant pathogens, as well as most seeds. If you want to make sure your pile gets hot enough, invest in a good compost thermometer and check the temperature daily.

One sign that a compost pile is done “cooking” and the contents are ready to spread on the garden, is a drop in the pile’s temperature. Finished compost will not be hot.

The amount of time it takes for a compost pile to finish decomposing depends on several factors, including the particle size and C:N ratio of the ingredients, the moisture content of the pile, and how often the pile was aerated. You can get finished compost in as little as four weeks, if you pay attention to all of the factors discussed in this compost how to guide.

A word on pile-it-up-and-wait composting

Before you tell me that you’ve always gotten wonderful compost by simply dumping whatever ingredients you have in a pile somewhere, I should let you know that this pile-it-up-and-wait method is technically called “cold” or “slow” composting. Since all organic materials will eventually decompose, it’s a legitimate way to compost, and it’s part of many a compost how to guide. However, though the finished compost may be dark and crumbly, the C:N ratio is probably not ideal. And, extreme caution should be used when “cold” composting with animal manures as these piles don’t get hot enough to kill human pathogens, including E. coli, nor do they get hot enough to kill most plant pathogens and weed seeds.

Related post: Feeding your soil: 12 creative ways to use autumn leaves

We’d love to hear about your composting process. Tell us about it in the comment section below.

Pin it! Keep science on your side and make better compost faster, with this compost how to guide.

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22 Responses to A simple compost how-to guide where science reigns supreme

  1. Ron Mitchell says:

    When turning the compost, does it have to be done keeping the original layers of brown and green in layers?

    • Great question, Ron. No; you do not need to keep the original layers intact. In fact, when you turn the compost pile, everything gets mixed together which the microbes like even more than having all the materials in distinct layers.

  2. Dave Chapman says:

    OK I have my leaves, but does the 3x measure apply to shredded or unshredded leaves? I find shredding reduces the volume to 1/4 of the start volume.

    • Great question, Dave. The 3x rule still applies, no matter what state the leaves are in when they’re added to the pile. The size of the material definitely influences the speed of decomposition, but you still need to aim for the 30:1 C:N ratio. If you use whole leaves, try to squish out as much air space as possible when packing them into the buckets for measuring. Then, when you put them into the pile, you can spread them out a bit to give the microbes oxygen.

  3. Lottie E. says:

    What about during the winter? Does the compost pile need turning?

    • Another excellent question! Yes, you should still turn your compost in the winter. And, you should continue to add kitchen scraps and other items to the pile. If you keep the ratio of ingredients correct, as the pile decomposes, it generates enough heat to keep the microbes happy and working hard.

  4. Mike says:

    You missed sort of the idea of fermenting in a car.
    It’s faster.
    Toss green material in a garbage bag. Squeeze out all air, attach a tie wrap leave in car (your solar oven).

    Materials moistened throughly 30:1 ratio of carbon to nitrogen. Put in heavy duty garbage bag, squeeze out excess air, seal tightly with wire tie and put in a warm location. 🙂 use a car.

  5. lf you take some time to cut your materials into short pieces you will not need to turn it, and if it is kept covered it heats more quickly and rarely needs to be watered.

    • Even if the particle size of the materials is small, you still need to turn the pile to introduce oxygen to the center. Otherwise the decomposition process will switch to anaerobic.

  6. Karen Brown says:

    Our compost recently was hit with salt water from a high tide in Long island sound. Any problems we should address?

  7. Jake says:

    This is the most helpful article I’ve read on composting, thank you. I built a compost bin out of pallets over a year ago and it’s been very rewarding! My question is: do you keep separate piles or continue to mix fresh materials into the partially composted pile? Do you create some kind of assembly line of piles in different stages? When’s the best time to STOP adding new materials into a mix and let it finish so that it can be ready to use?

    • I fill a bin until it’s full and then stop adding new materials to it. I turn it regularly until it’s finished “cooking”, then I harvest the compost and start filling it again. I have two different bins so I always have one working pile I can add new materials to. Assembly line is a great way to describe it!

  8. Julie says:

    My question is if i’m using potting soil for my potted garden, should i be keeping the potting soil for next year and just adding a bag of compost to the soil each year or just throwing the soil away and buying more. I’m new at this and did reuse the potting soil from last year but mixed new potting soil into the existing soil. If i’m understanding correctly i should be adding compost to the soil, not new potting soil, so it gets nuitrients?

    • I always recommend filling containers with 50% new potting soil and 50% compost at the start of the growing season. I recycle the previous year’s potting soil by using it to pot up perennial divisions to share with friends or tossing it into my potato-growing bins with lots of aged manure. If you can’t afford to buy new potting soil each year, you can reuse it, but do be aware that some diseases will overwinter in the potting soil (especially tomato and vegetable diseases) and infect the next season’s plants. If you reuse it from year to year, definitely add lots of compost and a complete organic granular fertilizer prior to planting the pot.

  9. Linda Green says:

    How do you get “carbon” material during the summer? Everything going into the compost during the summer is green! Do you keep piles of dead leaves to add in summer?

    • I do keep some leaves in bags right next to the compost pile, but for me, a bale of straw is the best choice. I just have one sitting next to the compost bin and I toss some in whenever I add my greens.

  10. Miguel Ramirez says:

    When creating layers of browns and greens at the ideal ratio of 30:1, do the browns go on the bottom and greens on top? Or how do you build your layers?

    • The order of the layers doesn’t matter as they’ll get mixed up in the turning process every few weeks anyway. I just layer them as they’re available, trying my best to keep the proper C:N ration.

  11. David Chapman says:

    To add carbon, I shred all my boxboard (cereal boxes, etc.) into small pieces and throw them in the countertop green bin with kitchen scraps. Eventually this goes to the composter and gets covered with a layer of dry leaves, but I don’t need so much.

  12. Barry Roberts says:

    Hi i’m just starting this composting and i’m getting confused with peoples different versions, one says more Greens than Brown, then other people say more Brown than Green which one is the right one? and could you tell me how many of my containers to use for each please?
    I’m using a 10L container of veg scraps, peelings and coffee grounds, and a 10L of shredded paper, cardboard, newspaper, brown leaves.
    Thank You.
    Barry (ENG)

    • Hi Barry –
      We agree that there’s lots of mis-information out there about proper composting technique. However, the information found in this article is accurate and up-to-date with the current science on composting. In it, you’ll find all the answers to your questions. Happy composting!

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