Active Composting vs. Passive Composting

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There’s a composting style for every personality; active composting is for those gardeners who want to produce consistent piles of high-quality black gold, while passive composting is for those gardeners who just want a place to toss grass clippings and hope for the best.

The main difference between active and passive composting is the time that it takes for organic matter to decompose. Active composting is faster but requires more work and monitoring. Passive composting is slower but needs little to no management.

In this article we will outline the key differences, benefits, and ingredients needed for successful active or passive composting.

The Basics of Active Composting

If a gardener is putting effort into building and maintaining a compost pile, they are encouraging active composting. Most books, tutorials, videos, and other instructables about how to compost are geared towards cultivating an active compost pile.

Ingredients

A compost pile is considered active if the microbial population is going through the cycle of growth, reproduction, and die-off each time the pile is turned. In order to maintain this cycle, you must start with the right carbon-to-nitrogen ratio.

Ideally, your compost pile will be 25 parts carbon to 1 part nitrogen by content. All compost ingredients will contain nitrogen and carbon, but green, fleshy material will be higher in nitrogen than woody, dead material.

Each ingredient is unique, so while dead leaves and wood chips are both considered to be high-carbon ingredients, they have very different ratios of carbon to nitrogen.

This is why the more common recommended C:N (carbon-to-nitrogen) ratio is 2:1. This number does not refer to actual carbon or nitrogen content, but rather refers to the volume of high-carbon ingredients to high-nitrogen ingredients.

In general, you can maintain an active compost pile by layering brown (carbon) ingredients twice as thick as green (nitrogen) ingredients, and end up with roughly 25:1 C:N by content.

Recommended Reading: Compost Ingredients: Lists, Ratios, & Cautions For Beginners

Temperature

An active compost pile will go through periods of high heat, followed by a tapering off as bacteria begin to die from lack of nitrogen (source). This cycle only happens in active compost piles.

A pile is active as long as the microbial population are provided the right environment to continue to break down plant material. However, if a compost pile dries out and decomposition slows, the pile transitions from active to passive.

You can encourage your pile to resume active decomposition by turning it with a pitchfork and wetting it down. If the internal temperature begins to rise, microbial activity resumes (source).

Compost is not harmed by switching from active to passive and back again, but the nutrient quality in the mature product may not be as high.

Moisture

Bacteria require water to move and break down plant material. If a pile dries out, bacteria are unable to move on to new sources of nitrogen and carbon, bringing decomposition to a halt.

If too much water is present, anaerobic bacteria take over, and begin breaking down plant material into less desirable and potentially toxic elements. The key to a healthy, active compost pile is a 50/50 balance between water and oxygen content.

Mature Compost

Active compost can be mature and ready to use within a few months, while passive compost piles may take years (source).

Mature compost from an active pile will be more nutrient-dense and have a lighter texture. Since it’s only exposed to the elements for a few months, most of the nutrients are retained in the compost. A passive pile, however, will be exposed to rain, wind, and other elements that may cause leaching.

Overall, an active compost pile will provide a more high-quality finished product.

The Basics of Passive Composting

Passive composting is essentially piling up organic material and letting nature take its course. In an arid, windy climate this may take years. In warm, humid climates it may only take a few months.

Passive composting does provide a way to recycle organic materials, and in some scenarios, it is a better option than active composting.

Ingredients

The ingredients for a passive compost pile are exactly the same as for an active compost pile. However, for gardeners who are less concerned with quick results, the C:N ratio is more flexible.

Generally speaking, passive composting is more like slow-motion composting, which happens in carbon-rich compost piles. Large piles of sticks, twigs, woodchips, dead leaves, and other brown materials will decompose, but will not generate the heat necessary to break down quickly.

These piles tend to be bulkier when finished, and are better for mulches and to improve clay soils.

Temperature

Temperature is a non-consideration for passive compost piles. The only reason to monitor temperatures is if you were trying to maintain an active compost pile.

Passive composting is slow, and the bacteria never reach high enough populations to generate high heat. Therefore, passive piles are generally the same temperature as their surroundings.

Decomposition begins at 50ᐤ and increases with rising temperatures, so the warmer your climate, the faster passive piles will decompose.

Moisture

Moisture is necessary for microbial activity. Passive piles are largely unattended, so moisture content will depend on the weather. Piles will decompose faster in humid climates, and slower in dry climates. Of course, this will also depend on the temperature.

Dense piles of smaller ingredients will retain more moisture than piles with large twigs and branches. If you live in a dry climate, chop your materials into smaller pieces before piling them up. This could be the difference between finished compost in a year or in a decade.

Mature Compost

Passive compost piles are usually very high in carbon, so the finished product will be bulky and may still have recognizable plant material. Adding this compost into garden soil may tie up nitrogen as the remaining chunks of plant matter continue to decay.

Plus, decomposed twigs and dead leaves are lower in nutrients than compost from an active, balanced compost pile.

However, bulky compost is perfect for mulch because it will take longer to break down, and its main purpose is to retain water, not provide nutrition.

This compost is also a good amendment for clay soils; it will help improve porosity, and since clay soils are usually very high in nutrients and resist nitrogen leaching, adding a bulky compost will improve texture without causing a nitrogen deficiency.

Pros & Cons

There are some clear-cut pros and cons for active and passive composting, but there’s also a huge grey area. Piles are active when they are managed consistently, and passive when they are left alone.

Many gardeners aim for active compost piles, but usually end up reviving passive piles over and over again because they let a few weeks go by between turning.

There’s nothing wrong with this method; it just takes longer. But it does make it more difficult to determine the benefits or drawbacks to either method.

Time

Active composting produces a finished product in a few months. Passive composting is totally dependent on the climate, and may take years before you have a usable product.

However, managing an active compost pile takes time. You need to turn a compost pile every 3-6 days in order to keep the decomposition process moving.

Preparing piles takes time; you need to shred or chip large materials, collect large amounts of green and brown ingredients, and find space to put each pile where it is easily accessible.

Piles can be ready to use in a few months, so you need to repeat this process 3-4 times per year in order to keep a compost pile going.

Passive compost piles are pretty low-maintenance. Just toss everything in a heap and check it when you’re bored.  You will have a less reliable source of compost, but you also won’t have to spend time maintaining or building piles.

Effort

Active piles are generally much more time and labor-consuming than passive piles. It takes time to find the right ratio of ingredients, monitor activity, and incorporate a consistent supply of compost into your soil.

However, it is easier to have a consistent supply of compost in your backyard instead of driving to the hardware store and spending $7 per bag on Black Gold.

Plus, you can put nearly half of your waste into a compost pile instead of a trash can, and you can recycle all of your yard waste.

Uses

Active and passive compost piles both produce a better soil amendment than any synthetic option.

Compost from an active pile is fluffier, more nutrient dense, and better suited for topdressing, seed starting, and incorporating into any garden soil.

Compost from a passive pile is bulkier, less nutrient dense, and better suited for mulch and as a soil conditioner for clay soils.

However, most compost will come as a result of both active and passive composting in the same pile. There is no such thing as using compost in a way that damages the soil.

While some compost may be better suited for some applications than others, using compost is always better than not using anything.