Tony O’Neill, gardener and author of the popular “Composting Masterclass” and “Your First Vegetable Garden,” combines lifelong passion and expert knowledge to simplify the art of gardening. His mission? Helping you cultivate a thriving garden. More on Tony O’Neill
It’s regrettable but true. We live in a world where more has been taken from nature than has been put back. But the tide is turning. Like you, more and more gardeners are making a concerted effort to return as much as possible to their garden – to feed it.
Trench composting is an easy and effective way to replenish the soil’s vitality using food scraps in the garden. When combined with fermenting processes, like Bokashi, trench composting offers maximum returns for minimum effort.
Even without pre-fermentation, trench composting is an effective way to increase your garden’s fertility, but it has some limitations. This post looks at the pros and cons of this anaerobic composting method.
Hot and Fast vs. Cold and Slow Compost
Cold and Slow Composting
Organic material buried in a furrow can take a year to stimulate a healthy soil food web. The diversity of microorganisms is what makes your soil more productive. Their waste is the plant’s food and vice versa; the plant’s carbon released via its roots is what the microorganisms thrive on in your garden soil.
Different composting techniques stimulate the proliferation of diverse microorganisms. Leaf mold, for instance, promotes the growth of fungi. In contrast, hot composting encourages thermophilic organisms (and many others).
Trench composting is a cold, anaerobic process performed by microorganisms that thrive without oxygen (or less than 6 percent).
With time, their presence attracts other local microorganisms to join the process. But there’s a way to speed the process up.
Making Slow Trench Composting Faster
There is a way to influence the available microorganisms to do the job faster. By pre-fermenting the food scraps and organic kitchen waste, we inoculate the leftovers with effective microorganisms (EM®).
These microbes are carefully selected to create a tipping point for other microorganisms to perform better, reducing the decomposition time from 52 weeks (a year) to two weeks.
EM® is used in Bokashi composting, but alternative combinations of lactic acid bacteria and yeast spores have a similar effect. If, for instance, you add fungi mycelium (fungi root), your trench compost process will accelerate significantly.
Hot and Fast Composting in Contrast
There’s the hot composting process on the other end of the spectrum. Also referred to as traditional composting, this process requires a mix of carbon- and nitrogen-rich materials. The carbon-rich materials are generally brown organic materials like wood, dried leaves, sawdust, and shredded paper. The nitrogen-rich materials are green plant matter.
A combination of about one part of nitrogen to 33 parts of carbon is an approximate mix. Adding water and air in a layered pile will activate the three phases of the decomposition as different microorganisms are activated and then enter a state of dormancy.
The composting process starts with mesophilic microorganisms helping the pile reach temperatures between 68- and 113 degrees Fahrenheit, breaking the organic matter down in the process.
As temperatures increase, the thermophilic organism could push the temperature as high as 145 degrees Fahrenheit. As this will destroy vital microorganisms, the pile needs to be cooled down.
This is done by “turning” it, a process of redistributing the layers in a pile to ensure equal exposure to active thermophilic microorganisms, moving what was in the center to the outside and vice versa. This second phase could take several days or weeks, depending on ambient temperatures, microorganisms’ activity, and several other factors. The only way of knowing the progress is by measuring temperatures.
How Does Composting Trench Work?
A compost trench is one of the older forms of composting in a garden. Early on, it was recognized that decomposing organic matter was a health risk, so it was buried in a pit dug in the garden. The plants around the holes where organic material was buried flourished, leading to doing it strategically.
First Variation – The Pit
The first form of strategic composting was the pit method, the lazy man’s alternative to trench composting – an abbreviated version. It remains a viable option in areas where you want to enrich the soil and where tree roots restrict digging a trench. Digging a pit under the tree’s drip zone is the best option.
The pros are that it’s straightforward and requires no turning or a need for carbon-rich browns (though adding some dry leaves will add value). The cons are that you must dig a new hole about a foot deep for every addition. This would be impractical in winter (unless you pre-dig in summer).
The Pit Evolved – The Trench
The trench is an expanded strategic solution to the pit. It offers the advantage of being able to add food daily. After adding the food scraps to the one- to two-foot-deep furrow, cover it to prevent rodents and other pests from having access to it.
Trench composting can take a while to decompose the buried waste. While some writers advise planting directly on the recently filled trench, I have found that this causes root rot and other pathogenic diseases. Remember, the only active microorganisms are those on the food and the microbes already in the soil.
Trench Composting Requires Time. But How Long?
Unless the food scraps were inoculated with external microorganisms, like fermenters in Bokashi, it takes a while for microorganisms’ activity to break the food down. In highly anaerobic soil, buried food can take years to break down like clay.
Suppose your soil is classified as loam (a balance of sand and silt and a lesser portion of clay). Your buried food waste will stimulate local microorganisms’ activity in that case. You could decompose your food scraps in six to seven months with increased microorganisms activity.
My advice is that you commit to an annual rotational plan:
Fill a trench where you want to establish a garden, ornamental garden, or productive garden.
- Dig a new trench one foot wide, about 18 inches deep and start filling it with kitchen scraps. You can add about 25% shredded dry leaves or leaf mold if you have any.
- Ensure that the added organic matter (food scraps) allows you to backfill the furrow will at least 8 inches of soil covering the food waste.
- The new furrow should be parallel to the one made the previous year.
- Leave about a foot between the two.
- Use the covered trench that was composted the year before as a path.
- Using minimum tillage, plant on top of last year’s path. Minimum tillage prevents you from destroying the newly established microorganisms’ ecosystem.
- The previous year’s trench is now your pathway
- Parallel to that, start your year three composting trench.
Having parallel trenches throughout your garden allows you to cultivate healthy soil biota systematically. The result will be a healthy, productive garden that, using kitchen scraps, produces fresh food for the same kitchen.
Trench Composting How Long
A friend once told me that a German expression translates as “A horse never jumps higher than it has to.” The meaning I took from that is that overexerting yourself is of no gain.
What I do with my trenches is I dig them a yard or two long at a time. That way, I limit the damage I may cause to an established soil food web. Removing the soil from the trench disrupts the ecosystem that I need for breaking the food down to compost.
Suppose I limit the time of that disruption. In that case, I can use the residential microorganisms to complement the ones already involved in causing the food to decay. I don’t want the food to rot. I like the leftovers as food for beneficial microorganisms – microbes that enrich our soil.
I realize it’s called composting (as it relates to decomposition), but you’re farming with microorganisms. Compost is a visible byproduct of microorganism farming. It is host to billions of beneficial microbes essential to soil well-being a healthy, productive plants.
These microorganisms are also responsible for plant resilience to disease and even pests.
What Can You Add To The Soil When Trench Composting?
Trench composting is a product of the ancient practice of using pits to dump organic matter that could rot, attract rats and flies, and cause diseases. These trenches were often burial grounds for animals that died of conditions (that couldn’t be eaten) and other organic waste.
Often the process included both incineration and burial. The pits were also used to dispose of litter, though our ancient forefathers were less messy in some ways—their main portion of trash was ash and food. Resources were scarce, so hides, horns, and hooves were used as shelter, tools, and other purposes.
I make this point because trench composting effectively uses all food scraps to benefit the soil. That said, here are some exclusions:
- If you’re burying bones, two feet deep is the minimum to hide the smell from scavenger animals. I bury them in a hole even deeper than two feet.
- Exclude yard plants that seem diseased. A good motto is – if in doubt, leave it out.
- Exclude anything that has been treated with insecticide. Food waste that is added should not have been treated with an antibacterial spray or solvent.
- Do not add weeds that have gone to seed. Why make trouble for yourself? Weeds are defined as plants that spend most of their energy reproducing. Their contribution to soil carbon is only 20 percent compared to trees which contribute 80 percent, and vegetables, which contribute 60 percent. The microorganisms need carbon.
- Exclude pet waste that may contain diseases.
- Exclude adding cut flowers that come from a florist. These are usually treated with a fungicide that will kill the fungi, an essential component of your decomposition process.
How Do You Dig A Compost Trench?
The average spade is six to eight inches wide and nine to eleven inches long. It would help if you had a trench about 18 inches deep and 12 inches wide – nearly twice as deep as your spade blade length and a bit wider than a spade.
Use a space that does not have a lot of foot traffic but is also close enough to monitor any rodent or raccoon activity. Rats and other scavengers won’t be a problem if you bury your plant or food scraps sufficiently deep to avoid the smell attracting them to the trench composting zone.
Vegetable Garden Trench Composting
If you use your established vegetable garden, the area surrounding the food-scrap-filled trench will show benefits within the first year. This happens as the local microorganisms get activated by the increased food availability.
Trench Composting Around Shrubs
Your shrubs can also benefit from trench composting. Test the soil around the scrub to establish where the root system is at its extremity.
Dig a trench around the shrub about a foot from the end of the root system, avoiding damaging the roots.
With shrubs, I would dig a trench around the scrub to allow water accumulation and the recovery of any damaged roots.
As you fill the trench with food scraps, water the channel before adding the scraps. This promotes the mobility of surrounding microorganisms and speeds the decomposition process up, making them available for the plants in your garden.
Trench Composting Around Trees
When using trench compost around trees, don’t damage the root system. Generally, I prefer to make a selection of pits around the tree. Here I also regularly water the holes before adding scraps to the pit.
Pits can be dug using an auger, but you must be sure you’re at least at the tree’s drip line. The drip line is on the verge of the tree’s rain shadow, where rain caught by the leaves primarily lands.
If it is safe to do so, dig all the holes beforehand. If, however, you have pets or children that could hurt themselves running in the area of the holes, delay making them until they’re needed or cordon the area off.
How Deep to Dig When You Bury Scraps?
Depending on the content of what you add greens to the trench, depths can vary from a foot deep to as much as 30 inches.
Bokashi Trench Composting
If you add fermented kitchen scraps to your trench, a product of EM® and Bokashi, then trench composting requires a different approach. Here are the main differences:
- Bokashi composting doesn’t attract pests are the smell is unappealing to them. This means you can get away with only burying your fermented scaps in a 12-inch deep trench.
- With fermented kitchen scraps, you mix the fermented mass with some soil before burying it. Mixing your food waste with soil is generally not required in trench composting.
- The most significant difference is that you can plant on the trench within a month, after two weeks.
- Remember to mix a portion of the backfill soil with the Bokashi fermented food waste AND then cover that mix with unmixed dirt to further limit rodent curiosity.
Kitchen Scraps without Meat, Fat, or Bones in Trench Composting
Suppose your kitchen separates the scraps from the holding bin to exclude meat-related products. In that case, you can also get away with having a 12-inch deep hole – 4 inches scraps and 8-inch backfill soil previously removed.
Remember to add water to the trench before adding the waste to speed the process up. After adding the backfill dirt, some compaction will help limit oxygen getting to the process.
Kitchen Scraps with Meat Products
Suppose your kitchen scraps include meat, small bones, fat, or other meat-related products. In that case, you must bury your organic material in a hole at least 18 inches deep – with 12 inches of backfill dirt on top of the kitchen scraps.
Kitchen Scraps with Bigger Bones included
If you regularly dispose of larger bones (for instance, from roasts), you must dig a 24-inch trench. Adding leaf mold will accelerate the decomposition process. Leaf mold has a dense fungi population that decomposes more rigid materials like cellulose, lignin, and bones.
With any meat-related scraps, it’s important not to mix in any fat, gravies, or juices on the soil used to cover the organic matter. The whole idea of burying it deep is to hide any trace from rodents, raccoons, or even bears. Their survival depends on their olfactory glands (sense of smell), so they only need a hint to uncover your buried treasure.
Does Trench Composting Attract Rats?
If you follow my advice above, you should not have any problems with rodents, stray cats, or other scavengers. The essential factor is to reduce air to and air from the buried scraps.
Air to the organic matter is because we rely on anaerobic microorganisms to do their work – and they die when oxygen gets to them. Air from the scraps carries smells.
Just a reminder – both depth and side cover is essential. Make sure that you bury the food and cover the side of the food sufficiently. What I do is accumulate waste in an airtight container in my fridge. Once the container is complete – about a gallon – I add it to my trench to maximize the continuation of organic material.
Follow these rules, and you will avoid attracting pests into your garden and protect your valuable compost piles.
Check out my tips on keeping rats out of your compost bins below
Pros Cons / Advantages Disadvantages Of Composting Using a Trench
Below are the pros and cons of trench compost. I aim to provide you with all the information so you can make a more informed decision if this is the right process.
Trench Composting Advantages
The greatest attraction of trench composting is the ability to incorporate waste into your soil – with almost no hassle. It’s an easy process and a perfect solution for gardeners with small spaces.
Traditional composting requires a mix of brown and green material – trench compost does not require space. Static piles need space to be kept – trench compost is a case of bury in a hole and forget.
Trench composting generally reaches more profound than other forms of composting. This helps improve soil from the bottom up.
Trench Composting Disadvantages
There are two downsides – probably a reflection on my personality rather than on the process. Firstly, the process is lengthy. I’m not the best at delayed gratitude for gardening, so I use the Bokashi fermentation to speed the process up. The Bokashi fermentation compost reduces the composting time twenty-six times from 52 weeks to a mere two weeks.
Secondly, it requires digging trenches. I don’t mind digging in my garden in wetter seasons, but in winter, it’s horrid.
Trench Composting In Winter
While digging trenches in the winter is a form of torture, suppose you dig your trenches in warmer weather and bury your scraps over winter. In that case, the process will not be negatively affected. It will merely freeze until warmer weather.
Trench compost benefits that other composting methods seldom achieve – an effective solution to compaction. Trench composting occurs at depths not generally reached through other compost methods – at depths where mature plant roots go. This means that at the height of a plant’s fruit production phase, it has access to fresh nutrients.
It is also a very effective way to eliminate food waste in your garden that you generally would not add to a compost bin – like meats, fish, and fats. Requiring limited space, trench compost is in a gardener’s composting regime.
I sincerely hope you got a fresh appreciation of trench composting in this article as we delved deeper into composting topics (sorry, I couldn’t resist). If you did find it informative and would like to be updated on other events and offers, please add your email address to the form below—Happy gardening, folks.