4 Bay Compact Composting

You don’t need a huge open pile to break down organic materials from your garden. Try this small but mighty four-bay system for homemade “black gold.”

article image
by Matt Rees-Warren
A small, modular composting system may be just what your small property needs.

You don’t need a huge open pile to break down organic materials from your garden. Try this small but mighty DIY compost bin system for homemade “black gold.”

In a woodland, the forest floor absorbs everything that dies and turns it into soil, which then gives sustenance to what is living. Composting is about emulating that process, speeding up the lengthy cycles of decay and renewal from many decades to mere weeks or months.

Compost is the heart and soul of the garden, and its proper management is a core principle of gardening organically and in harmony with nature. It’s also deviously complex for its outwardly simple appearance, because we don’t understand everything about soil or compost. Scientists regularly learn more details about soil’s infinite variety and its microscopic marvels. Compost is a living, breathing, organic, natural machine that can feed the soil and sustain our world. We only need to follow the recipe.

Anything derived from an organic source can be composted, be that leather gloves, fish heads, toenails, or dead flies. The better you get at composting, the more you’ll be able to add to the compost heap. The materials you’ll most likely compost in your garden can be divided into one of two categories: those that supply carbon (known as “brown”) and those that supply nitrogen (“green”) to the pile. (See “Common Compost Ingredients,” below.) When building a pile, aim for a 50/50 ratio between the carbon and nitrogen ingredients.

How Compost Works

The two main composting methods are aerobic (with oxygen) and anaerobic (without oxygen). When done correctly, both lead to the same outcome of nutrient-rich compost for your garden. Think of aerobic composting as what happens on the forest floor, open to the elements of rain, air, heat, and microorganism and animal activity. Anaerobic composting corresponds to a bog or sediment layer, where decaying matter is trapped beneath the water or surface and starved of air and oxygen.

You might also sometimes hear compost referred to as “hot” or “cold.” Both refer to aerobic composting. Cold aerobic composting is passive, and occurs when you leave a pile of materials untouched, either in bins or on the ground, to mature into nutrient-rich compost. This process takes one to two years. Hot aerobic composting is an active process, in which you speed up the action by heating the pile through correct management. This can be achieved by turning and aerating the pile periodically, and by using specific ingredients in an appropriate ratio. My DIY Diamond compost bay design uses the hot aerobic method. Your job is to assist the microorganisms–mainly bacteria and fungi–that do the actual composting of your leaves, scraps, and clippings. Their primary requirements are water, oxygen, and heat. Water comes from nitrogen-rich ingredients and weather, while oxygen can be increased by adding carbon materials to the pile and by turning it.

Heat is a natural byproduct of the microorganisms’ own actions, and is generated in a two-phase process. First, the mesophilic (moderate-temperature) phase, which can last a couple of days, brings the temperature up to a threshold of about 111 degrees Fahrenheit, at which point the thermophilic (high-temperature) stage can begin, and the hot composting starts (between 113 and 149 degrees). This critical stage must be prolonged by turning or aerating the pile for up to two weeks to kill most pathogens and seeds. With no further intervention, the compost will cool down, and the microorganisms will revert back to the mesophilic stage to further mature the compost and decompose the matter. Your compost can be considered finished when the organic matter has turned to a consistently dark brown color. This can take as few as five to eight weeks using a hot aerobic composting system.

I’ve created many different compost piles and systems over the years. When managed correctly, they soon become the lifeblood of the garden.

Materials

  • 10-inch boards, 20 feet
  • 12 metal corner braces, 4-inch or larger

DIY Diamond Compost Bays

I devised this four-bay system as a small-scale alternative to the large open-bay method. The simplified design and compact piles make it easier to turn the materials and achieve the required temperature for hot composting. Its compact footprint–less than 3-1/2 feet tall by 30 inches square–makes this bin adaptable to the smallest of gardens. Plus, it’s easily taken apart and rebuilt, should you need to move it. For an economical project, I recommend you source recycled scaffolding planks from a reclamation yard or a scaffolding company, as I did in the pictured build. The following instructions are for 10-inch-wide dimensional lumber planks, but of course, you can scale up or down depending on the materials you have on hand.

From the lumber, mark and cut three 40-inch lengths, and six 19-1/4-inch lengths. Using the corner braces, fix two of the shorter boards to the center of each longer board at a 90-degree angle. You should end up with three assembled X-shaped sections. Place the first assembled section on level ground in your garden. Stack the next section directly on top, and repeat with the remaining section. Secure the stack with vertical stakes driven into the ground; fix the stakes to the boards with screws if needed.

Before you begin, save both nitrogen (green) and carbon (brown) elements. Chop up as much of the organic matter as you can–especially roots, branches, twigs, and other woody materials. Now you’re ready to start composting! Here’s how to make compost with your four-bay bin.

Select one bay to be your starting point; this will always be your first bay, where you add the freshest material. Fill the first bay with consecutive layers of brown and green organic material. After about a week, turn this pile into the adjoining bay, which will always be your second bay. Whether you move clockwise or counterclockwise around the bin is up to you, but be sure to stick to the direction you choose with all subsequent turnings. Use a pitchfork to turn the piles, breaking up any clumped material as you go.

After turning the material into the second bay, fill up the newly empty first bay with fresh organic materials (again, alternating layers of brown and green) until full.

After another week or so, turn the composting pile that’s in the second bay into its adjoining empty (third) bay. Turn the pile from the first bay into the second bay, and then fill up the first bay again with new material.

Finally, at least one week later, turn the existing piles into their adjoining bays and fill up the first bay with new brown and green layers. Every bay in the bin should now be full.

I recommend acquiring a compost temperature gauge to track the temperatures of the piles as they travel through the bays. Add moisture if needed. When the compost in the final bay has matured and is ready to use, remove it and begin the process all over again.

The gauge will help you ensure the pile or piles reach the desired temperature of between 113 and 149 degrees. This should be easily reached by balancing the carbon and nitrogen ingredients, and regular turning of the pile. Be careful not to let the pile get too hot — over 158 degrees — because this will kill beneficial microorganisms. If you’re going through drought, add nitrogen or greens to wet the pile, or soak it with greywater or harvested rainwater. If it’s raining hard and frequently, consider covering the pile; if the pile is soaking wet, add more carbon material. You can also add a panel roof if your pile is getting too wet.

Common Compost Ingredients

Carbon (Brown)

  • Dry leaves
  • Soil
  • Twigs
  • Cardboard
  • Newspaper
  • Wood ash

Nitrogen (Green)

  • Grass clippings
  • Soft plants
  • Vegetable scraps
  • Fruit peels
  • Pond weeds
  • Coffee grounds

Matt Rees-Warren is a gardener, designer, and writer. This is an excerpt from his book The Ecological Gardener: How to Create Beauty and Biodiversity from the Soil Up (reprinted with permission from Chelsea Green Publishing).