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.
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Adding compost to your garden isn’t as much about the carbon as it’s about introducing microbes to create healthy soil that hosts resilient, productive plants.
The organisms responsible for breaking down organic materials get their energy from carbon and nitrogen materials. The former (carbon) they burn up as energy, and the nitrogen they use as a protein source for cell growth and multiplication, growing their numbers.
Table of Contents
How Composting Works
Depending on the availability of both carbon and nitrogen (in the right proportions), these microscopic life forms can burn through tons of organic matter.
Their survival requirement is not much different from humans’, needing carbs, proteins (nitrogen), water and oxygen. Some anaerobic microbes don’t need air, but that’s a story for a different article.
A significant difference between single-cell microbes and mammals is their reproduction cycles and times required. Single cells organisms divide, with a hundred becoming two hundred and two hundred becoming four hundred, etcetera.
Your compost pile can move from a few hundred microorganisms to trillions in a few days. All they need is air, water, carbon and nitrogen in specific ratios.
We’ll get into why they’re so crucial to your garden; first, we’re interested in getting these little bugs to work for us, breaking down our garden and kitchen waste into what some call black gold.
Aerobic Composting Recipe
Making this black gold requires green (nitrogen-rich) organic materials and brown (carbon-rich materials) organic materials.
As the aerobic in the title hints, the third essential element is air. The final ingredient is water, fueling chemical reactions, improving microbial mobility, and helping to manage temperatures.
The Story of Nitrogen-Rich Materials
The word organic refers to natural, carbon-containing matter. To a lesser or greater extent, all organic (carbon-containing) material also contains some nitrogen – even cardboard.
Nitrogen is the most ubiquitous gas in the atmosphere (78%), and keeping nitrogen in soil and plant matter is a continuous struggle. Like Steven Spielberg’s E.T., all nitrogen wants to go home, back into the atmosphere.
While a plant is alive, it produces sugars (carbohydrates) through photosynthesis. This energy produces amino acids (nitrogen), lipids (fats), and other chemicals during plant photorespiration.
Once the plant is no longer actively growing, it starts losing nitrogen. Take fresh lawn clippings from a recently mowed lawn as an example.
The fresh grass clippings are loaded with nitrogen, but over a few days, they will start turning brown, indicating that the volatile nitrogen has started dissipating. The result is an organic material with decreasing nitrogen availability.
Below is a table with a list of common compost materials and their respective carbon-to-nitrogen ratios in ascending order (increasing carbon ratios)
| Material | Average C: N Ratio |
|---|---|
| Fresh vegetable scraps | 10:1 to 20:1 |
| Hairy vetch cover crop | 11:1 |
| Grass clippings | 12:1 to 25:1 |
| Poultry litter | 13:1 to 18:1 |
| Young Alphalpha Hay | 13:1 |
| Hay | 15:1 to 30:1 |
| Cow manure | 17:1 |
| Legume Hay | 17:1 |
| Potatoes | 18:1 |
| Coffee grounds | 20:1 |
| Rotted Banyard Manure | 20:1 |
| Horse manure | 25:1 |
| Mature Alfalfa hay | 25:1 |
| Pea Straw | 29:1 |
| Fall Leaves | 30:1 to 80:1 |
| Corn Silage | 40:1 |
| Straw | 40:1 to 100:1 |
| Corn stalks | 60:1 |
| Oat Straw | 70:1 |
| Wheat Straw | 80:1 |
| Rye Straw | 82:1 |
| Bark | 100:1 to 130:1 |
| Wood chips and sawdust | 100:1 to 500:1 |
| Brush, Wood Chips | 100:1 to 500:1 |
| Shredded Office Paper | 150:1 to 200:1 |
| Newspaper | 400:1 to 800:1 |
| Cardboard | 500:1 |
Ideal Ratios of Compost Materials
Plants capture the sun’s energy and, using CO2 and water, produce carbohydrates with some water and oxygen to boot. The plant uses carbohydrates (sugars and starch) for growth, storing carbon.
When that plant dies, its stored carbon is decomposed by microorganisms, releasing CO2 and water. The microbes feed on carbon and need nitrogen for cellular growth (and reproduction).
They need much more carbon than nitrogen, ideally thirty times more. A mix of compost materials with 3.3% available nitrogen will spark a reproduction and feeding frenzy, releasing CO2 and water vapor and reducing the total weight of the initial organic material by about 70%.
Keeping the End in Mind
The compost you add to your garden must have enough available carbon to support continued biota activity but not so much that the microbes need to use all the available nitrogen to break it down, immobilizing plant nitrogen availability.
A garden bed with excessively carbon-rich materials will set microorganisms to action, and their demand for nitrogen will be prioritized at the cost of the plant’s health.
How The Carbon-to-Nitrogen Ratio Affects Aerobic Composting
All fresh organic matter like leaves, grass, yard debris, kitchen scraps, vegetable scraps, straw, hay, wood chips, sawdust, and paper has a higher carbon content and lower nitrogen content.
Fresher, greener materials have higher nitrogen content, and older, drier materials have a lower nitrogen content. You want to start with a ratio of 30 parts of carbon to one of nitrogen (30:1), or as close as possible.
Green materials (higher in nitrogen) include grass clippings, coffee grounds, and food scraps. Brown materials (carbon-rich materials) are drier and more brittle, including dry leaves, straws, wood chips, dry branches, and newspaper.
While we aim to get a C: N starting ratio of 30:1, we also have to manage our mix’s humidity and air levels. A good example, provided below, is composting with freshly mowed lawn clippings and fall leaves.
Four Basic Composting Factors
For ease of illustration, let’s use a simple composting mix of nitrogen-rich material (recently mowed lawn clippings) and carbon-rich material (dead leaves). Four factors increase your composting success probabilities:
Factor A – Air
Your composting batch, which should be at least a cubic yard (0.9 m3) big to work, must contain at least 6% air internally. Like in soil, air availability is a product of pore size and waterlogging.
To avoid anaerobic conditions, the moisture content of the material in your compost pile should be less than 60%. The interplay of particles of different sizes influences the pore size.
Aerobic microorganisms cannot survive without oxygen. Anaerobic microbes take over the compost pile if adequate oxygen is not provided, slowing down the composting process.
A tell-tale of anaerobic conditions is if your batch emits foul odors. It can be remedied by adding some dry materials like shredded dry leaves.
Regular pile turning improves pile aeration. Balancing moisture and airflow is essential.
Factor B – Water
As with all living creatures, microorganisms require water. A thin layer of water on the surface of the materials provides the microbes with a means to move and spread. Water also serves as a platform for chemical and biosynthesis activity.
We saw earlier that too much water in a batch would leave no space for oxygen, cutting off the air supply to the microorganisms. When aerobic microorganisms drown, anaerobic ones take over, releasing methane, sulfurs, and ammonia – all smelly stuff.
For composting, the ideal moisture content should balance microbe functionality with maintained aeration – between 40 and 65 percent. When the moisture content drops below 15%, all microorganisms hibernate.
Evaporation and precipitation change the compost pile’s moisture content during the process. Enough water and porosity must be maintained at all times during the process.
Moisture content decreases during composting, and additional water may be needed depending on the climate. Moisture content considerations are also influenced by the material used in the compost mix.
For example, moisture content can be higher in porous materials than in densely packed ones. Fresh-cut grass is excellent for upping nitrogen, but it is usually very moist, way above the maximum of 65%.
Moisture also serves as a cooling mechanism. In the process, the microbial activity heats the compost pile’s air and compost material. Compost piles may overheat and even ignite if the compost pile becomes too dry.
Factor C – Carbon-to-Nitrogen Ratio
Soil is ultimately a biological system. Regarding soil health, the goal should be to create a habitat ideal for soil microorganisms. The following is a good starting point for creating a microorganisms breeding colony.
Carbon
Carbon is the primary food source for aerobic decomposition microorganisms. Some available carbon provides energy, and some are combined with nitrogen for cellular growth (body maintenance).
The average microorganism’s cell mass is approximately 88% carbon, and humans, in comparison, are 23% carbon. Microorganisms have an average C: N ratio is 8:1, something they need to maintain to stay alive – even in the soil.
While you may start a compost pile with the standard 30 parts of carbon to every nitrogen part (30:1), ending with a mere 10:1 ratio is typical (and ideal). In the process, some carbon dioxide is released during respiration.
As a result, the carbon content of a compost pile is continuously decreasing (and the nitrogen percentage is increasing). Not all weight reduction is due to carbon loss; some are water vapor loss.
The starting ratio of 30 to 33:1 carbon to nitrogen is essential because:
- The lowers acceptable starting C-to-N ratio is 24:1. At that ratio, the microorganisms will consume all the carbon, 16 units for energy and eight units to maintain their cell structure. That means that the carbon available is too low to sustain microbes in the soil.
- In the suggested 30:1 ratio, the same 24 carbon units are consumed, leaving six available for other in-soil microbial activity.
- If your carbon content is too high, the process will be slow to start and take long as there are insufficient nitrogen-rich materials to boost microbe populations.
- Adding finished compost with a 10:1 ratio to your garden will result in a brief nitrogen deficit. This is why we always try to achieve a 30:1 C: N ratio starting ratio.
- A 30:1 carbon-to-nitrogen ratio allows resident microbes to readily decompose the organic matter, creating a finished compost pile that provides the soil with enough partially decomposed organic matter for continued activity without tying nitrogen up in the process.
Nitrogen
Nitrogen is the most abundant gas on earth and represents 78% of the atmosphere. Every breath you take is mainly nitrogen.
Sixteen percent of protein content is nitrogen in the form of amino acids. You may have heard me referring to adding a urine solution to leaf composting. Well, the average human’s urine is 11 to 18% nitrogen.
Cellular material, amino acids, and proteins are all made from nitrogen, continuously recycled by microorganisms. Nitrogen incorporated into microorganisms’ cells is released when the organism dies – a way of fixing this volatile compound in the soil.
An initial C: N ratio of 24:1 to 40:1 is recommended for rapid composting. The process is slowed if the C: N ratio is above the optimal range because of excess carbon.
A lack of nitrogen prevents microorganisms from consuming the carbon, so the C: N ratio must be reduced to a more appropriate level through several life cycles of organisms.
Your end product will be carbon deficient if you have too much nitrogen (and limited carbon) in your raw material. Ammonia (NH3) or ammonium (NH4+), both varying forms of nitrogen, will form if protein decomposition is interrupted due to a lack of carbon.
Toxic concentrations of gaseous ammonia or nitrogen leachate could contaminate nearby groundwater or surface waters.
Please note that composting is both a science and an art form. We can try and micromanage the whole process, but several factors influence speed composting.
Even with a C: N ratio of 30:1, there could be some delay as the bioavailability of carbon plays a part. For instance. carbon materials in hardwood with high lignin contents degrade slower than softwood carbon-rich materials.
Woody material bound by decay-resistant lignins is more difficult to decompose, whereas material containing simple sugars like fruit waste decomposes quickly. Keratin is the only nitrogen source that is resistant to decay.
Keratin is the fibrous protein tissue used in DNA testing – hair and nails. It is also found in the animal kingdom’s horns, feathers, and wool.
Factor D – Distribution
A compost pile’s efficiency is influenced by the material’s physical characteristics, mainly porosity, surface area, and compost pile structure.
Compost Air Porosity
The amount of air space in a compost mixture impacts airflow, which may be restricted when the pores in a material become water-clogged or if they collapse.
As the amount of oxygen available to microorganisms decreases, anaerobic activity takes over. Combining materials of different shapes and sizes improves porosity.
Larger particles help improve airflow but decompose slower because of their limited exposed surface area.
Surface Area
Two new surfaces are created every time an object is halved, increasing the surface area. Is decomposition, this means more surface area the microorganisms can act upon.
Combining big and small particles also reduces the substrate’s inclination to compact.
Structure
Structure refers to a particle’s ability to withstand compacting and settling during transportation. It’s essential for composting because it helps keep the material porous.
The composting process slows when the decomposing material settles and closes off air spaces in a pile. Less porous compost piles, like those made of grass clippings and leaves, lose structure more quickly than highly absorbent material.
Composting with Fresh Grass Clippings – A Practical Guide
Below is a step-by-step guide to making a successful compost pile using grass clippings and fall leaves. Fall leaves are generally abundant and should be harvested, shredded, dried and stored, ready for compost or leaf mold, or as a mulch.
With a C: N ratio of between
The key metrics are:
- Air availability
- Moisture >15% and <60
- A C: N ratio of about 30:1
- Optimal surface area
- Non-collapsing pile structure
- The compost pile should be about a cubic yard – 3 x 3 x 3 feet. A larger pile is excellent, and it just requires more manual labor.
Balancing Moisture, Carbon, and Nitrogen
Moisture
To calculate a substrate content, take the weight of the freshly cut grass clippings and compare it to the dry weight of the same grass to determine the moisture percentage.
I’vI’vene this a couple of times, and the average is 77%. It’s a surprise that lawn cuttings start rotting if you leave a pile standing for a day or two.
If your batch is too dry, add more crass cuttings; if it is too wet, add more leaves. You need to know the value of the following variables to balance the moisture levels.
| Reference Symbol | Description | Value |
|---|---|---|
| Q1 | The weight of the lawn clippings | 10 |
| G | Moisture goal | 60% |
| M1 | Moisture of Q1 | 77% |
| Q2 | Shredded leaves weight | ? |
| M2 | Shredded leaves moisture | 35% |
So what is the weight of the dry leaves that needs to be added to the wet lawn cuttings to move that moisture needle down from 77% to the required 60%, taking into account that the leaves also have a moisture content of 35%?
Q2 = the absolute value of (10 pounds x 60) – (10 pounds x 77) / (35 – 60)
That equals 170 / 25, meaning we need to add 6.8 pounds of dry leaves to every ten pounds of lawn cuttings to get a 60% moisture mix. You will use fewer leaves if they are thoroughly dried, shredded, and stored.
That would sort our moisture problem out, but what would it affect the carbon/nitrogen ratio? We have a mix of ten pounds of clippings and 6.8 pounds of shredded leaves. Note: The same principle would apply to the metric system – 10 kg of clippings and 6.8 kg of leaves.
Calculating C: N Ratios for Composting Nitrogen
To satisfy Factor C, a 30:1 C: N ratio, initially ignore the moisture in the grass clippings. We will start with the high-nitrogen materials and calculate how much carbon materials need to be added to get the right balance.
For a handy, comprehensive list of C: N ratios and moisture contents of different materials, check out the On-Farm Composting Handbook Appendix A: Table A.1 hosted on the Cornell University Website
At a C: N ratio of between 30 to 70:1, fall leaves hardly qualify as quality carbon-rich ingredients, compared to cardboard or newspaper at 400 to 500:1. Still, compared to lawn clipping at 12:1, they are comparatively carbon-rich materials.
The longer lawn clippings stand before being mixed into a compost batch where it can feed microorganisms, the lower their nitrogen content. Because nitrogen is volatile, dried grass clippings have a much lower nitrogen content than a new version.
Dry leaves have a nitrogen level of 0.75%, and carbon content is 50% carbon (the rest is cellulose and tough lignin). The C: N ratio of our batch of dry leaves is 66:1.
I calculated the ratio by dividing 50 by 0.75. What would the standard shredded leaves to lawn clippings be to obtain a C: N ratio of 30:0?
Let’s work it out. The average glass clippings C: N ratio is 17.5:1, with a nitrogen content between 2.4 and 3. Let’s assume we have an average batch of 2.8% nitrogen and 49% carbon.
We want to solve the number of leaves we need to add to the 10 pounds of clippings to get a C:N ratio of 30:1.
The values of the variables we will need are:
| Reference Symbol | Description | Value |
|---|---|---|
| Q1 | Lawn clippings quantity | 10 |
| N1 | Lawn clippings N level | 2.8% |
| R | Ratio desired | 30 |
| C1 | Lawn clipping C level | 49% |
| M1 | Lawn clippings moisture | 77% |
| Q2 | Dried levels required | ? |
| N2 | Leaves N level | 0.75% |
| C2 | Leaves C level | 50% |
| M2 | Leaves moisture | 35% |
Using the formula in my book, I can calculate that we need 4.5 pounds of shredded leaves per 10 pounds of grass clippings to get our desired carbon-to-nitrogen mix of 30:1.
So, to get to a 30:1 ratio, for every 10 pounds of lawn clippings, you only need about 4.6 pounds of dry leaves.
But as we established, we need 6.8 pounds of leaves to manage moisture levels. What is more important? C: N ratio or moisture content?
What will our C: N ratio be if we opt for the safer dryer conditions and add the leaves required to get the moisture level below 60%?
The 6.8 pounds (kg) of shredded leaves would give us a C: N ratio of approximately 37:1. This is okay, but the amount of carbon in your finished compost will be a problem for plant-available nitrogen. The finished compost mix ratio ought to be about 10:1 once it is stabilized.
If, however, we opt to add fewer leaves, focussing on starting with the optimal 30 to 33:1 ratio, our moisture will be more than the crucial 60 percent.
It won’t be long before the distinct rot of anaerobic conditions become evident. In that case, you will need to add wood shavings – more dry-matter cats. It’s better to start with a mixture with a total humidity factor of less than 60 percent.
Composting progress is influenced by both physical and chemical factors. Varying temperatures at different stages of the process are critical in determining the success of composting.
The ingredients’ moisture content and particle size affect the rate at which composting occurs. In addition, the system’s size, shape, and content affect the rate and type of aeration compost’sompost’s tendency to retain or dissipate heat.
All these factors are influenced by the ratios of carbon and nitrogen in the materials (stock) being composted. Phosphorous and potassium also play a role, though their presence is generally assumed in using vegetation.
Building a Compost Pile
Composting is probably the least appreciated and most beneficial gardening strategy and a way to make a valuable soil amendment.
If it is done onsite, the benefits are even more substantial as the organisms responsible for the process are more resilient to local pathogens, a trait passed on to local crops.
During the active composting period, the compost pile experiences a wide range of temperatures. Some microorganisms cannot survive when the temperature changes, while others thrive in new conditions.
A home composting system has three temperature ranges during the active composting period, known as psychrophilic, mesophilic, and thermophilic, based on the types of prominent microorganisms in a pile at those temperatures.
Temperatures lower than 50 °F (10 °C) are considered psychrophilic. Psychrophilic organisms are most prevalent at the initiation of the process and during curing.
The mesophilic organisms are active at temperatures between 50 and 105 °F (10 – 40 °C), while thermophilic microorganisms are responsible for temperatures above that and can reach temperatures high enough to ignite a dry batch.
Ideally, we want to keep temperatures below 167 °F (75 °C) but occasionally above 155 °F (68 °C). These temperatures are generally achieved in the batch center first and should be closely monitored.
The self-insulating compost heap traps heat generated by microbial activity as the population degrades the most readily degradable material and grows. As the microbial populations grow and diversify, the temperature increases steadily through the psychrophilic and mesophilic temperature ranges.
Depending on the operation, compost piles can take anywhere from 2 to 3 days to transition from mesophilic activity to thermophilic (hot).
Depending on the process, the compost pile can stay in the thermophilic range for anywhere from 10 to 60 days. Aerate the pile to reactivate active composting once the temperature drops below 105 ° F (40 °C).
Active composting is never determined to be finished at a specific point. When the pile conditions are such that microbial activity cannot increase enough to reheat the pile, it is usually considered complete and ready for curing.