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Use our Best Compost Calculator to check the C:N ratio of your current pile and find what other materials you should add!
Finding the right balance for your compost pile can feel a bit daunting. First you need to figure out the Carbon:Nitrogen (C:N) ratio in each type of material you have, then figure out the total ratio when you combine everything together and compare that number to the optimal ratio needed for a speedy compost. Next, calculate how much more carbon or nitrogen you need to add, then figure out what additional materials to mix in based on that … oh it’s all so confusing! And let’s be honest, in the end you’ll find yourself just dumping everything in a pile, throwing some leaves on top, and hoping for the best; Of course days later you’ll go back only to find a smelly and cold heap of rotting mush.
Although it may have caused frustration in the past, getting your compost pile to the right C:N ratio is very important. Once the perfect ratio is reached (generally considered to be around 30:1), it creates the perfect environment for microorganisms to thrive. Given a certain amount of carbon for energy and a proportionate amount of nitrogen for protein, within a short amount of time billions of microbes (such as bacteria, fungi, and actinomycetes) will vigorously eat, reproduce, and use enzymes to breakdown your pile until the organic matter is reduced to its most basic macronutrients and micronutrients (i.e. nitrogen, phosphorous, iron & zinc). The smoke and heat is evidence that the microbes are working at its optimal speed and the rising temperature is a natural byproduct from all the activity.
So what’s the best way to get your pile to reach this elusive ideal ratio of 30 parts carbon for every one part nitrogen (30:1)? The answer is first doing a little bit of math; Let’s take a look at the Pearson Square method.
The Pearson Square is a fairly easy to understand mathematical technique that farmers often use to find the right portion between two feeds to create the perfect balance of nutrition to give to their animals. The same method can be used between any two numbers that will be combined to meet a predetermined numeric goal.
In our case, the two numbers are ratios that will come together to make the ideal ratio for composting.
Ratio 1 is going to represent all of the organic waste you have accumulated at your home or business that you want to compost such as food scraps, weeds, and used paper.
Ratio 2 is going to be additional items mixed in afterwards that have a C:N either high in nitrogen or high in carbon – whichever is opposite to Ratio 1. This means that if Ratio 1 is high in carbon (higher than 30:1), then you need to mix in additional items that are high in nitrogen (having a ratio lower than 30:1).
Run the two ratios through Pearson Square and out comes what percentages you should for each to result in the ideal 30:1.
Before using our calculator, let’s walk through a simple example using Pearson Square so you can understand the math behind it.
We’re going to calculate how much grass clippings and leaves to combine for a balanced compost pile.
Draw a square, then draw a small circle in the center of it with our goal of the ideal ratio “30:1” written inside. Place Ratio 1 (in this case 15:1 for grass clippings) in the top left corner of the square and Ratio 2 (60:1 for leaves) in the bottom left corner.
1Calculate the (absolute value*) difference between the two ratios. This number will be the “Dividing Factor”.
|15 – 60| = 45
2Starting at the top left, get the absolute value* of the difference between the Ratio 1 and the Ideal Ratio.
|15 – 30| = 15
3Following the arrow diagonally downward, divide the difference by the Dividing Factor.
15 / 45 = 0.33
4Multiply by 100 and the result represents what percentage of your final compost pile should be made up of leaves.
0.33 x 100 = 33%
5Move on to the bottom left, get the absolute value of the difference between the Ratio 2 and the Ideal Ratio.
|60 – 30| = 30
6Following the arrow diagonally upward, divide the difference by the Dividing Factor.
30 / 45 = 0.67
7Multiply by 100 and the result represents what percentage of your final compost pile should be made up of grass clippings.
0.67 x 100 = 67%
*Absolute value is the numerical output without regard to whether the sign is negative or positive.
67% Grass Clippings and 33% Leaves creates the ideal ratio of 30:1
Note: 67 divided by 33 is 2, meaning for every two parts grass clippings, add in one part leaves
You may be saying to yourself, “I need to compost a lot more than just grass and how would I know all of the different ratios for everything?” Worry no more! It just so happens that our Best Compost Calculator has a feature that allows you to add multiple materials to determine the overall C:N ratio for what you have accumulated.
Instead of typing in your pile’s ratio, you can click on ‘Add item(s)’ to bring up a list of commonly used materials and their carbon-nitrogen ratios. And for each material, you can choose the amount or number of portions that you have.
What is a portion? It is a part of a whole, meaning its actual size/volume can only be determined when compared to others that also take up portions within the same space. With our calculator, you can assign up to 20 portions per material; For example if you choose two portions of coffee grounds and eight portions of straw, that means that your pile has four times the amount of straw than coffee grounds. The unit of measurement can really be anything – that can mean 2 and 8 gallons, 2 and 8 cups, or even 2 and 8 handfuls – as long as it’s all relative to each other.
It may surprise you that when composting all of your organic waste at home, you may not need to add that many more items to balance it out. Fruit is generally 35:1 while vegetables are around 25:1, two of the most common scraps you’ll probably have and already so close to the ideal 30:1.
Of course the Pearson Square should be kept as a guideline rather than a blueprint. Once you have initially mixed all of the materials and let it sit for a certain amount of time, the look and feel of your pile is what’s really going to tell you if it’s working. And just because your pile is not heating up, that doesn’t mean that your ratio is necessarily off. There are so many other factors that can affect the health of your pile such as size, moisture, drainage, and airflow.
Without further ado, here is…
The Best Compost Calculator
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RATIO OF YOUR PILE
RATIO OF ITEMS TO ADD
The Best Compost Calculator
Using the Pearson Square Method
Materials Already in Your Pile
Materials to Add to Your Pile
How to use:
1 For the box in the top left corner, type in the C:N ratio of your current pile (if known) or choose “Add Item(s)” to select the materials you currently have. (If you are only adding one item, the number of portions is irrelevant).
2For the box in the bottom left corner, you will be alerted if your pile needs more Carbon or Nitrogen. Type in a ratio or choose “Add Item(s)” to choose from a list of appropriate materials that will help balance out your pile. (If you are only adding one item, the number of portions is irrelevant).
3Percentages will appear showing how much of your current pile and additional items you should mix together.
The Carbon-Nitrogen Ratios used in the calculator are based on an average from multiple sources. These sources include:
Carry On Composting, WebLife, WebPal, Morning Chores, Planet Natural, Home Composting Made Easy, Energypedia, Permaculture Information