The cornerstone of good soil health

A great gardener is not a gardener of plants, they are a gardener of microorganisms. With this in mind, let’s explore the art and science of composting.

“Composting is the ‘art and science of mixing various organic materials in a pile and controlling conditions so that soil organisms break down the raw organic matter into humus”

(Dunne, 2015, p.51)

Humus (often called compost) is the dark, fragrant, spongy material resulting from composting. It is the cornerstone of soil health holding crucial nutrients for plant growth. Unlike applied synthetic fertilizers, humus breaks down slowly, releasing these nutrients gradually so they are less likely to be leached from the soil. It also improves soil tilth, aeration and water holding capacity. When we compost we are engaging in a natural process in which we work to produce soils like the incredibly fertile ones we find in a forest floor, known as A horizon soil.

Composting equation

Composting is easy. Just put organic materials in a pile, let it sit for a year or so, and you will have humus to apply in your garden. Or chop and drop- when your garden plant is dying or dead, chop if off at ground level leaving the roots intact below ground, chop the plant into smallish pieces and drop on the ground where the soil micro organisms will turn them into humus.

But if you want to speed the process up, then controlling inputs and conditions is important. You can control the amount and type of organic yard and food waste you input, and the amount of oxygen and water in your compost pile. The soil organisms will come in on the organic material you add and will already be present at your composting site. Just take a moment to consider this, a single leaf has 100 million organisms on it, representing at least 1000 species (Grinberg M. et al, 2019). Your job as a composter and a gardener is to create the right conditions for these soil microbes to thrive and do their work.

Balancing your inputs

When reading about composting inputs, you will often hear the words ‘brown’ and ‘green’ in reference to the two categories of inputs. It’s important to understand the difference between the two because you’re going to try to balance those inputs. Brown refers to those organic inputs with high carbon content while green refers to organic materials with high nitrogen content. They are confusing terms though, because while many high nitrogen inputs like freshly mown grass are ‘green’, some are most definitely brown, like coffee grounds. So lets use the terms ‘high carbon inputs’ and ‘high nitrogen inputs’ instead.

High carbon inputs include things like dry leaves, straw, cardboard, and wood chips. Like the high nitrogen inputs, they also have other nutrients but are especially high in carbon. The other thing to notice about them is that they are dry, they don’t hold much water and will be much lighter than if they were ‘fresh’.

High nitrogen inputs include things like household waste, grass clippings, fresh manure, and coffee grounds. All these materials include many different nutrients including carbon, but they are especially high in nitrogen. The other thing you should notice about them is that much of their weight is in water.

Here’s why we care about carbon and nitrogen. All organisms require both to survive, including the microbes we’re trying to encourage to do the decomposition work in our compost piles. Carbon and nitrogen are key elements in their diets. Carbon is their energy source, nitrogen allows them to make digestive enzymes. Lots of carbon in relation to nitrogen and the microbes can’t really get to work digesting. Too much nitrogen and you might get the wrong kind of microbes and a smelly pile (Pavlis. R., 2023, p.39).

C/N ratios

Your job if you want to speed the composting process up is to balance the Carbon/Nitrogen or C/N ratio. This is the WEIGHT of carbon compared to the WEIGHT of nitrogen (not the weight of the bit of organic matter you are adding). All organic matter contains both carbon and nitrogen along with other nutrients. Each type of material however, has a different ratio. You can find tables that list the approximate ratios on lots of sites. Check these tables out at Compost Magazine https://www.compostmagazine.com/carbon-nitrogen-ratio-tables/ . And yes, there’s a magazine just about composting in case you’re keen on becoming a composting nerd!

For optimal composting we should try to get a C/N ratio of 30:1 in our inputs. This will result in a ratio of 24 to 15:1 in the finished compost but by adding more of carbon to begin we’ll keep the microbes happier longer (Pavlis, 2023, p.40). I’m not going to try to explain all the ways, some more accurate than others, that you might attempt to get a ratio of 30:1 ’cause this is Composting 101- you just want to get started. If you want to get really good at accurate C/N ratio inputs, I highly recommend you pick up Robert Pavlis’ book Composting Science for Gardeners. For our purposes, a good rule of thumb is to add 2 or 3 VOLUME parts of high carbon inputs to 1 VOLUME part of high nitrogen inputs. It’s much easier to think in volumes when we’re composting then to pull out tables and calculate weights. So add 2 or 3 buckets of high carbon inputs to 1 bucket of high nitrogen inputs. It’s 2 or 3 because it will depend a little bit on how much you pack down that lighter, drier high carbon input but the nitrogen inputs tend to be wetter and therefore heavier.

Problematic additions

The home composter should avoid adding the following;

• meat, bones, fish, milk products, eggs, oils
• pet feces
• diseased plant material or plant material that has been treated with herbicides or pesticides
• seed heads and roots of ‘pushy’ plants who are not welcome in your garden
• inorganic material
• bbq ashes and coals
• dishwater

The role of oxygen and water in composting

Oxygen is crucial for aerobic composting. Microbes need oxygen to survive and do their work, so occasionally turning your pile will help bring oxygen in. Once a week is a good rule of thumb. Different size particles of organic matter additions will help bring in oxygen.

Water is another crucial element for microbe survival. The pile must be damp with a moisture content between 40 and 60 %. If you take a handful and squeeze, it should feel like a wrung-out sponge. If the pile is too wet or dry, the organisms will go dormant or die.

Everything you do in the composting process is aimed at the good health of soil organisms- they are the key to creating humus. They need nutrients, oxygen, and water to thrive as they break down organic matter into humus. A warm compost pile is a sign that soil organisms are at work, heat is one of their byproducts but even in a cool pile, organisms are working.

Your microherd- The Soil Food Web

There are 5 billion microorganisms in a couple of tablespoons of soil! When you add organic matter into your compost pile, you are adding microorganisms. Two of these microbe types are bacteria and fungi, part of the second trophic level in the diagram above. Here’s what each are doing in your soil and in your compost pile

In the soil or compost pile, bacteria are exuding chemicals while they digest organic material. These chemicals are useful to plants. Bacteria live in water (that’s why your compost pile needs to be damp), where they absorb nutrients from organic material through their skin. They exude a substance that will break the matter down so its particles are small enough for them to absorb, using some of it but leaving the rest for others including plants. The result of their work is a soupy stew in the soil which improves soil aggregation (Pavlis, 2023, p.33).

Fungi have long filament structures or hairs below ground through which they send out digestive juices amongst other things. These filaments can burrow into plant roots often at the invitation of the plant. There they establish mutually beneficial associations with those plants. Plants are autotrophs, the first trophic level in the soil food web. They can make sugars from the Sun’s energy which they share with the fungi they are in association with. Fungi can digest some mineral particles that other microorganisms can’t and provide them to plants (Pavlis, 2023, p.35).

These microorganisms work on the surface of organic matter so more surface area equals faster decomposition; chopping up pile additions will result in faster humus production.

Factors to consider when building your compost pile

• your pile should ideally be at least 1 meter x 1 meter x 1 meter (1 cubic meter). Smaller piles can’t get hot enough while much larger is difficult to manage especially to aerate.
• level, well-drained ground
• near raw materials
• near a water source
• room to stockpile ingredients
• away from your house, sheds, trees and shrubs
• you may want the ability to screen it

Which composting method?

Consider;

• aesthetics
• amount and kinds of organic matter to be composted
• how much space is available?
• how much finished compost is needed?
• how quickly is it needed?
• time and money available
• your municipality’s by-laws

Realistically most of us will at best achieve WARM piles that’s why our list of inputs is limited. But think about how involved you want to be in this process when choosing your composter/composting method. The prices included below were accurate in March of 2024, are not necessarily from the cheapest source. They include CofK (City of Kingston), LV (Lee Valley), HD (Home Depot). There are of course, lots of other places from which you can purchase these items. It is worth doing a price comparison.

Is it ready?

• has your pile shrunk to half its original size?
• is it hard to distinguish any of the original additions?
• is it a uniform dark brown with a uniform texture?
• does it have a pleasant earthy smell?
• has it cooled?

Yes to all these questions? Your humus is ready to use?

Remember “finished” compost will continue to decompose for another 5 years or so after application.

Using your finished humus

• Soil application: spread it widely rather than concentrating in one spot, at a yearly rate of a couple of centimeters on top of your soil
• Mulch: no more than 5 to 8 cms around shrubs or trees, not touching the tree’s bark or feed at the canopy’s edge
• Seed starting: no more than 25-30% of your seedling mix
• Outdoor potted plants- no more than half of your potting soil
• Top dressing your lawn: a thin layer in early spring or late fall
• Compost tea: soak a bag of compost in water and use it to water your plants or as a foliar spray; read more about this here; https://1000islandsmastergardeners.ca/2021/09/21/foliar-sprays/

I hope you feel empowered now to get composting. And remember when you compost not only are you improving your garden’s soil, you are contributing to carbon capture and to waste diversion. You’re helping all of us and all our relations too! Especially the manidooshag, our tiny little soil friends.

References

Unless otherwise noted, information from this blogpost came from the Reference Manual for Ontario Master Gardeners 3rd Edition 2020

Unless otherwise noted, images in this blogpost came from Canva. Featured image: Judith Horvath on Pixabay

Dunne, Niall (ed). (2015). Healthy Soils for Sustainable Gardens. Brooklyn Botanical Garden.

Grinberg, M., Orevi, T., Steinberg, S., & Kashtan, N. (2019). Bacterial survival in microscopic surface wetness. eLife, 8, e48508. https://doi.org/10.7554/eLife.48508

Pavils, R. (2023). Composting Science for Gardeners. New Society Publishers.