The Difference Between Compost and Fertilizer

A common queston for farmers and gardeners alike is what is the difference between compost and fertilizer.

To answer this question, we’ll first have to differentiate between inorganic and organic fertlizers and then explore what compost is in greater deatail. Let’s get started.


A fertilizer, in basic terms, is plant food. It’s a compound that is meant to help plants grow by providing macro- and micro-nutrients.

Fertilizers come in two main forms- inorganic and organic. Organic nutrients are generally derived from plants or animals, hence the name “organic” meaning derived from living matter. These fertilizers tend to deliver nutrients to plants in slow-release form because they depend on the soil life to help make their components plant-available.

Inorganic fertilzers are composed of chemicals or minerals that are not derived from living matter (“in” meaning not, organic defined above). Typically fast-acting, inorganic fertilizers can feed the plant directly because they come in water-soluble form. Once applied to the soil, the plant is able to absorb the nutrients through its roots.

At first glance, you may think fast-acting nutrient delivery would be ideal for your plants, but these inorganic fertlizers come with a few negative “side effects” for your soil.

Inorganic Fertlizers

The chemical makeup that allows inorganic fertilizers to be water soluble also causes quite a few problems for soil life. This is because soluble nutrients are in fact SALTS.

Salts are detrimental to your soil in many ways, but first and foremost, salts take water and make it unavailable for plants. This can be explained using the ocean as an example.

Soluble nutrients are salts
All soluble nutrients are salts (not all are table salt)

Despite being plentiful, ocean water is salty and unavailable to humans as drinking water unless desalinated. That’s because salts in excess quantities are toxic to humans, and the same is true of plants.

The main salt is oceans is sodium chloride (NaCl), also known as table salt. Without getting too deep into the chemistry, sodium chloride (and all salts) are readily available to bond with water molecules due to the charges associated with the bonds between the salt atoms. When salt is added to water (H2O), it bonds with the either the oxygen atom or the two hydrogen atoms of the water molecule and renders that water unavilable to humans and plants.

Well, the same thing happens in soils when soluble nutrients in the form of inorganic fertilizers are added to soil. The salts bind with the water in the soil and make it unavailable to the plants. Over time and with repeated applications, the soil becomes more and more salty, which can lead to desertification as water becomes unavailble to the plants despite being present in the soil.

The difference between compost and soil- tractor spreading inorganic fertilizer
Adding salts in the form of inorganic fertilizers decreases water availability

On top of that, the soluble nutrients now bonded to water molecules are able to be washed away in rain events, causing the inorganic nutrients to leech away from where they were applied into the local watersheds. Downstream, huge problems are created as excess nutrients can cause algal blooms, as evidenced in the Gulf of Mexico and along other great river basins where conventional agriculture is the norm.

Just this fact alone should be reason not to use inorganic fertlizers, as it does not follow the ethic of Earth Care as taught in permaculture design. But there’s more problems with inorganic fertilizers.

Side Effects on the Soil Food Web

The soil is a living organism made up of bacteria & fungi, their predators, their predator’s predators, and so on up to worms, ants, beetles, bugs, spiders and more. Together, these organisms are known as the soil food web and are responsible for making nutrients within the soil available to your plants.

Inorganic fertlizers cause major damage to the delicate structure of the soil and the organisms living within it.

Take Gypsum, a common soil additive/fertlizer used in agriculture. In chemical terms, gypsum is Calcium Sulphate (CaSO4). Sulphates are commonly used in industrial agriculture as a fungicide, or a chemical that kills fungi. By adding gypsum to your soil, you’re adding a fungicide that will have a very detrimental effect on the fungi population within the soil.

Gypsum contains sulphate, a fungicide that damages the soil food web
Sulphates are a fungicide and damage fungal networks

“So what?,” you might ask. Well, fungi, unlike bacteria, take longer to colonize soil but play a much larger role in long-term nutrient availability and carbon storage within the soil. Plus, fungi do a great job at building soil structure across large areas of land. By applying a fungicide (anything ending in -sulphate), you’re decreasing available nutrient in your soil, destroying soil structure, and decreasing your carbon sequestration potential.

This is one example of the side effects that one inorganic fertilizer has. Adding more would simply be repeating the same idea over and over. The general message is that inorganic fertilizers cause major damage to both the soil structure and the organisms that work hard to make sure your plants have what they need to grow abundantly.

“Green” Revolution

Most of today’s industrial conventional agriculture’s dependence on inorganic fertlizers dates back to the end of World War II.

Ammunitions manufacturers suddenly lost the majority of the demand for their product and were left with huge stockpiles of nitrogen-rich materials to make gunpowder, mainly in the forms of sulphur and potassium nitrate.

Being savvy business owners and with the help of the government, these companies recognized that their raw materials could provide the key macronutrients that plants need to grow: Nitrogen, Potassium, and Sulphur. And thus, the beginnings of the green revolution in agriculture.

the difference between compost and fertilizer- corn powered by chemicals
Corn production powered by inorganic fertilizers

Record production of corn and soy followed appications of the newly made fertilizers, and record profits began to be recorded in peacetime by the same companies that profited off of the wartime efforts.

Little did they know (or did they?) the new industrial food production boom would lead to the destruction of soils and environments throughout the United States and across the globe.

Organic Fertilizers

Put simply, organic fertilizers are nutrients derived from living organic matter, usually from plants or animals.

Organic fertlizers and soil ammendments depend on soil life to break down the component within the fertlizers, and thus are considered “slow release.” You can think of organic fertlizers essentially as concentrated organic matter high in essential plant nutrients.

So the focus here is still on feeding the plant, rather than the soil, but those nutrients need to be processed by the soil food web in order to become plant-available (Keep this point in mind as you continue reading).

While most organic fertilizers tend to be more beneficial than inorganic fertlizers, there are some things to note when considering whether to use them on your property.


Peat is one of the most common organic fertlizers/soil ammendments out there and is used abundantly. It doesn’t necesssarily add nutrients but it helps add organic material to the soil and increases its water holding capacity.

The problem with using peat is that it’s typically mined from peat bogs. These ecosystems are very fragile, unique, and take a very long time to develop, making them a limited resource similar to fossil fuels. Sure, we can mine them now and use it while it’s in abundance, but the degradation of these ecosystems will come at a cost.

Peat harvested from a peat bog
Peat bogs are fragile ecosystems that take a long time to develop

In general, if destroying ecosystems to mine a product that we can easily replace in our systems should be avoided. Adding organic matter sourced locally would work just as well and would have a much smaller ecological impact. If you happen to live on a peat bog and use the resource wisely, you may be the exception to the rule here!

Animal-based products

Many organic fertlizers are derived from animals, like feather, blood, and bone meals. There’s nothing inherently wrong with using these byproducts from animal processing, but it’s worth noting a few things.

Most (not all) animal-based organic fertilizers originate from industrial scale animal farming and processing facilities. These CAFOs and factory farms are ethically questionable and often have huge negative climate impacts.

Cows at a CAFO
A CAFO is a controlled animal feeding operation, often used on factory farms.

Buying products from these facilities indirectly supports the continuation of these practices and should be questioned. Sure, you’re keeping these products from going to a landfill and that’s commendable, but is it worth the global price we pay for continuing to support these facilties.

The same general idea goes for manure products. It’s up the buyer to choose where he or she stands on the spectrum. If you’re going to use an organic animal-based fertilizer, try to understand where it came from. Is there a small local farm that produces similar products that you can source from? Can you find farms that are practicing regenerative agriculture and are certified-humane instead? These are just questions to guide you through your decision making process on using these organic fertlizers.

Sewage Sludge

It may sound like the last thing you’d put on your garden, but the byproducts from wastewater facilities are high in organic material and are often available for free or very cheap and advertised as a fertilizer.

If your local wastewater facility is offering this product, they will (hopefully) have to prove that their product does not contain pathogens or negative chemicals in the mix. That being said, I personally would not use these products directly on my garden just knowing where it originated from.

I would, however, use it as a base material for a compost pile, which brings us at last to…

Compost and the Soil Food Web

By now, you’ve heard me mention the soil food web a few times. The reason behind this is that compost cannot exist without the soil food web.

So what is the soil food web? For the sake of this post I’ll simplify and say that the soil food web is the group of aerobic organisms that are responsible for primary nutrient cycling in soils.

Similar to the food webs you learned about in grade school, the soil food web consists of primary consumers (bacteria and fungi), secondary consumers (protozoa, nematodes, etc.), tertiary consumers (micro-arthropods, etc.), and so on.

Knowing that, we can move onto the definition of compost and at last find out the difference between compost and fertilizer.


Compost is the result of aerobic decomposition of a mix of organic material. Let’s break that down.

Aerobic means that oxygen is present in adequate amounts (greater than 6ppm). Decomposition means breaking down and is carried out by the microorganisms in the soil food web. These first two terms imply that the soil food web must an aerobic environment and does not function in anaerobic conditions.

A mix of organic material allows for a large diversity of foods for the soil food web to chow down on. The organic material is the food that the bacteria and fungi feed on within compost piles.

The bacteria and fungi are then consumed by their predators. When the predators eat their fill and digest, they need to release the waste material (much as we need to do after a big meal). The resulting waste, or “poop” to put it colloquially, is in the form or plant-available nutrients.

Active aerobic compost pile
Compost turns waste organic matter into garden gold

So compost is the result of an aerobic composting process where the organisms from the soil food web break down organic matter and turn it into plant-available nutrient.

What’s responsible for the breakdown of the organic material? Bacteria and Fungi. What’s responsible for making nutrients available for the plants? The predators of the bacteria and fungi. We call this process the “poop loop.”

So compost is really an inoculum of beneficial organisms plus organic material that will take your parent mineral material (sand/silt/clay) plus soil organic matter and turn it into plant food.

Soil Food Web

Based on the definition above, it should be apparent that in nature, the soil food web is what feeds your plants. They take the raw minerals from your soil and whatever organic material is present and turn it into plant-available nutrients.

And you don’t even need to pay them for their work! Once established, all they need is a regular input of organic material (mulch or leaves in the fall), enough water to keep things going, and plants to occupy the soil.

The Difference Between Compost and Fertilizer

By now, I expect you can begin to understand the difference between compost and fertilizer.

Fertlizer is plant food in organic or inorganic form. It focuses on feeding the PLANT, not the soil. Organic fertilizers add raw “food” to your soil for the organisms to break down, but it’s really the organisms themselves that are doing the work.

Compost is a product of aerobic decomposition of a mix of organic material that, when processed properly, serves as an inoculum of beneficial microbes from the soil food web coupled with organic matter.

Compost is the ULTIMATE “fertilizer” as it adds the microbes that create plant-available nutrients from the soil minerals and organic matter.

Focusing on the SOIL using the biological approach instead of focusing on the plants will increase production, minimize disease, improve soil structure and water retention, increase carbon storage, among other benefits.

Dr. Elaine Ingham's Soil Food Web School
Focusing on soil biology and the soil food web will benefit your garden for years to come.

If you’re interested in learning more, I would highly recommend Dr. Elaine Ingham’s course on the Soil Food Web. I took this course personally and found it to be the most valuable education I’ve ever received. Understaning the soil food web and its role in food production, mitigating cliamte change, and improving water quality is exactly what is needed in today’s climate.

If you have questions about the course or want more information, please reach out in the comments or on the contact page. Please note that I am an affiliate for Dr. Ingham’s course, and if you sign up through my link here or above, I’ll receive a kickback which helps support this site and the work I’m doing.

Wrapping up: The Difference Between Compost and Fertilizer

I hope you’ve enjoyed today’s post and have a better understanding of the difference between compost and fertilizer.

If you have any questions, please feel free to leave them in the comments or head on over to the contact page.

Thanks for reading!