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Can Compost Replace Fertilizer? Here’s What the Experts Say

I’ve been researching and writing about gardening for a few years now and one of the most common questions I come across is if compost is sufficient as fertilizer. To help break this down, I did some digging. Here’s what I found.

According to the University of Massachusetts Amherst, compost can replace fertilizer. While plants aren’t picky when it comes to a source of nutrients, compost and manure contain organic carbon, which is vital for soil health—assisting in the plant’s growth, immune system, and nutrient absorption.

So, while compost is healthy for plants, can it really replace fertilizer? And what’s the best way to use compost? Let’s take a closer look.

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the inside of a composting container

Can You Use Compost Instead of Fertilizer?

Compost vs Fertilizer

CompostFertilizer
Nutrients are released slowlyNutrients are usually released quickly
Increases water retentionLimited nutrient profile
Has more absorbable nutrientsPrevents the exchange of nutrients between beneficial soil life and the plant

In summary, compost benefits the soil’s health, which directly impacts the plant’s health. Synthetic, or chemical, fertilizers, while effective in the short term, usually have long-term consequences. Most notably, chemical fertilizers short-circuit the nutrient exchange between the beneficial soil life and the plant.

Let’s start by first looking at the pros and cons of fertilizer.

Fertilizer

Fertilizer ProsFertilizer Cons
Fast release nutrients (can also be a con)Limited nutrient profile
Widely availableDamages water retention
Easy to handle and applyRenders beneficial soil life obsolete

Everyone knows that chemical fertilizers are great at delivering fast-acting nutrients in quantity.

However, it’s the fertilizer’s quality that’s the root of the problem.

There are three main concerns when using chemical fertilizers:

  1. Limited range of nutrients
  2. Commonly creates a water-repelling crust on top of the soil
  3. Prevents the exchange of nutrients between beneficial soil life and the plant

First, compared to organic materials such as compost, manure, and bone meal, chemical fertilizers tend to have a limited range of nutrients.

Inorganic fertilizers usually contain only a few nutrients – generally nitrogen, phosphorus, potassium, sulfur and sometimes micronutrients, either singly or in combination. These nutrients are in a form readily available to plants. However, since they are lost from the soil quickly, you may have to fertilize plants several times during the growing season unless you use a specially formulated, slow-release type.

Ross Penhallegon, Horticulture Extension Specialist, OSU Cooperative Extension

Aside from the range of nutrients being limited, the nutrients are also in their most basic, chemical form. For example, imagine if instead of eating food, we could only get our nutrients and minerals from multi-vitamins.

Nutrients from fertilizers also typically leach through the soil quickly, creating the need to buy and apply more. While there are slow-release fertilizers available, they still commonly cause other issues such as damaging the water retention ability of soils as well as killing off the healthy soil life.

To see how fertilizer damages the water retention and life of the soil, let’s take a look at how these two aspects are supposed to function (and how compost promotes them).

Compost

Compost ProsCompost Cons
Increases water retentionSlow-release nutrients
Feeds beneficial soil lifeComposting process can take months
Has more absorbable nutrientsMore difficult to store and distribute

Compost is the decomposition of organic material, such as plant and animal matter. Common sources of compost include yard scraps, animal byproducts (such as feathers and blood meal), and manure. Unlike chemical fertilizers, compost also provides sufficient trace and secondary nutrients including copper, zinc, and selenium (source).

So, how is compost generally better for our plants?

It first has to do with fungi.

According to the National Resources Conservation Service, beneficial fungi in the soil promotes water retention, nutrient uptake, and disease resistance for plants.

And fungi are just one of the types of helpful soil life. Earthworms, bacteria, and other helpful organisms feed on organic materials, breaking them down into smaller particles.

These smaller particles are fantastic at being dissolved by the soil’s slightly acidic pH into nutrients for the plants (source), as well as holding more water. These organisms’ waste products also contribute to the organic matter in the soil.

And as the organic matter grows, the soil becomes richer, with every 1% increase in the soil’s richness holding an additional 20,000 gallons of water per acre (source).

But the soil life doesn’t just get its food from processing clumps of organic matter—they also give nutrients to the plant’s roots in exchange for sugar in the form of carbon (obtained by the plant from photosynthesis).

 In exchange for carbon from the plant, mycorrhizal fungi help solubilize phosphorus and bring soil nutrients (phosphorus, nitrogen, micronutrients, and perhaps water) to the plant.

NRCS, USDA

So, the plant gets quality nutrients directly from the compost, and by trading with beneficial soil life, such as mycorrhizal fungi.

The problem begins when we use chemical fertilizers. Since plants will get the majority of what they need from the fertilizer, there’s no reason for them to continue trading with the soil life for nutrients. And when the soil organisms’ source of food is cut off, they die off. Unfortunately, so does the soil’s water retention and other numerous benefits like increased disease resistance for the plant.

The Potential Problem With Compost

While compost might seem like the magic solution, some soils are devoid of specific nutrients more than others. For this reason, compost alone might not fill every need.

To help with this, you can first test your soil with your local cooperative extension and supplement an organic amendment for whichever nutrient is lacking. For example, if you’re trying to boost any of the three primary plant nutrients (NPK), consider using amendments such as feathers and blood meal (nitrogen), bone meal (phosphorus), and kelp (potassium).

Here’s a table with more details.

Organic FertilizersNPK (Nitrogen, Phosphorus, and Potassium)
Feathers15% N
Blood meal12.5% N
Bone meal15-27% P
Kelp4-13% K
Source: OSU Cooperative Extension

If you prefer to not use animal products as fertilizer, there are other plant-based options.

Should You Make or Buy Compost?

Store-bought compost is often good for soils, but it can be overly processed and the nutrients degrade the longer it sits on the shelf. Since homemade compost has freshly decomposed matter, its nutrients usually result in more nutrient uptake and healthier soils.

When making your own compost, the three main methods are hot, cold, and vermicomposting. 

Hot composting is when the compost pile’s heat from decomposition is maintained between 135-160ºF (source). Cold composting is what it sounds like—composting at a cooler temperature. Lastly, vermicomposting is composting with a type of composting worm such as red wigglers.

While hot composting is the quickest to decompose, cold composting is the easiest. All you have to do to cold compost is throw food scraps and yard waste into a pile and let it decompose. That’s it! However, I would suggest putting a layer of dry dirt or carbon such as leaves, straw, or sawdust on top to prevent smells, mold, and flies.

Vermicomposting is in-between hot and cold composting as it’s faster than cold composting, but you don’t need to turn or maintain a high temperature as with hot composting. And you can even have a vermicompost bin in an apartment.

When I lived in an apartment, I got some red wigglers from the pet store, a bin from Target, and leaves from outside. After drilling holes in the bin’s lid for air, and placing leaves for the bedding, I simply added organic material like kitchen scraps to the bin and the worms did the rest! Every now and then I’d apply this compost to my garden and the plants absolutely loved it. After applying it, new growth on the plants started almost immediately.

Keep in mind that nitrogen-rich materials such as greens, coffee grounds, and yard waste need to be balanced with carbon materials such as leaves, wood chips, or hay. This is called the carbon-nitrogen ratio. Too much or too little carbon or nitrogen results in problems with the compost. This is true for all forms of compost.

According to Cornell University, the ideal carbon-nitrogen ratio of compost is 30:1 (source).

Normally, compost works well enough on its own, but it can be difficult to get in larger volumes. So, if you’re working with larger pastures, fresh manure and pasture rotation might be the better option. There are some exceptions such as if you’re doing the deep litter method for your livestock and you have large deposits of composting materials.

Here’s a helpful table that not only shows the average NPK of the different animal manures but also if it’s hot or cold (generally, hot manure needs to be composted before applying it to the garden).

LivestockAverage NPK of Manure*Hot or Cold Manure
Chicken1.1-1.4-0.6Hot
Cow0.6-0.2-0.5Hot
Horse0.7-0.3-0.6Hot
Pig0.5-0.3-0.5Hot
Goat0.7-0.3-0.9Cold
Sheep0.7-0.3-0.9Hot
Rabbit2.4-1.4-0.6Cold
*NPK values can vary – Source: alpacasofmontana.com

How and When To Use Compost

Generally, apply 2-4 inches of compost every 1-2 months on top of the soil. Keep it at least 3 inches from the plant’s trunk or stem. While compost won’t chemically burn the plant, its moisture can introduce mold if it remains on the trunk or stem. For best results, also add 4 inches of mulch on top of the compost.

I’d recommend only using compost and mulch if your plant’s soil is well-draining, as these materials can make the drainage worse.

If your soil does have poor drainage, you can amend it by planting in a mound of soil above the ground. For potted plants with poor drainage, repotting it with fresh potting soil is often the best approach.

Final Thoughts

So, while chemical fertilizers are good at delivering nutrients in quantity, their nutrient quality typically causes more harm than good.

Compost provides the soil with organic carbon, feeding the soil life—which increases water retention, extra nutrients, and disease resistance for the plant.

Here’s another way to look at it—plants have been getting all of their own nutrients for millions of years. Long before chemical fertilizers were invented in 1903. So, if plants didn’t need chemical fertilizers then, why would they need them now?