Dirt vs. Soil
Let’s get this straight: dirt and soil are not the same thing.
Dirt is lifeless. It’s mostly crushed-up rock—minerals with no organic matter, no microbial activity, and no system of support for plant life.
Soil, on the other hand, is alive. It’s full of microorganisms, bacteria, fungi, decayed leaves, plant roots, and—if you’re lucky—a whole colony of hardworking earthworms.
If you were to bend down and touch the forest floor, you’re not touching dirt. You’re touching living soil.
Living soil—that’s what I aim to create in every kitchen garden I build. And guess what? You can turn dirt into soil by giving it the ingredients it’s missing—organic matter and microbial life. And earthworm castings are a big part of how we do that.
What Are Earthworm Castings?
Earthworm castings are the polite term for… worm poop. Yes, really.
Earthworms ingest organic material like decomposing vegetation and kitchen scraps, and then mix that material with their digestive enzymes. The result is excreted as nutrient-rich castings. It may sound simple (and a little gross), but worm castings are packed with billions of beneficial microbes. Think of worm castings as a probiotic for your garden—they introduce good bacteria and fungi that help plants thrive.
When you add them to your soil, you’re not just adding nutrients. You’re seeding the soil with living organisms that will begin rebuilding the ecosystem underground.
In case you're wondering, handling worm castings is not as gross as it may sound. Castings look like a dark, crumbly form of compost.
The Benefits of Earthworm Castings
Castings by any other name may just be worm poop, but they’re basically superfood for your soil. Unlike regular compost, worm castings contain highly concentrated nutrients, beneficial microbes, and natural plant growth stimulators that make soil more fertile and plants healthier. Worm castings are so much more than fertilizer. They’re one of the most complete, biologically rich soil amendments you can add to your garden.
Here are the many benefits of adding worm casting to your soil:
Add Nutrients to Soil
Worm castings contain nitrogen, phosphorus, and potassium (NPK), plus tons of micronutrients. These nutrients are in a form that's more available to your plants than synthetic chemicals.
Enhance Plant Growth
Research has shown that plants grown with worm castings develop stronger root systems, produce higher yields, and demonstrate greater resistance to pests and disease compared to those grown in untreated soil.
Fight Plant Disease
The beneficial microorganisms in worm castings help your plants fight off soil-borne diseases like powdery mildew, blossom end rot, and other fungal pathogens.
Release Nutrients Slowly
One of the biggest benefits of worm castings is their slow-release nutrients. Traditional fertilizers (even organic ones) can break down quickly, causing nutrients to wash away before plants can absorb them. But worm castings contain nutrients that are wrapped in a protective microbial coating, allowing them to be released gradually as plants need them.
Improve Soil Structure
Worm castings supercharge soil biology. These tiny, nutrient-rich granules are loaded with beneficial microbes and enzymes that improve soil structure so that it's more porous. Because of their fine, crumbly consistency, worm castings help loosen compacted soil, improve aeration, and increase water-holding capacity. This means sandy soils stay moist longer, and clay-heavy soils drain more efficiently. Worm castings are, in fact, one of the best soil amendments for clay soil.
Prevent Pest Issues
Worm castings contain an enzyme called chitinase that dissolves the chitin of pests like whiteflies, aphids, and spider mites. You can mulch with worm castings to protect your plants from these pests without using a single pesticide.
Improve Germination
Worm castings not only improve seed germination, they can also protect your seedlings from damping off, a soil-borne disease common in indoor seed starting, according to research at Cornell University.
Provide Benefits to Plants in All Stages of Growth
I use worm castings for many gardening purposes, including top dressing, enhancing potting mixes, and adding nutrients to seed starting mixes.
With all these benefits, it’s no surprise that worm castings are one of the most sought-after organic fertilizers for gardeners and farmers alike.
How to Add Castings to Garden Soil
Most of us already know that seeing worms in our soil is a good sign. But here’s the thing: worms don’t just show up because you built a raised bed. They come when the environment says, “Hey, there’s something here for you.” Worms follow the microbial buffet. And earthworm castings are the invitation.
Microbes + Organic Matter + Worms = Living Soil
Here’s where the magic happens.
When you mix earthworm castings with compost, the castings introduce the microbes, and the compost supplies the food. This kickstarts the process of decomposition and regeneration. As the microbes multiply and feed on the organic matter in compost—leaves, stems, roots, and plant debris—they start transforming that material into nutrient-rich humus.
Once the buffet is set, the worms arrive. They eat, digest, tunnel, and aerate the soil. Their tunnels allow air and water to flow, and their castings keep enriching the space. This is the natural cycle that takes plain dirt and transforms it into living, breathing, fertile soil.
So what's the best way to add compost and worm castings to soil?
The 103 Soil Blend
Every time I install a new garden bed, I create a foundation of compost and soil, and then mix in a generous helping of earthworm castings. This is my 103 Soil Blend:
- 1/3 compost
- 1/3 topsoil
- 1/3 coarse sand
- a handful of castings for that life-giving extra 4%
This is the same blend I share in my book Kitchen Garden Revival, and recently, I delved deeper into the why and how inside my new course, Gardenary Soil School.
7 More Ways to Use Worm Castings
Besides putting worm castings in soil as you fill your garden beds, you can use worm castings in so many ways.
Add worm castings to potting mixes
Worm castings make an excellent addition to store-bought potting mixes. I typically fill pots with 1 scoop of compost for every scoop of potting mix, and then mix a scoop of worm castings into the top inch of soil before planting.
Welcome transplants with worm castings
Add about 1/4 cup of worm castings to the bottom of a planting hole when you're moving transplants to your garden.
Boost seed health in garden
When you're digging a trench for sowing seeds, just make the trench a little deeper than it needs to be. Sprinkle about 1/4 inch of worm castings in the bottom of the trench before sowing your seeds.
Top dress plants
Sprinkle some worm castings around the base of fruiting plants and vegetables every 2 to 3 months, or whenever they look like they could use a little nutrient boost.
Make Your Own Liquid Fertilizer
You can make worm tea—like compost tea but with worm castings—to provide a quick nutrient boost for fruiting plants, large veggies, and houseplants. Simply brew worm castings in water for 12 to 24 hours and then filter out the castings (you can add the castings to your garden or compost pile). Pour the worm tea into a watering can or spray bottle to use in your garden.
Start Your Compost Pile
Worm castings work as a great starter for compost bins and piles. The microorganisms inside stimulate the ecosystem of your pile. It's like using a scoby to start your next batch of kombucha.
Provide Nutrients for Seedlings
Plants don't need much right after they've germinated. But once they've produced their first couple sets of leaves, they benefit from additional nutrients. I like to add 1 part castings to every 3 parts of my organic seed starting mix to give my seedlings everything they need for strong, healthy growth. You can also top dress seedlings with castings if they need more nutrients before being transplanted to the garden.
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How to Source Worm Castings
Worm castings can be expensive to buy, but you can also make your own with a simple vermicomposting setup. You can find bags of worm castings at big box stores, garden centers, and local nurseries.
If you're shopping online, here are some of my favorite worm casting soil amendment products to buy on Amazon:
Worm Castings FAQs
What are the disadvantages of worm castings?
One disadvantage would definitely be the cost. While you can grab a 40lb. bag of compost or manure for under $10, just 1 quart of worm castings will set you back at least $10. You can't really buy worm castings in a quantity larger than 20 quarts, so this is not something easy to buy in bulk. Fertilizing a large vegetable garden with worm castings is definitely an investment in the health of your soil.
If you wanted to make your own (vermicomposting), you'd need to buy materials for the worm bin setup, plus the right worms. Then, you'd have to perform routine maintenance on your worm bin, which involves handling the worms.
What plants benefit from worm castings?
Worm castings are so rich in nutrients that pretty much every plant can benefit from an application. That includes vegetables, herbs, flowers, and even indoor houseplants. Worm castings will promote healthy growth, improve soil structure, and boost flowering and fruiting.
Elevate your backyard veggie patch into a sophisticated and stylish work of art
Consider this your modern guide to setting up and planting an edible garden that's not only productive, but beautiful, too. Kitchen Garden Revival will forever change the way you think about growing a little bit of your own food.
Can you use too much worm castings?
Unlike synthetic fertilizers, a high concentration of worm castings won't burn (or overstimulate) your plants. That being said, a little bit of worm castings goes a long way. Instead of filling an entire seed starting cell or pot with worm castings, you would just use a dash to promote plant growth.
Can you plant directly into worm castings?
On their own, worm castings don't provide good drainage for your plants, so that's one of the reasons why we mix castings with topsoil, compost, and coarse sand.
Final Thoughts: It Starts Below the Surface
Before you can harvest a single cherry tomato, sprig of basil, or head of lettuce, your garden has to come alive—beneath the surface.
Adding earthworm castings is a simple, powerful way to begin that process.
It’s not flashy. It’s not complicated. It may really gross you out.
But it’s the difference between dirt that sits there and soil that grows something. It's the most natural alternative to fertilizing plants, and it'll make all the difference in the size and quality of your garden harvests.
Add compost. Add castings. And let the worms do their work. Nature will take it from there.
Want to learn more about building life-filled soil from scratch?
Check out my book Kitchen Garden Revival, or join me inside Gardenary Soil School, where we break down everything you need to build a garden that grows with you—starting from the soil up.
Learn the Gardenary Soil Method
Get the step by step to create, maintain, and enhance your own organic garden soil inside Gardenary's Soil School.
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