How Much Worm Castings Should You Add to Soil?

Starting seeds and growing a garden is an economical way to produce food for your family and I want to share a tip I’ve learned over the years. Add organic worm castings (vermicompost) to your seed starting mix before sowing your seeds and you’re sure to have healthy seedlings pop up! But how much worm castings should you add to your soil?

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In order to have the most success with seedlings, among other things you should give them a proper diet.

Worm castings are a superfood for all plants. Seedlings all the way up to established plants will benefit if you add this nutrient-rich soil amendment.

I usually think of my seedlings as babies. They’re my baby plants and I want to take care of them so that they will grow big and strong! I noticed an improvement in germination rate when I started adding worm castings to my seed starting mix years ago.

What Are Worm Castings?

Earthworm castings (vermicompost) are the organic material (a.k.a. worm poop) produced by earthworms after they eat.

Earthworms eat organic matter including food waste, old plant material, even spent coffee grounds. If you’re a vermicomposter, you probably have a bin of red wigglers going.

Worm castings are a non-toxic and natural soil amendment that’s safe for children, pets, and pollinators. Just like compost (a.k.a. black gold), worm castings are in the realm of slow-release fertilizers because the nutrients are slowly released over time. It also improves the soil structure by improving soil aeration and water retention. Additionally, it supports a healthy and biologically diverse active soil food web thanks to all the beneficial microbes and fungi.

You may have heard that healthy soil equals healthy plants and it’s true. Worm castings help fight off plant diseases!

It also boosts chitinase (enzyme), humic acid (decomposed material), essential nutrients, and minerals. For example, it’s high in nitrogen, phosphorus, phosphate, potassium, potash, sulfur, magnesium, calcium, sodium, iron, manganese, copper, zinc, and boron.

Study: Growing Tomatoes in Worm Castings

I found a fascinating study published in 2017 by the U.S. National Institutes of Health (NIH) that compared (1) growing tomatoes in soil amended with vermicompost, (2) growing tomatoes in soil with chicken manure compost, (3) growing tomatoes in plain soil using traditional chemical fertilizers with urea, and (4) growing tomatoes in plain soil with no added amendments.

The study showed that the tomato plants grown in soil with vermicompost were significantly taller, with thicker stems, more fruit per plant, and the fruits were better quality. They also found that the worm castings improved the soil more than the other methods.

I can testify to the benefits of adding worm castings because my tomato plants grow by leaps and bounds every year!

Study: Growing Gourds in Worm Castings

Another study published in 2018 by the NIH compared (1) growing gourds in soil with vermicompost from cattle manure, (2) growing gourds in soil with only cattle manure, and (3) growing gourds in plain soil with no added amendments.

The number of gourds, the number of leaves, and the number of flowers was significantly higher when the gourds were grown in soil amended with worm castings. But also the nutrition and the number of seeds inside the gourds was higher!

There were 2.5 times more vermicompost gourds than cow poop gourds. And there were 20 times more vermicompost gourds than plain soil gourds! The mineral content in the roots, leaves, and gourds was also highest in the vermicompost gourds.

Did you know that heavy metals in soil are absorbed by plant roots just like nutrients and heavy metals can be found in roots, stems, leaves, and fruits?

I thought it was interesting that they mentioned long use of animal manure can lead to the accumulation of heavy metals in soil and negatively impact plant growth, soil microbial diversity and activity, and agricultural productivity.

But vermicomposting animal waste will bring down the heavy metal levels because worms bind them up when it passes through their bodies so that the heavy metals can’t be absorbed by the plants anymore! I didn’t know that worms could do that!!

How Much Worm Castings Should You Add to Soil?

While you can safely plant your seeds and mature plants in 100% pure worm castings, the general consensus is to use regular soil and add 20% of worm castings or less. This is because there’s not much of a growth difference when you add more than 20%. And unless you’re making a ton of vermicompost at home, worm castings are expensive to buy.

Here’s how I add worm castings to my seed starting mix.

When I’m getting ready to start seeds, I get out a bag of seed starting potting soil, a bag of worm castings, and a tray of pots ready.

Then I will get out my big Rubbermaid tub and a quart-sized plastic cup. My seed starting area is in my basement and so is my laundry room where there’s a sink.

Doesn’t everyone do laundry and start seeds at the same time?

I don’t usually measure the seed starting mix because the bag usually indicates if it’s 8 quarts or 16 quarts and I typically use an entire bag and then some when I’m starting seeds.

Following the 20% general guideline, I will add 2 plastic quarts full of worm castings for every 8 quarts of seed starting mix.

Then I will mix it together with my hand while adding water.

There’s no bad smell to the worm castings just like there’s no bad smell to finished compost. The worm castings are noticeably more granular and almost black when compared with the seed starting mix.

Ways to Add Worm Castings to Soil

There are a few different ways to add worm castings to your soil for improved plant growth and production.

Potting mix. Mix 4 parts of soil and 1 part worm castings to fill your seed starting trays and pots. This is what I always do when I’m starting my vegetable and herb seeds.

Add it to your planting hole. If you’re transplanting your seedlings outside, why not add some worm castings directly into the bottom of your planting hole to give the roots of your plants a great start?

It’s been shown that adding some worm manure to your planting hole reduces transplant shock and helps your plants deal with environmental stress.

Compost tea. You can make compost tea, or worm tea, by adding worm castings to a bucket of water and letting it sit overnight for best results.

It’s a good idea to put the worm castings into a porous bag like a pillowcase first, so that after the “tea” is prepared, you can simply pull the bag out and be left with rich compost tea. Then the castings won’t clog your sprayer when you spray the compost tea on your plants or directly on the soil at the roots.

Top dressing. As your plants grow, you can add some fresh worm castings directly around the base of your plants and scratch it in a little bit. This is a fast and easy way to feed the mature plants in vegetable gardens.

You can also use worm castings as a top dressing in flower beds and under fruit trees. It’s a great, natural fertilizer for any indoor plants, too!

A good rule of thumb is to add two pounds of vermicompost per ten square feet and then work it into the top of the soil gently. You can add it more often but it’s generally done once per growing season.

If you’re adding worm castings to mature bushes or trees, add a half pound per one square foot.

Leave a comment.

Leave a comment and let me know if you use worm castings. If you live in an urban area or if garden space is at a premium, I want to encourage you to add some worm castings and get more production out of your garden!

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