Worm Castings 101: Benefits to Plants and Soil

Welcome to your crash course in Worm Castings 101! If you’ve poked around our neck of the woods much, you likely know that we’re mega-fans of worm castings here at Homestead and Chill. We use this form of natural fertilizer extensively in our organic gardens – including for seedlings, raised vegetable beds, fruit trees, perennials, and more. If I had to choose just one, I’d say earthworm castings are my top-favorite soil amendment! 

Read along to learn answers to frequently asked questions about worm castings, including what they are, the benefits they offer soil and plants, how and when to use them in the garden, where to get worm castings, and more. If you’re looking for information on how to start your very own worm compost bin at home, see this step-by-step tutorial, followed by this guide on how to harvest worm castings. 

Homemade or store-bought, I know you’ll want to get your hands on some worm castings after hearing all these poo-tastic facts! So let’s dig in.

What are worm castings?

In the simplest terms, worm castings are worm poop. But keeping it that brief is greatly selling them short, because worm poo is extra-special excrement. As worms digest food or other material they consume, they break down complex nutrients into more bioavailable forms then found within the castings. When worm castings are added to soil, it provides valuable nutrition to plants as well as enhances the overall soil quality and structure. 

Because of the vast array of outstanding benefits they offer (described more to follow), earthworm castings are often called “black gold” in the horticulture world. You may also hear them referred to as vermicast or vermicastings. The prefix “vermi” means worm, so the process of composting with worms is known as vermicomposting.

Where do worm castings come from?

Worm castings used for horticulture either come from commercial compost worm farms, or from a personal worm bin that is maintained at home – like we do! All earthworms generate castings, but these specialized compost systems generally utilize a certain species of worms (Eisenia fetida, aka “reg wigglers”) that have a faster metabolism and larger appetite than common earthworms. This makes them a prized choice for vermicompost systems. They can really pump out the poo! We get our compost worms from Uncle Jim’s Worm Farm.

Depending on the size and type of worm farm, compost worms are kept in containers, bins, large beds or windrows. There, the worms are maintained and fed select food waste and/or green waste. After they work their magic to digest and excrete it all, the castings can be harvested and used or sold. Learn how to harvest worm castings from a home-scale worm bin here (video included). 

What do worm castings look like

To the untrained eye, worm castings look a lot like very rich uniform soil. Worm poops are tiny, oblong, dark brown to black dots (almost like coffee grounds, but soft and without sharp edges). A handful of well-maintained finished castings will be moist but not soggy, similar to the consistency of a wrung-out sponge. When clenched together in your hand, the castings should form a clump that holds together but also easily crumbles back apart. 

Sometimes there are leftover bits of food waste, bedding (e.g. shredded newspaper or dry leaves), worms, other insects, or debris mixed in the castings. That is totally okay and normal, though it’s best if the majority of the material is castings when you apply it to the garden. 

Why use worm castings? A breakdown of the benefits for plants and soil 

• Worm castings are a gentle, effective, natural fertilizer that provide essential nutrients to plants. Scientific studies show that using worm castings in the garden can help plants grow significantly larger, stronger, and produce more abundant and better-quality flowers, fruit, and vegetables. It can also increase a plant’s resilience to pests, drought, heat, disease, and other stress. 
  
  
• Because they’re slow-release, it’s incredibly difficult to “overdo it” with earthworm castings. But a little also goes a long way!
  
  
• In addition to nutrients, worm poo is loaded with beneficial bacteria and fungi that support other organisms in the soil food web – a very important component of organic gardening.
  
  
• Worm castings also improve soil structure by increasing aeration, improving drainage, and enhancing moisture retention all at the same time!
  
  
• By supporting worm farms or maintaining a worm compost bin at home, you’re also participating in a sustainable closed-loop system. Vermicomposting reduces the amount of garden and food waste going to the landfill and creates stellar free organic fertilizer instead. Talk about a double whammy of awesome. 

So the better question is, why not use worm castings?

Are worm castings good for all types of plants?

Absolutely! Because worm castings are mild yet nutrient-rich, all types of plants will benefit from a little worm poo lovin’. We use them on flowers, veggies, perrenials, houseplants, fruit trees, cannabis, herbs, for starting seedlings and more. 

Are worm castings considered fertilizer?

Yes, worm castings are an organic and 100% natural form of fertilizer… but that’s not all! They’re also a fantastic soil amendment. According to the University of California, “fertilizers improve the supply of nutrients in the soil, directly affecting plant growth”. This includes both chemical or synthetic fertilizers as well as more eco-friendly derived from natural materials. Related yet different, “soil amendments improve a soil’s physical condition (e.g. soil structure, water infiltration), indirectly affecting plant growth.”

Do they contain nitrogen?

Studies show that earthworm castings are rich in a variety of essential nutrients including nitrogen, phosphorus, and potassium (NPK) as well as iron, calcium, sulfur, and humic acid. However, because it can be difficult to determine the exact nutrient content of homemade worm castings (and it varies brand-to-brand), we consider them a supplement to our soil amendment routine and also use other natural slow-release fertilizers such as alfalfa meal, kelp meal, neem seed meal – or something like this organic all-purpose fertilizer. And don’t forget the mycorrhizae!

Can worm castings burn seedlings or plants?

Nope! Pure worm castings should not shock or burn seedlings or plants like some high-nitrogen fertilizers can. That’s one of the best things about them: earthworm castings are a gentle, slow-release fertilizer, despite the fact that they contain a higher concentration of nutrients than bulk compost. As food and other material passes through the worm’s digestive system, the castings are coated with a film of mucus that in turn slows the breakdown and release of nutrients in the soil. Like other slow-release fertilizers, you can safely apply a larger volume but less frequently than fast-acting fertilizers. 

When to add worm castings to garden soil 

There are numerous opportunities to use worm castings in the garden: add some to your seedling start mix, when filling a new raised garden bed with soil, directly in the planting hole when transplanting seedlings into the garden, sprinkled on top of soil (lightly scratched in) after planting, or around the base of established plants like a layer of mulch. We do all of the above!

In addition to using worm castings in their fresh natural form, consider turning them into compost tea! If you aren’t familiar with compost tea, it is exactly what it sounds like: a liquid solution or “tea” created by steeping compost in water. It’s a great way to make a small amount of compost go a lot farther. Once the nutrients in the compost seep into the water, you can feed many plants or garden beds with it. We love to make actively aerated compost tea (as opposed to passive steeping) which also boosts the beneficial microbes in the tea. Learn how to brew compost tea here!

How much worm castings do I use?

A small amount of worm castings can make a big impact! Yet again, it won’t harm anyone to add a little extra either, so don’t fret too much about precise measurements. We generally eyeball the following:

• Filling new pots or seedling containers: mix in about ¼ to ⅕ of the total soil volume in worm castings.
• Transplanting plants outside: Add a modest handful (about ¼ to ½ cup) per planting hole when transplanting small vegetable seedlings, a cup or two for larger shrubs (in one to three gallon pots), and several cups when planting trees from 5 to 15 gallon containers.
• Already-potted plants and houseplants: spread one inch of castings on top of the soil. Gently scratch it into the surface and then water it in. Refresh once or twice per year, up to every quarter.  
• Established garden beds (vegetables, flowers, perennials, and/or shrubs): top dress with 1 to 3 inches and gently incorporate into the top layer of the soil. Repeat in spring and fall. 

Which worm castings are best? Where can I buy them?

I have to admit, I have a soft spot for our homemade worm castings. They can’t get any fresher! Plus, we know the exact inputs to our system including the bedding and what the worms are fed. Since it’s all nutrient-dense and organic, the castings will reflect that in their quality.  

However, I realize not everyone has the ability or time to keep their own worm compost bin at home. Good thing there are plenty of other worthy options out there! Wiggle Worm is a great reputable company that sells pure worm castings online and in stores. They have stellar reviews for freshness, quality, and happy plants! Their worm farm is located in Wisconsin.

Last but not least, check around to see if you happen to have a local worm farm near you. We were fortunate enough to live a short drive from one for many years. It makes for a fun family outing to go pick up castings or worms.

Cheers to Black Gold!

Well, if you came to this post because you were curious about earthworm castings, I hope you’re leaving EXCITED about them – and with all of your questions answered! Please drop a comment below if there is something I didn’t cover. All in all, whether you make your own or buy worm casting, your plants and soil will thank you with bountiful growth in return. If you found the information in this article to be useful, please spread the worm poo love by pinning and sharing this post!

You may enjoy these related articles:

• Composting 101: What, Why & How to Compost at Home (6 different methods!)
• How to Create and Maintain a Simple Worm Compost Bin
• How to Make Actively Aerated Compost Tea
• The Benefits of Using Mycorrhizae in the Garden
• How to Amend & Fertilize Garden Soil Before Planting (No-Till Style)