Welcome to the Urban Worm Company's Guide to Worm Castings!

This article will do a deep dive on worm castings, nature's most potent fertilizer.

We'll detail the benefits of worm castings to your plants and soil and cover how to make, harvest, and apply worm castings. We'll also provide links to academic research and talk about how to judge the quality of worm castings.

Finally, for those of you hoping to purchase worm castings, whether in bulk or in smaller quantities, we'll provide several ways to do that.

What Are Worm Castings?

Worm castings are, in the most straightforward sense, worm poop. Also called vermicast in some circles, the term worm castings is understandably – but kind of incorrectly – used interchangeably with the term vermicompost.

Worm castings make up the “poop” fraction of vermicompost, a veritable poo-pourri of microbes, undigested organic matter, and yes, worm castings.

How Are Worm Castings Made?

Worm castings are produced through the process of vermicomposting, the consumption and decomposition of organic waste via earthworms and microbes, producing a humus rich in organic matter, microorganisms, and worm castings!

As organic matter is consumed by the worm, it travels the length of the worm's digestive tract before being excreted as a bacteria-enriched, pill-shaped cast.

This worm poop is coated in a mucus filled with even more bacteria.

(My apologies if you were eating while reading this!)

Worm castings can be produced at small scale in the home or in much larger quantities using indoor continuous flow vermicomposting systems, windrows, or adaptations of windrows.

This blog is dedicated primarily to vermicomposting, so you've got some incredible vermicomposting resources at your fingertips.

What Are the Benefits of Worm Castings in Plants and Soil?

Worm castings have seemingly magical benefits to both plants and soil.

But not all worm castings are created equal.

Differences in feedstock, maintenance of proper climactic conditions, and proper storage and handling practices can all greatly affect the benefits – or lack thereof – that worm castings can provide.

So let me start with the caveat that there is no guarantee that simply adding castings will give your garden incredible results. But reams of academic literature make a compelling case that worm castings have incredible benefits to plants and soil.

Soil Benefits of Worm Castings

Increased Organic Matter

The addition of worm castings – like the addition of other manures – gives the soil a boost of the organic matter that serves as a food source for bacteria, fungi, and other soil microbes. Organic matter should comprise 5% or more of healthy soil.

Soil organic matter is crucial for water retention

But modern farming techniques have helped rob the soil of this precious resource, reducing that percentage to 1% or less in some cases, resulting in worm-free, mostly lifeless soil. Adding worm castings, compost, or other animal manures is an excellent way to increase organic matter in the soil.

In other words, adding worm castings actually feeds the living, breathing ecosystem underneath your feet!

Soil Structure and Water Retention

Worm castings aid in soil aggregation, acting as a glue to help soil particles like sand, silt, and clay stick together creating the pose space between them to help store water. In fact, just a 1% increase in soil organic matter will allow a single acre of soil to hold and additional 25,000 gallons of water.

By making soil more absorbent, you help decrease runoff, topsoil erosion, and algal blooms as nitrogen-rich synthetic fertilizers will remain in soil rather than ending up in our waterways.

Nutrient Delivery – slow release and plant available due to microbes

While nitrogen is plentiful in both the atmosphere and in the soil, it is not plentiful in forms that plants can actually use, namely ammonia and secondarily, nitrate.

The microbial life within vermicompost is insanely effective at turning nitrogen into ammonia, and then into nitrates.

Benefits of Worm Castings on Plants

Faster Germination

Faster germination and seedling growth from worm castings
Worm castings produce faster germination and seedling growth!

While conventional wisdom says the seed should germinate with only its own nutrient reserves, the addition of worm castings to a seed starting mix is shown to improve seed germination.

Studies suggest that no more than 20% worm castings by volume should be used in seed starting mixes.

Quicker Seedling Growth

After a quicker germination, mountains of academic research suggests that seedling growth is also much faster in soil that has been amended with biologically-active vermicompost.

Stronger Root Growth

Black Diamond Vermicompost produced stronger root growth in the basil plant on the left.

A plant's root structure, responsible for pumping nutrients and water into the plant, can be much healthier in soil amended with vermicompost. Roots are thicker and will fan further out into the surrounding soil.

The accompanying image, courtesy of Black Diamond Vermicompost, shows the difference in root growth in basil plants. As you can image, the basil plant on the left was grown in a partial vermicompost mix.

Improved Pathogen Suppression

This is an interesting one. There are several mechanisms by which worm castings can help defeat the effect of soil pathogens. But when it comes to the common pathogen seedling-killing disease called pythium, the presence of vermicompost acts as a smoke screen.

Pythium zoospores normally sniff out both germinating seedlings and mature roots by detecting root exudates and then penetrating the seed or plant. But vermicompost creates such “noise” in the soil, that the zoospores cannot zero in on the exudate-producing root or seedling.

The accompanying image from a study at Cornell University shows 4 groups of cucumber seedlings. The left column shows growth in the seedlings that have not been inoculated with pythium. The difference in gowth between soil and vermicompost is certainly noticeable.

But check out the difference between soil and vermicompost in the right column where the cucumber has been intentionally inoculated with pythium. The results are stark.

Worm poop for the win!

Improved Pest Suppression

Aphids attacking a fruit tree

The presence of vermicompost in soil can repel unwanted pests like aphids and mealy bugs by way of chitinase. 

Chitinase is an enzyme present in some vermicomposts which breaks down chitin, a glucose derivative which forms the backbone of insect exoskeletons.

Studies suggest a 20-40% lower incidence of hard-shelled pest attacks on plants grown in soil treated with vermicompost.

Higher Yield

Strawberries, peppers, and tomatoes have shown a higher marketable yield when grown in 10-20% vermicompost mixes.

Most research around vermicompost deals with the resulting boost in yield from plants grown in soil that has been amended with vermicompost.

There's a dizzying array of data out there and it varies depending on plant species, what organic material is being vermicomposted, and more.

But the correlation between yield and the presence of vermicompost is crystal clear.

Better Pollination

This is a really interesting one.

Research indicates that bees pollinate plants grown in vermicompost-treated soil much more effectively.

They take less time to discover the plants, visit them more frequently, and stay for a longer duration on each visit.

How to Apply Worm Castings to Your Garden

The best way to apply worm castings depends on the stage of growth of the plant.

Worm poop is an incredible addition to soil, but there are no tightly-defined application rates as you might find in conventional synthetic fertilizers.

Most of the academic literature we've seen suggests that the lion's share of benefits are captured when vermicompost makes up roughly 10% of the medium. In other words, if your planting medium is 10 gallons, then 1 gallon of worm castings is likely enough.

At application rates approaching 20%, you will reach a point of diminishing returns where more worm castings will provide little added benefit. Beyond 20%, you may begin to see negative effects on the growth, yield, and health of your plants.

Results, however, are highly dependent upon the plant, its stage of growth, and the quality of the vermicompost applied.

Use the following rules of thumb to guide you in your worm castings application efforts!

This guidance is mainly borrowed from my friends at Black Diamond Vermicompost in Paso Robles, CA.

Germinating Seeds

Most of what a seed needs in oder to germinate is already inside the seed. But studies suggest that germination is faster in a soil medium that has been treated with a small amount of vermicompost.

In the case of a germinating seed, each seedling should only receive a pinch of vermicompost.

Transplants

image of seedling

Seedling transplants don't need much vermicompost either. When your seedlings are ready to be transplanted into the garden, each hole should get 1-2 teaspoons of worm castings.

For mature transplants, add a handful (probably about 1/2 to 1 cup) to the hole to reduce transplant stress and establish roots more quickly.

Established Plants

Expect to add 1 to 2 cups of worm castings to an established plant. Other guidance suggests 1/4 cup for every 6-inch- diameter of growing area. To apply as a top dressing, simply layer the worm castings on to the surface and scratch it in to get the goodies closer to the root zone.

UV light is the top killer of microbes in vermicompost, so getting it into the soil quickly is preferable to spreading it on top and simply leaving it alone.

Lawns and Turf

Guidance on worm castings application rates on turf are highly variable. Black Diamond suggests rates as high as one pound per square foot where other sources offer rates closer to one per per 10 square feet.

My general thinking is that direct application of worm castings to turf grass can be very expensive. Perhaps a more economical route would be a worm tea application, if it's available in your area.

Worm Tea Application

worm tea image

Worm castings can be relatively expensive, but one way to make castings go a long way is to produce worm tea. Actively-aerated compost tea (or AACT) is brewed over a 24-48 hour period using worm castings in a tightly-woven mesh bag suspended in water.

This water is agitated in order to introduce oxygen to the system to feed microbes.

To further boost the microbe population, worm tea brewing will often feature alfalfa, kelp meal, or fish hydrolysate to act as a food source for microbes. Molasses is also a common ingredient but can elicit a short- term boom and bust of microbe activity.

Worm tea can be applied as a foliar spray or root drench and actively-aerated tea should be used within just a few hours after brewing is complete.

Judging Quality in Worm Castings

Worm castings are not a commodity. There can be huge wide variations in the quality of the product you think you're buying. And without a microscope or expensive tests, it will be tough to know you're getting a quality product.

With that in mind, use the following characterisitcs

Nutrient Values

Expect worm castings to have an NPK value of 1-0-0

I mention this first because the metric that comes to mind for conventional fertilizer is nutrient level, specially N-P-K or nitrogen, phosphorous, and potassium.

But this measure is fairly useless when judging vermicompost which will normally have a 1-0-0 NPK value. Check commercially-bagged worm castings and you are likely to see this number.

The easiest way to explain the distinction between worm castings and fertilizer is that fertilizer feeds plants.

But worm castings feed the soil.

So you would not expect vermicompost to have high levels of nitrogen.

But you might find that what little nitrogen it does have will be in the ammonia form which can taken up by the plants.

As a measure of worm castings quality, N-P-K may be one of the least helpful, so let's get it out of the way!

Microbial Population

Beauty may be in the eye of the beholder.

But that beholder's eyes will need to be looking down the lens of a 400x microscope when it comes to worm castings quality.

Excellent after-the-fact results might indicate the presence of a healthy microbe population. But you'll need someone with microscope skills to prove you've got a thriving population of nematodes, fungi, flagellates, or the tiniest of microbes….bacteria.

My friend Heather Rinaldi of the Texas Worm Ranch has some thoughts on judging microbe populations in vermicompost, inspired by her studies of the Soil Food Web under Elaine Ingham.

When under a microscope, the sample should feature one beneficial nematode per slide and one large fungal hyphae every 5th view. You'll also want at least one aerobic protozoa every 5th view and a 1:1 fungi-to-bacteria ratio.

Bacterial dominant composts promote the growth of annuals, weeds, and lower succession plant species.

Heather-Rinaldi-Texas-Worm-Ranch
Heather Rinaldi of the Texas Worm Ranch near Dallas, TX

Composts or soils with a healthy fungal content benefit higher succession plans like trees, shrubs.

Heather mentions that her personal goalposts are “multiple beneficial nematodes/slide with at least one aerobic protozoa every 3 views. She's also looking for “at least one good quality fungal strand every 3 views, and the same 1:1 rato between fungi and bacteria.

I also want to see high levels of humic compounds, because humic acid helps with chelation (pulling large mineral particles through root cell walls) along with being food for fungi AND a benefit in remediating toxins like chemical or heavy metal pollution.”

Presence of Plant Growth Hormones

Some vermicomposts feature plant growth hormones, which can have an outsized effect on plant growth, root growth, and yield. Academic research is rapidly advancing when it comes to identifying these hormones which normally fall into the categories of:

  • Auxins
  • Gibberellins
  • Cytokynins
  • Abscisic acid

It's beyond the scope of this article – and my own science chops! – to get too deep into plant growth hormones.

So you're better off reading this easy-to-read resource from Untamed Science on the descriptions and functions of plant growth hormones.

Humic Acid Levels

Humic acids are effective in delivering both nutrients and water to the plant, and are critical to the plant's health when there are insufficient mycorrhizal (fungal) relationships in the root zone.

The positively-charged humic acids are attracted to the negatively-charged roots, allowing for more efficient nutrient uptake by the plant.

Moisture Content

Worm castings that have been stored on a shelf for a long time may have retained insufficient water levels, either killing the microbes or turning them dormant. Worm castings you buy should have a moisture content around 55-70%.

When moisture levels get too low, it can actually make the castings become hydrophobic. This makes it more – not less – difficult for the castings to retain water.

Color

This is a subjective measure of worm castings quality, but Heather is adamant that worm castings should have a brown, cocoa color.

Black worm castings, she says, indicate possibly anaerobic conditions during the vermicomposting process.

The worm castings we produce at the Urban Worm Company have a lighter color than the dark brown or black color of store-bought worm castings.

So we've made Heather happy with this one!

Granularity

The shape of the individual casts are not necessarily a measure of quality.

But it's important to know that quality worm castings can have different shapes and textures. For instance, red wigglers produce a more elongated, irregularly-shaped, stickier cast.

But African nightcrawlers produce a more rounded, granular cast that can flow from your hands like sand despite having excellent moisture content.

Visual Comparison of Worm Castings

The image below compares the Urban Worm Company's worm castings to those produced by our friends at Wisconsin-based Lush Farms. While you can see the clear difference in color and granularity, both of us are happy with our soil test results.

Red wiggler castings produced at Urban Worm HQ near Philadelphia. (Larger flecks of material are grain hulls.)
African Nightcrawler castings from our bulk supplier, Lush Farms in Watertown, Wisconsin

Below are the results from my most recent compost test. I also received a biological assay from Earthfort. I link to both below.

All measures for the compost test are within the range I would expect……except for my pH. This may be due to some fermentation of my very potent grain and wood chip mix in the early stages of our aerated static precomposting process.

This one is a puzzle because a pH that low should have some anti-bacterial properties. But as the tests show, my levels of biology, aside from a low fungal count are excellent.

I would rather treat the cause than the symptom. So instead of just adding lime, we're going to increase the airflow timer on the aerated static pile.

Unfortunately, it may be months before we know if that works.

The biological test result also indicate lower-than-desired (at least what I desire!) active fungal count.

Anecdotal evidence from my customers remains very positive for my worm castings.

So while this pH and evidence of anaerobic conditions in the aerated static pile gives me some concern, we at least have happy customers!

AnalysisUrban Worm ResultDesired Compost Range
Moisture66.88%40 – 60%
Total Nitrogen.95%1 – 2%
pH5.06.0 – 8.0
Soluble Salts (dS/m)5.02<5 (although research is evolving about higher levels)
Organic Matter22.13>20%
Total Organic Carbon11.07
Carbon-to-Nitrogen Ratio11.7:110 – 20%

Buying Worm Castings in Small Quantities

Garden Centers/Big Box Stores

Worm castings can be purchased in small quantities in person via gardening centers or big box hardware stores. But these castings will almost always be packaged in unbreathable plastic bags. And they were likely have been harvested months ago, likely reducing or killing the beneficial microbes inside.

These castings will likely be very consistent, but they may be consistently low quality.

Online Sellers

There are a growing number of smaller-scale producers selling worm castings online. Consistency and quality will be highly variable.

But it is a near certainty that most worm castings you can buy direct from small online sellers will be harvested more recently than those found in big box garden centers.

Local Sellers

While a local Google search may reveal a number of nearby sellers, you can also check out the Urban Worm Network. Here, you'll find a network of small-to-medium sized businesses and many will have freshly-harvested worm castings.

Buying Worm Castings in Large, Bulk Quantities

Castings in one-ton Super Sacks ready to ship to a client

Thanks to both the normal growth in interest in organic gardening methods and the COVID-inspired boom in pandemic gardening, bulk worm castings may be hard to come by. But entreprenuerial-minded composters are trying mightily to fill the gap!

Most purchasers of vermicompost in these quantities are buying for the purposes of making soil blends or for their own large-scale agricultural purposes.

Bulk castings are typically bought in 1000-2000lb quantities in large, white, sturdy shipping bags called Super Sacks. Because shipping 2000lbs is only marginally more expensive than 1000 lbs, you're often far better off to spring for 2000 lbs.

My bulk supplier in Wisconsin, Lush Farms is accelerating production rapidly and is currently selling castings by the ton. But depending on the season, you may find them in short supply!

Conclusion: Worm Castings Are Not a Commodity

Before Starbucks came along, most us saw coffee as a commodity.

We scoffed at the idea that someone would pay $4.00 for something you could get for 75 cents at the diner. Whether you're a Starbucks person or not, we can agree that coffee is no longer a singular thing.

Quality matters.

The idea that worm castings are a commodity is not helpful. It's not helpful to the small worm castings producers creating rich, biologically-active vermicompost as they try to compete with the lower-quality big boys selling months-old product in plastic bags.

It's also not helpful to the disappointed consumer who expected a bumper crop of tomatoes after adding store-bought worm castings.

Worm castings may be highly variable in texture, color, particle size, and properties like microbiology which can't be seen with the naked eye. And coupled with the differing methods of application, and differences in plant species and stage of growth, these variabilities may have an outsized effect on their performance in your garden!

Whenever possible, ask a vendor to produce test results. And if you're a vendor – or potential seller of worm castings, get your castings tested through your local extension office or a reputable lab like A&L Great Lakes or Midwest Labs.

For testing your soil biology, Earthfort can't be beat!

Whatever your interest in worm castings, whether it's to consume or produce, we wish you the best of luck!