This is Part 3 of a 3-part harvesting vermicast series. Be sure to also check out Part 1 Common Questions About Harvesting Worm Castings, and Part 2 How to Separate Worms from Their Castings.
Diverting our food scraps from the landfill and reducing our ecological footprint are both great reasons to compost with worms, but for many of us, an even bigger motivator might be the prospect of producing our very own top notch “black gold” - and for good reason!
In the first installment of our castings harvesting series, “Common Questions About Harvesting Worm Castings”, we hit on some of the key characteristics that make castings (also known as “vermicompost” and “vermicast”) so special.
Vermicast can:
- Increase the water-holding capacity, porosity and microbial biomass of soils.
- Significantly boost the growth of plants, even when all the plant’s nutritional needs are already being met!
- Offer protection against many plant pathogens, and even some arthropod pests.
- Have a major positive impact on plant growth/health even when very small quantities are used!
In part II of the series, “How to Separate Worms from Their Castings”, we explored specific harvesting methods in greater detail - leaving us with one major topic yet to explore: how to actually put worm castings (and their liquid extracts) to good use.
No spoilers needed today - this is of course the topic we’re exploring here!
Before we dig into the “how to”s of castings and their liquid kin, it’s important to look at the topic of castings “quality”. Unfortunately, it is beyond the scope of this article to fully flesh this multi-faceted, and important topic properly - hopefully something we’ll be able to do in the future - but it will be valuable to at least hit on some key points worth keeping in mind.
First and foremost, if there is one main take-away, it’s that not all worm castings are created equal - not even close!
The type of food and habitat materials used, the type of system, moisture / airflow balance, experience of the person doing the vermicomposting, duration of the vermicomposting process, and various other factors can all have a major impact on the quality of the end product.
Here are some other important “quality” related guidelines to keep in mind:
- The N-P-K and micro-nutrient profile of castings is only one piece of the “quality” puzzle - and not even the most important one. Most of the unique benefits have been linked to the microbiology and various growth factors found in the material.
- Be wary of claims not backed by actual data/research. E.g. Some suppliers claim to produce “100%” (“pure”) castings. This is virtually impossible to achieve, regardless of the system or harvesting/screening methods used. Some materials break down without going through the earthworm digestive system, and some materials don’t even break down at all before harvesting (e.g. highly resistant fibrous/woody materials, mineral-based materials etc). Castings (‘worm poop’) should absolutely make up the highest proportion of the end product, but claims of purity should be ignored (and the product potentially avoided).
NOTE: This probably helps to explain why many researchers in the vermicomposting field prefer the term “vermicompost” over “castings”.
- Bagged castings purchased from retail chains will almost always be lower quality than castings you make yourself or buy direct from a reputable supplier (one with a well-managed process and a focus on biology).
- “Quality” is also dependent on the planned usage. Are you growing blueberry bushes? Potted patio tomatoes? Giant pumpkins? There won’t necessarily be a single “perfect” type of castings for every situation.
None of this should leave you feeling stressed out or paranoid about using castings! We simply want our readers to be aware of some of the potential pitfalls and misconceptions associated with castings quality, as well as what makes them so special.
At the end of the day, most castings will offer at least some benefits for your plants, and their use will virtually always be a better choice than relying solely on inorganic fertilizers or using no amendments at all.
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Why the Microbes Matter
The incredible importance of microorganisms in castings is often overlooked, especially by those more familiar with using fertilizers to boost plant growth. Here are some good reasons why they definitely shouldn’t be ignored:
- They unlock nutrients in the soil, making them more plant-available.
- They work closely with plant roots - exchanging nutrients for sugars.
- They help suppress plant pathogens.
- They improve soil structure. Microbial glues help to form aggregates, which help with both water-retention and airflow.
- They produce plant growth promoters and regulators.
- Microbial biomass itself serves as a pool of slow-release nutrition for the plants.
Circling back to the quantity of castings you need to succeed…
When we go on and on (and on) about the fact that “a little goes a long way”, it’s for good reason!
Various academic studies have demonstrated that application rates even as low as 1-5% can have significant positive effects on plant growth and overall health. (Nonthapa et al., 2024; Massa et al., 2024; Arancon et al., 2004 - among others).
A reasonable guideline to keep in mind (for soil mixes etc) is aiming for ~ 5-20%. Once you get up past this range, you may start seeing the “law of diminishing returns” kick in, and eventually it’s not uncommon to end up with too much of a good thing when it comes to castings.
Numerous research studies have found that when concentrations of castings go up - especially above 40 or 50% - you can start to see a negative impact on plant growth. (Lim et al., 2015; Blouin et al. 2019 - among others)
NOTE: Just keep in mind that these sorts of attributes can once again vary quite a bit from one type of castings to the next. E.g. You are more likely to see the negative effects of castings from a smaller indoor system than castings from an outdoor system exposed to the elements when you’re using higher concentrations.
OK, with the important preliminaries out of the way, it is finally time to explore different ways you can put your castings to good use!
Simple Ways to Use (Solid) Worm Castings
Potting Mixes
One of the easiest, and most beneficial ways to put your castings to use is by making amended potting mixes. These blends can usually provide fairly consistent/predictable (and often impressive) growth benefits in pots, planters, raised beds, and seed-starting mixes.
Remember, castings biology tends to be highly correlated with the positive impact the material can have on plant growth/health. Make sure to protect that biology by avoiding any sort of solar/heat sterilization (e.g. keeping mixes in plastic bags out in the summer sun), providing some airflow, and avoiding the use of fertilizer-laden bagged soils as an ingredient in your blends (organic potting mixes work great).
We highly recommend experimenting with your castings % in these soil mixes but, as touched on earlier, aiming for ~ 5-20% (by volume) should be a great starting place! This works out to just over ¾ cup of castings per 1 gal of potting mix for a 5% application rate.
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In Planting Holes
A very easy way to use castings beneficially is by simply dropping modest amounts down in the bottom of your planting (or seed starting) holes or furrows. No need to make this into an exact science either! A small scoop/handful or two in smaller holes is great, and a small shovel-full for larger plants like young trees and shrubs should do the trick.
This should help to stimulate root growth significantly and establish a diverse community of beneficial microorganisms that can help your plants get the nutrients they need, while also potentially helping them ward off pathogens and pests!
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As a Top-Dressing
Even if you have castings down in the root zone, it never hurts to also add a thin layer on the soil surface around the base of your plants (or over top of where you’ve added seeds). This way you’re helping to ensure the beneficial biology is well-distributed, and that the plants are also getting an extra little “tea” boost every time you water.
This can be a great option for any type of potted plants (houseplants, planters etc) and garden plants. For outdoor plantings it is always best to cover the castings with some form of mulch to protect the microbes from UV radiation and drying out.
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Starting Seeds
We’ve touched on this one already, but it’s a topic worth looking at it in a bit more detail, since there are some important things to keep in mind.
- Seeds can vary widely in terms of their germination requirements and preferences - so don’t ever assume that castings are automatically going to boost germination rates (this can definitely be hit and miss).
- To compound the issue a bit, the real benefits of the castings may not show up until later in the plant’s lifecycle! E.g. you might actually see slower germination rates, but much more vigorous growth and vitality in the actual plant itself.
The key is to: 1) make sure you familiarize yourself with the specific requirements of the seeds you are working with (and try to track down relevant information relating to using castings with those seeds), 2) not be afraid to experiment with different approaches!
An easy starting place is to create high-quality seed starting mixes with a somewhat lower % of castings - say 5-10%, and take things from there.
NOTE: If our coverage of solid castings usage feels a tad“basic”, it’s because using worm castings effectively is a lot easier than most people think! Use high quality castings, in the way we’ve outlined, and you should see some great results! No need to overcomplicate it!
OK - it’s time to look at a closely related topic that can get a bit more complicated: liquid castings extracts!
“Worm Pee”, “Worm Tea”...and Other Terminology 😉
New vermicomposters who spend any amount of time trying to learn more about vermicomposting online can quickly end up overwhelmed with all manner of confusing terms and contradictory information - not to mention some pretty fierce debates.
If there is one sub-topic where this is especially bad - it’s the topic of “teas”!
For our purposes here, “tea” is being used as a blanket term for all liquid castings extracts, regardless of how they are produced. As you’ll see - similar to castings (likely even more so) - the range of “quality” in these liquids - in the context of plant growth and health - varies a lot!
On that note, a great starting place is to make our way through some of the common (even bizarre) terminology that can get tossed around.
“Worm Pee” - This is a (thankfully) less frequently used term that implies that the drainage liquid from vermicomposting systems is actually some form of beneficial earthworm urine. This is not the case at all - earthworms don’t actually pee (they excrete wastes in other ways). This is really just a misleading name for…
“Worm Teas” & “Leachates” - Both of these typically refer to the liquid that drains from a worm bin (a very common example being the liquid that ends up in the reservoir of stacking tray systems). People who see this liquid as a resource tend to use “worm tea”, while the critics (or at least those offering cautions) tend to use “leachate”.
Here at PC we prefer the term “leachate” simply because it’s the most accurate word to describe the liquid that drains from a composting system. In terms of its potential for (safe) use on plants, age matters a lot - along with other factors like oxygenation and dilution. It’s also worth pointing out that a lack of leachate from a system can actually be a good sign that moisture is being managed properly (just make sure conditions aren’t getting too dry)!
The older a worm composting system is, the higher the concentration of more stable, humified material (i.e. compost). So, naturally, the more stable and potentially-beneficial the drainage liquid will be (assuming that favorable conditions have been maintained).
The leachate draining down in a newer system mostly comes from decomposing, water-rich waste materials, so it will have a much higher potential for going anaerobic (quickly) and producing phytotoxic compounds after sitting and stagnating in a reservoir or catch tray. We recommend not using this “young” leachate, especially if it has a foul odour.
Even leachate from mature systems should be treated with a certain amount of caution. We recommend diluting it (at least 5:1) and only using it for outdoor ornamentals, shrubs and trees. It can also be added to a backyard composter. Always avoid using leachates that have a foul odour.
Worm Castings Teas & Castings Extracts - The key with high quality teas and extracts is to make them using high quality, finished castings, so we prefer terms that include reference to this. Both of these are technically liquid extracts, but there are some key differences between them (we will getting into in the next section). In case it isn’t obvious - these are the ones we recommend people make and use, not the various types of leachate.
Worm Castings Teas (“Vermicast Teas”, “Vermicompost Teas”) vs Castings Extracts
Once again, we don’t have an official, standardized glossary or rule book to help us out, - anyone can call something whatever they want - but a widely accepted distinction between these two among serious compost producers is that:
- Teas involve a brewing/extraction process - with vigorous aeration - along with the addition of certain “microbe foods” to help good microbes proliferate.
Extracts simply involve some form of (often specialized) extraction process.
- Again, yes teas are technically “extracts” too - but note the extra importance of aeration and additional microbe feeding.
There is considerable debate in the composting world re: extracts vs teas - in terms of which is “better” - and we’re not here to take a major stand, but some of the potential limitations/challenges relating to compost teas include:
- A more involved preparation process
- The potential for boosting the growth of the wrong microbes (e.g. pathogens)
The much more limited shelf life of the end product due to the added nutrients
- For experienced composters/vermicomposters - especially those who test their composts for pathogens, and who use high-end brewing equipment - brewing actual teas can offer some additional benefits. Apart from the boost in populations of beneficial microbes, it’s been said that teas can end up with more microbial “glues”, potentially making them a better choice for foliar applications.
All of this may beg the question…
What’s the best option for the average vermicomposter (or at least someone interested in testing the waters with castings teas & extracts)?
We recommend:
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Keeping things as simple as possible - There is no need to purchase expensive tea brewing or extraction equipment, or get too bent out of shape about doing everything “perfectly”. In light of this we also recommend…
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Opting for extracts vs brewed teas - Not only are they easier and faster to make, but you also greatly reduce the chances of ending up with a bloom of pathogens (or phytotoxins if you don’t provide sufficient aeration) in the end product.
- Making or buying really good stuff - Put more time and energy into producing high quality castings - or finding a good supplier if you don’t want to set up a system / wait for results - than trying to produce lots of “worm tea” (leachate) with your active systems.
Start with the highest quality worm castings you can make or purchase - When making your own castings, really take the time to learn the best methods for producing this material in a way that boosts microbial diversity (especially with an emphasis on increased fungal biomass).
The good news is that this is a major priority here at PC (and we have the microbiological results to prove it). All our educational content is written from this perspective - so be sure to spend some time reading our vermicomposting articles (and related resources) to learn about effective strategies.
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Use an extraction bag with 400-600 micron mesh - You might assume something like an old pillow case is a great option - and yes, you can indeed make an extract with beneficial nutrients, plant-growth promoting compounds and smaller microbes, but the openings are actually too small to allow a lot of the beneficial microbes through. Some paint strainer bags may fall into the right range.
Some possibilities include:
NOTE: In a pinch, something like a burlap sack could work too. The holes are on the big side, but at least you are ensuring that plenty of your amazing biology is making it into the extract. Just keep in mind that you will end up with a lot more particles in your extract, which can greatly increase the chance of clogging sprayers (if doing foliar applications), or even some watering cans.
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Use de-chlorinated/filtered tap water, well water, or rain water - We want to protect the microbes as best we can, so it is best to avoid water that contains anti-microbial chemicals. It’s important to mention that a lot of regions now use chloramine which, unfortunately, can’t be off-gassed the way chlorine can. In some cases, a really good filtration system may be the only way you can be sure the water is safe for microbes.
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Aim for 1-3 lb (~ 0.45 - 1.36 kg) of castings/compost for every 5 gallons of water - We want our extract to be as rich in biology as possible! That being said, just a friendly reminder that a little goes a long way when it comes to castings. So, work with whatever amount you happen to have on hand!
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Gentle massage vs vigorous shaking - Once again for the sake of limiting harm to our microbial friends, it is best to gently massage the material in the bag by hand until the water has a very rich dark brown colour.
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Waste not, want not - The extract definitely isn’t the only beneficial product from the extraction process! Make sure you also use the left over castings and any sludge that accumulates down in the bottom of your extraction vessel.
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Aim to use the extract within a couple of hours (assuming no aeration) - There are many who claim that you can basically store extracts for as long as you want. While this is somewhat true as far as the beneficial compounds and nutrients in these liquids go, in order to take care of all that beneficial aerobic biology, we are best to stick with a tighter time-line! NOTE: you can extend this window somewhat with aeration, but you should still use the extract the same day you make it.
If you follow these guidelines, alone, you will likely end up with extracts that are far superior to what even a lot of much more experienced vermicomposters are producing and using!
Before we chat about using extracts, it’s important to put things in perspective here. We realize that some of the information shared in this article might make castings/tea production and use seem a bit intimidating - maybe even something not worth bothering with, since you’re worried you will get it “wrong”.
As with many other aspects of composting, we encourage you to use the “Good - Better - Best” model for assessing your situation!
It’s (really) “Good” when you are even interested in vermicomposting and are wanting to put castings and liquid extracts to use!
It’s obviously “Better” when you actually try things out! It doesn’t need to be perfect. You can make mistakes, use methods/products that vary from recommendations…just generally “trial and error” your way towards getting some great results. NOTE: this is honestly where most people are - and that’s great!
“Best” is reserved for those who weren’t afraid to get out of their comfort zone and try things out - the compost geeks and pros who are now ready to really get things dialed in. Rest assured, you can easily achieve a lot of success without fully reaching this level!
How To Use Castings Extracts
Soil Drenches - This is likely the one approach where extracts really shine! You can think of this as an effective way to give established plants (whether in the ground, in planters, or in pots) a major microbial injection down in their root zone. Helping to establish a rich diversity of beneficial microbes in the root zone can greatly help your plants to make use of nutrients in the soil, and ward off pathogens - among numerous other benefits.
Worm castings can often be in somewhat limited supply (especially for small-scale home-based worm warriors), and we know that small amounts can provide big benefits, so don’t assume you need to be adding a lot of extract on a regular basis. In fact, even a single application can have a significant (lasting) impact on the growth and health of your plants!
And don’t hesitate to dilute your top notch extracts for the sake of being able to use them on more plants at once. (In the next section we break down our extract-making recommendations from earlier in terms of the concentration of the extracts you are going to end up with).
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Seed Starting - Once again, we will start by reminding you that there can be a lot of variability among different seeds in terms of how they respond to castings and extracts. We recommend getting familiar with the particular preferences/needs of the seeds you happen to be working with.
If you are working with seeds that respond well to soaking, using extracts can be great for this purpose. Similarly, extracts can be great for watering your seed starting media as well (potentially a better option than using solid castings).
Research seems to suggest that somewhat weaker extracts can be better for starting seeds. Arancon et al. (2012) found that soaking tomato seeds in 1% and 5% extracts had a positive impact on germination and seedling growth, while higher concentrations (10% and above) didn’t have a significant positive impact, or even had a negative impact on the success of the seedlings. What’s interesting, is that the lowest concentrations seemed to increase germination rates, while the somewhat higher concentrations seemed to promote more seedling growth.
To illustrate, using our recommended castings:water ratios (1-3 lb / 5 gal):
- 1 lb in 5 gal is ~ 2.4% (1:42)
- 2 lb in 5 gal is ~ 4.8% (1:21)
- 3 lb in 5 gal is ~ 7.2% (1:14)
Our suggestion would be to further dilute these somewhat (1:2 or 1:3 - extract:water should work well - but up to 5:1 should still leave you with a great seed starting liquid)
Once seedlings become well established, you should be ok to use the regular strength extracts!
NOTE: Always remember that the type of castings being used can play a major role in determining the properties of the extract. (e.g. chicken manure castings may have a much higher EC value than castings from a system that received a lot of bedding and modest amounts of kitchen scraps). This is a big part of why we recommend erring on the side of low concentrations of castings and extracts, especially when just starting out! You can always adjust from there! 😎
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Foliar Sprays - As touched on earlier, brewed castings/compost teas are often touted as a superior option for foliar applications, since they may offer more “stickiness” (adhering to plant surfaces more readily), but that’s not to say that extracts can’t be beneficial for this as well. This is another case where diluting your extracts may be a superior approach - not only so you are able to get a lot more coverage, but also simply to make sure you avoid “too much” of a build up of fine organic matter and microbes.
PRO TIP: If doing foliar applications outdoors, it’s best to avoid days/times when rain or hot, direct sunlight are expected (UV and drying being the main hazards). As always, we want to at least give our microbes a fighting chance! Timing your applications for early mornings, evenings, and on overcast (but not rainy) days can help with this.
A word or two about spray bottles - The process of spraying extracts and teas has the potential to be pretty traumatizing for microorganisms, especially certain groups like fungi, so you want to make sure you are using a sprayer that’s as gentle as possible.
You want to avoid high pressure models (aim for less than 70 psi), very fine mist/fog sprayers (remember we need to make sure openings are bigger than 400 microns), and any sort of electrostatic sprayer. Hopefully it goes without saying that you should never use a sprayer that has been previously used for spraying harsh chemicals (e.g. herbicides).
For small-scale applications (e.g. houseplants, planters etc), regular plant misting bottles (ideally ones specifically intended for plants) using a moderate nozzle setting may be OK. Just keep in mind that these can clog up pretty easily if you’ve sieved the castings using mesh with openings larger than 400-600 microns, and that some spray bottles may have too fine a spray to let all the beneficial microbes through.
Flushing out sprayers with a weak hydrogen peroxide solution (and then water) between uses can be very helpful for ensuring you don’t end up with a build up of unwanted microbial biofilm and other gunk.
NOTE: We’ve purposely avoided sharing exact quantities of extract to use, since there are so many variables involved. This is definitely yet another situation where experimentation (maybe starting with weaker extracts), keeping things simple, and not getting too concerned about “perfection” are all very helpful!
SUMMING UP
- Worm castings possess a number of beneficial properties that are unique in the world of compost.
- Two key benefits are that: 1) they have been found to significantly boost plant growth even when a plant’s nutritional needs are being fully met (indicating that the value of this material extends well beyond N-P-K), and 2) relatively small amounts are needed (especially in comparison to most composts) to see major benefits.
- Much of the unique value of worm castings can be linked to the microorganisms involved in the process and found in the end product.
- Really small amounts (as little as 1-2%) have been shown to have significant positive impact on plant growth and health, but application rates of 50% or more can reduce benefits or even have a negative impact (although, less likely with outdoor systems exposed to the elements).
- Castings quality can vary widely - depending on a wide range of factors - but most should provide at least some benefits.
- Aiming for 5-20% castings in soil mixes, and modest “handfuls” (or “scoopfuls”) down in planting holes or as a top dressing around the base of a plant can work very well.
- Many people view “worm teas” (leachate) as a highly valuable liquid amendment for plants. We encourage caution - using only from mature systems / dilution / outdoor ornamental plants, and completely avoiding the use of leachate from newer systems, and any leachate with a foul odour.
- Teas and extracts made from high quality castings will almost always be far superior to drainage liquids, so we encourage everyone to focus on making/purchasing high quality castings as a starting place.
- It is generally accepted that “compost teas” involve a brewing process with vigorous aeration and “microbe foods”, while “compost extracts” simply involve some form of specialized extraction process. The latter may be a better option for the average home composter, since they tend to be easier to make and there is less chance of ending up with pathogens in the finished product.
- It can be relatively easy to make good quality extracts at home using a bucket, a fine mesh bag (400-600 microns), and ~1-3 lb of good quality compost.
- This undiluted extract can be an excellent soil drench for well-established plants, but it’s a good idea to dilute extracts when you have a lot of plants (need more coverage), are starting seeds, or are using them as a foliar spray.
- Making and using castings effectively isn’t as complicated (or prone to major errors) as you might think - and the rewards can be amazing!
REFERENCES CITED
Arancon, N. Q., Edwards, C. A., Atiyeh, R., & Metzger, J. D. (2004). Effects of vermicomposts produced from food waste on the growth and yields of greenhouse peppers. Bioresource Technology, 93(2), 139–144.
Arancon, N. Q., Pant, A., Radovich, T., Hue, N. V., Potter, J. K., & Converse, C. E. (2012). Seed germination and seedling growth of tomato and lettuce as affected by vermicompost water extracts (teas). HortScience, 47(12), 1722–1728.
Blouin, M., Barrere, J., Meyer, N., Lartigue, S., Barot, S., & Mathieu, J. (2019). Vermicompost significantly affects plant growth: A meta-analysis. Agronomy for Sustainable Development, 39(4), Article 34.
Lim, S. L., Wu, T. Y., Lim, P. N., & Shak, K. P. Y. (2015). The use of vermicompost in organic farming: Overview, effects on soil and economics. Journal of the Science of Food and Agriculture, 95(6): 1143–1156.
Massa, D., Prisa, D., Incrocci, L., & Maggini, R. (2024). Low doses of vermicompost as a sustainable amendment for lettuce cultivation. Horticulturae, 10(4), 418.
Nonthapa, A., Iwai, C. B., Chankaew, S., & Falab, S. (2024). Dual-purpose vermicompost for the growth promotion and suppression of damping-off disease on potted vegetable soybean. Plants (Basel), 13(12), 1607.
Helpful Related Resources
Common Questions About Harvesting Worm Castings
How to Separate Worms from Their Castings
What Is The Best Way to Store Finished Worm Castings?
How to Feed Your Worms to Get The Best Results From Your Worm Bin
Living Materials
Bedding - The Most Important Material in Your Worm Bin?
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