How to Make Worm Castings + Benefits for Plants and Soil

It’s a known fact that you should make a point to regularly fertilize your garden, but many people aren’t sure how to go about it without using chemical fertilizers. We’re going to outline exactly how to accomplish this task using worm castings, touch on how they help your garden, how to apply them, how to make them, and more below.

1 Earthworms
You can easily get worms from your own yard to create your worm castings or you can buy them from your local bait and tackle shop. Earthworms! By Yun Huang Yong / CC BY 2.0

Worm Castings—The Basics

Worm castings is a fancy way of saying worm poop. These castings are some of the richest natural fertilizer you can get, and they have a huge list of nutrients and minerals in quantities that outdo other organic manures and materials. The nitrogen, phosphate, and potash levels in worm castings are through the roof, and they all contribute to strong plant growth.

Livestock manure will add volume to your soil, but it’s easy to accidentally burn the plants if you add them in excess or before you give the composting process time to finish. Manures are a slow-release fertilizer, and worm castings are the opposite as they are immediately bioavailable to your plants when you apply them.

Also, manure has a higher chance of being contaminated with herbicides, and this can destroy the soil and result in a failure for your crops. In some instances, this is leading backyard compost bins to become contaminated with herbicide, even if you don’t use them in your garden.

Many gardeners also buy bulk or bagged compost without fully understanding that even compost soils that are organic approved can have herbicide contaminants in it. While stray, manure, hay, and grass clippings tend to have herbicide contamination frequently, food scraps don’t, and this is why keeping a worm bin is a smart idea.

There is no maximum amount of worm castings you can apply safely to your garden, so you can’t apply too much. Even a tablespoon of worm castings for every plant would be enough to improve the plant’s growth habit and health, and it will boost the amount of beneficial organisms present in the soil.

Applying worm castings is a way to protect your plants from diseases and help them grow quickly by forming relationships with the plant’s roots. They also ensure that the soil retains moisture.

How to Make Worm Castings

Worm castings come about through a process known as vermicomposting. This is where earthworms and microbes consume and break down organic waste to give you a humus with a high organic matter content with worm castings. The worm will eat the organic matter, the matter travels through the worm’s digestive tract before it excretes it as a pill-shaped, bacteria-enriched cast.

The worm poop has a mucus coating that has even more bacteria. You can produce them in small quantities at home or in large quantities using an indoor continuous vermicomposting system.

2 Making Worm Castings
Making worm castings isn’t hard, and you can even set up a worm bin in your greenhouse or garage with minimal effort. Earthworms by Amit Patel / CC BY 2.0

Benefits of Worm Castings

Worm castings have a host of benefits for the soil and plants. But, not all worm castings are as effective as others. Differences in the climate, feedstock, and proper storage and handling practices can impact the potential benefits that these castings can provide to your garden. So, there is no 100% guarantee that adding castings will give your garden fantastic results. However, if you get it right, you can reap the following benefits:

Faster Germination Times

Conventional wisdom will tell you that your seeds should germinate using its own nutrient reserves, but adding worm castings to your seed starter mix will improve your germination ratios. For seed starting mix, you should try to add around 20% or less worm castings by volume for the best results.

Higher Yield

Most research surrounding vermicompost highlights the plant yield boost when you grow them in vermicompost-enriched soil. Your success rate will vary based on the plant species you add and what material you’re vermicomposting.

Improved Pathogen Suppression

There are several ways that worm castings can help neutralize soil pathogens that can harm your plants. But, when you’re talking about pythium, this is a common seed-killing disease, having vermicompost works like a smoke screen. The zoospores will find mature roots and germinating seeds by detecting root exudates to penetrate the plant or seeds. Vermicompost wll add so much “noise” to the soil around the plants that this disease can’t find the seedlings or roots.

Improved Pest Suppression

Vermicompost in your soil can repel an array of unwanted pests like mealy bugs or aphids using chitinase. This is an enzyme that you’ll find in some vermicomposts that breaks down chitin, and this is a glucose derivative that forms exoskeletons for insects. So, this can ward off hard-shelled pests by 20% to 40% when you apply it.

Increased Organic Matter

Adding worm castings, just like any other type of manure, gives the soil an organic matter boost that works as a food source for fungi, bacteria, and microbes. In a healthy soil, the organic matter should be 5% or more. However, modern farming techniques greatly reduced this resource in the soil, and the percentage can drop as far as 1%. In turn, you’ll end it with a virtually lifeless soil that is worm-free. Adding compost, worm castings, or other manures is a great way to boost the organic matter in the soil.

Nutrient Delivery

Nitrogen is very plentiful in the soil and the atmosphere, but it’s not abundant in forms that plants can readily use, including nitrate and ammonia. The microbial life inside of worm castings is very effective at taking nitrogen and turning it into ammonia before turning it into nitrates the plants can use.

Quicker Seedling Growth

After a much faster germination process, scientists found that seedling growth will also be much faster than normal if they’re in soil that you added worm castings to. In turn, you can get a faster harvest.

Soil Structure and Water Retention

Worm castings help with soil aggregation, and they function a little like glue to help various soil particles, including clay, silt, and sand to stitch together and create a space between them to hold water. Just a 1% increase in the organic matter in the soil can allow one acre of land to hold 25,000 gallons of water. By making the soil much more absorbent with worm castings, you decrease topsoil erosion, runoff, and algae blooms because nitrogen-rich fertilizers will stay in the soil and not wash out.

Stronger Root Growth

The root structure on the plant is responsible for pumping nutrients and water into your plants, and the roots will be much healthier if they’re in soil with worm castings. Roots are much thicker, and they’ll fan out much further into the soil.

3 Worm Casting Benefits
There are several benefits to applying worm castings, but they benefit both the plants and the soil, unlike fertilizer that only feeds the plants. Worm Castings by Doug Beckers / CC BY-SA 2.0

How to Apply Worm Castings

Worm castings are great additions to your soil, but there are no exact application rates with it like you’d get with traditional synthetic fertilizer. Most of the research done into worm castings suggests that you have them making up right around 10% of the medium. So, if your planting medium is 10 gallons, you’d add a single gallon of worm castings to enrich it.

If your application rates get around 20%, you’ll notice that there are diminishing returns on your worm castings, and adding even more won’t benefit the soil much. Over 20%, you can actually see negative effects surrounding yields, growth, and the health of your crops. The results are very dependent on the plant, the growth stage, and the quality of your worm castings when you apply them.

Established Plants

To an established plant, you should add one or two cups of worm castings, or you could add ¼ cup for every growing area that is six inches in diameter. To apply it as a top dressing, all you have to do is layer the worm castings on the surface and scratch it to get it much closer to the root zone. UV light is the number one killer of microbes in worm castings, so getting it safe in the soil very fast is better than spreading it on top and leaving it be.

Germinating Seeds

Most of the resources your seeds need to germinate are already in the seed. However, adding worm castings to the starter compost can result in quicker germination. In the instance that you want to add it to germinating seeds, you’ll only need a small pinch of the worm castings around the soil.

Lawns and Turf

There is no hard and fast amount of worm castings to use when you’re dealing with lawn or turf, unlike when you use synthetic lawn fertilizers. Some companies recommend a ration of one pound of worm castings per square foot while others increase them to one point of worm castings per 10 square feet. As you can see, this can get expensive very quickly, so another option is to add a worm tea instead.


Seedling transplants won’t require a lot of worm castings to do well. When the seedlings are ready to transplant into your garden, add a teaspoon or two into each hole. Mature plants will require slightly more worm castings at a ratio of ½ to 1 cup to the hole to help reduce stress and encourage quick root establishment.

Worm Tea Application

Even though worm castings are relatively inexpensive, one way to make them go a long way is to create a worm tea. AACT (Actively-Aerated Compost Tea) gets brewed over a 24 to 48-hour period by putting worm castings in a tightly-woven mesh bag and suspending it over water. The water gets agitated over the brewing time to introduce oxygen to the system to feed all of the microbes.

To increase the microbe population more, this method often features the use of kelp meal, alfalfa, or fish hydrolysate to work as a food source. Molasses is also a common additive, but it can encourage a very short-term growth explosion for the microbes that then die off. You can apply worm tea as a foliar spray or root drench, and any actively aerated tea should get used within a few hours of finishing the brewing process.

4 Worm Casting Applications
There are several ways you can use worm castings to benefit your plants, and it’s easy to apply it to several different plant types. Seedlings by Derek Bridges / CC BY 2.0

How to Make Worm Castings

You’ll need earthworms to make worm castings. This whole process is relatively easy and straightforward as worms aren’t high maintenance. You’ll also need to get a few shallow wooden boxes or small plastic bins to keep your worms. As a guide, each box should be roughly 10 inches deep because worms thrive in humid, warm, and dark environments. So, the goal is to recreate this as closely as possible. We’ll outline the basic steps to follow to create your own worm castings below.

Building a Worm Bin – Step by Step

  • Drill roughly 50 holes in the bottom of each bin you use using a ¼-inch drill bit. This works for drainage, and it’s how the worms will migrate upwards to the empty bins as they need to.
  • Drill roughly 60 holes just under the top edge of the bin using a ⅛-inch drill bit, and drill 50 holes in one of the lids. These holes will act like ventilation.
  • Add half of your shredded paper to the bin that you moisten using a spray bottle. You want to make it feel like a wrung out sponge, and this will act like the bedding.
  • Add your worms. You can add a few handfuls of loose garden soil if you like but this isn’t strictly necessary.
  • Add your food scraps and spread it out very evenly.
  • Top the mix with the other half of the shredded paper and moisten it very well with your spray bottle.
  • Find a location to set your bin permanently. The worms hate extreme temperatures, including really hot winters and cold summers. You’re aiming to put it in an area that is 55°F to 75°F. If you have a greenhouse or attached garage, they’d do well there.
  • Put the lid without holes on the ground and put a brick inside each corner before setting the bin on top of the bricks.
  • To feed the worms, pull away the layer of shredded paper on top, pour in half of a cup of food, and replace the shredded paper to cover all of the food.

Worm Bin Care

Only feed the worms in the worm bin a cup of food per week for the first few months you set it up. As the worms start to multiply and look bigger, you can increase how many food scraps you add each week. Over time, the shredded paper bedding will break down. When this happens, just add more shredded paper and moisten it using a spray bottle.

Carefully monitor your moisture level in your worm bin. The contents should always have that wrung-out sponge feel to it. If you see a lot of liquid draining out of the bottom of the bin, this could be an indicator that you added too much water. If so, just allow the moisture levels to balance naturally by lifting the lid once a day. If the bin develops an issue with mites or fruit flies, you’re probably adding scraps too fast. Stop feeding them for a month and give them time to break down what is in the bin before adding more.

5 Worm Bin
Worm bins are popular to make to have worm castings for your garden or plants, and most of them are small-scale setups. Worms! By Will Luo / CC BY-NC-ND 2.0

Judging Worm Casting Quality

Since worm casting isn’t a commodity, they don’t have strict quality guidelines you can rely on. There can easily be a huge quality variation from the produce you really want. WIthout expensive tests or a microscope, it’s hard to tell if you’re getting quality worm castings or not. The following characteristics can help you judge the quality:


The shape of your worm castings don’t necessarily measure quality. However, you should know that quality worm castings can have different textures and shapes. For example, red wigglers will give you a more irregularly-shaped, elongated casting. African nightcrawlers will produce a more granular, rounded cast

Humic Acid Levels

Humic acids work to bring water and nutrients to your plant, and they’re something your plant needs to stay healthy when there aren’t enough fungal (mycorrhizal) relationships around the root zone. The humic acids have a positive charge that attracts to the negatively-charged roots to allow the plant to have a more efficient nutrient uptake.

Microbial Population

You’ll need a microscope to see if your worm castings are high quality or not if you want to go by the microbial population. You’ll want to see populations of fungi, nematodes, bacteria, and flagellates when you look at your slide. The sample should have one nematode per slide when you look at them, and you’ll want to see one big fungal hyphae every fifth view. There should be a minimum of one aerobic protozoa every fifth view, and the worm castings should show a 1:1 fungi-to-bacteria ratio. If the bacteria is too high, this will promote the growth of weeds and annuals.

Moisture Content

Worm castings that you stored on a shelf for a year or two most likely won’t have the water content they needed, and this can turn them dormant or kill the microbes. When you buy them, the moisture content should be between 55% and 70%. If the levels dip too low, it can make the castings become hydrophobic. If this happens, it makes the castings have trouble retaining water.

Nutrient Values

When you think about traditional fertilizers, the nutrient values are listed as N-P-K for nitrogen, phosphorus, and potassium. This metric is virtually useless when you’re trying to judge the quality of your worm castings as it usually has a 1-0-0 NPK ratio. If you look at commercially-bagged worm castings, you’ll see this number.

The easiest way to explain what sets fertilizer apart from worm castings is that you have to remember that fertilizer feeds the plants. Worm castings feed the soil instead. So, it won’t have higher levels of nitrogen, but the levels of nitrogen it does have are in ammonia form that your plants can take up.

Plant Growth Hormones

Some worm castings have plant growth hormones, and this can have an impact on the plant’s growth, yield, and root growth. These hormones you should look for will fall into these categories:

  • Abscisic acid
  • Auxins
  • Cytokinins
  • Gibberellins

Bottom Line

Worm castings have a host of benefits for your soil and plants if you use them correctly and get good-quality products. We’ve told you how they’re made, what they are, and how you can make your own. You can use this quick guide to create worm castings to enrich the soil and boost your plant growth this season.

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