Can You Use Worm Castings and Fertilizer Together?

When choosing between fertilizer and worm castings, you can’t help but feel confused, given their similarities in appearance and purpose. Is it possible to use them together? 

You should avoid using worm castings and fertilizer together because if your fertilizer is too chemically harsh, it may harm your worm population. Worm castings are a chemical-free, organic alternative to fertilizer. If you decide to use both, you need to exercise the necessary precautions. 

Whether you should use worm castings or fertilizer in your garden is a matter of personal preference, but it’s best to avoid using the two together. In this article, I’ll discuss these products individually so you can decide which one works best for your gardening needs. Then, I’ll discuss the safest ways to use both if you choose.

Similarities and Differences Between Worm Castings and Fertilizers 

It’s important to understand the similarities and differences between worm castings and fertilizers. While they appear to work the same way, these two products aren’t necessarily interchangeable. 

Fertilizer is a synthetic product that provides plants with essential macronutrients (nitrogen, phosphorus, and potassium) and micronutrients (boron, copper, iron, and zinc). These elements are needed for plant growth.

To help understand how worm castings can work with fertilizers, I need to explain what vermicomposting is first. Vermicomposting is the process of using earthworms to transform organic waste into a nutrient-rich fertilizer called vermicompost. Within this mixture, there will be worm castings.

When hydrated, the cocoons within these worm castings will help produce a large population of worms. These worms help with aeration and nutrients in your garden or compost things like coffee grounds, egg shells, decomposing fruit, and vegetables in a bin. It’s an alternative to both fertilizer and composting in the traditional sense. 

When you go the vermicompost route, you’ll need to add a few things to your garden via the exchange between the bin and the garden bed.

Vermicompost contains many things that plants need, such as nitrogen. However, it also contains other elements (e.g., calcium) which can potentially stunt plant growth if applied directly on top of the soil. 

For this reason, it’s common practice for users of worm castings to mix them with other forms of organic matter before applying them directly onto plants or the soil around their roots.

How Fertilizer Is Made vs. How Vermicompost Is Made

The first thing you should know is that fertilizer is a man-made substance, whereas worm castings are a natural substance. Fertilizer is made of chemicals and is still often used by organic farmers, more so than pesticides or herbicides. These chemicals aren’t off-the-wall or anything to fear. Typically, fertilizer is made with natural gasses and other elements. 

It’s also important to note that fertilizers often contain other ingredients besides nitrogen, phosphorus, and potassium. For example, some fertilizers are essentially recycled from sewage waste (i.e., they’re “biosolids”), which isn’t a problem on its own. However, since sewage often contains ingredients like arsenic, lead, and mercury (which are poisonous), the Clean Water Act is in place to regulate the use of biosolids. 

On the other hand, worm castings are made by worms that eat decomposing matter and excrete the nutrients they take in. This excrement can then be applied directly to the soil to increase its nutrient value and promote healthy plant growth. 

Additionally, this excrement often has cocoons in it full of worm eggs. These eggs will grow into worms given the right conditions. Specifically, the worms need to have access to essential nutrients, as well as grow in an environment with the right temperature and moisture levels. 

Though worm castings are more natural and organic than fertilizer, that doesn’t necessarily mean they don’t have man-made elements. They’re often dried out so they can be stored for later usage, and this processing isn’t technically natural or organic. 

How To Use Worm Castings and Fertilizer Together

I’ve already described the similarities and differences between worm castings and fertilizers. Both are used to enrich your soil. If you want to use worm castings and fertilizer together, there are some things to keep in mind. 

Don’t Mix the Two Right off the Bat

Getting a bag of worm castings and adding some fertilizer (or vice versa) will be detrimental to the living microbes within your worm castings or any potential worm eggs. If you want to use fertilizer, you should apply it before vermicomposting for several weeks. 

The fertilizer will help break down any organic material in the soil and make it much more nutritious for the growing plants. The worms will then feed on this new and improved soil, adding even more nutrients to your garden.

Worm Castings Mix Better With Water Than Fertilizers

Fertilizers don’t work well when mixed with water. On the other hand, worm castings are designed to be mixed with water. They can quickly break down any organic material present in your soil and turn it into rich nutrients for your plants to absorb. This method is preferred because it maximizes its effectiveness.

Mixing Fertilizers and Worm Castings Can Save You Money

On top of maximizing their effects when used together, mixing fertilizers and worm castings has another benefit: It allows you to save money! Combining these two products will reduce your overall cost significantly compared to buying only one product at full price. 

Again, you should remember not to combine them right away. Otherwise, you’ll end up wasting both.

If you want to vermicompost fertilizer, you’ll still need to wait several weeks until your worms are healthy. As mentioned above, you shouldn’t do it this way because it can kill the worms living within the garden. 

Use the Right Amount of Fertilizer or Worm Castings

If you’re just going to pick between or the other, there are rules of thumb for both. Don’t overdo it with either product. Too much worm casting or fertilizer can deplete your soil’s nutrient levels rather than increase them. In particular, you should be careful when applying these products to plants that have been recently transplanted or seeded because they may not be ready for fertilization yet.

Should You Use Fertilizers or Worm Castings?

You should choose either fertilizers or worm castings for your garden, but do not use both at the same time. If you plan to mix them, take care not to harm the living organisms within the worm castings and in your vermicompost. 

So how do you decide which one to use? You’ll need to put your budget, resources, and abilities to the test for both. Vermicompost and worm castings are the gift that just keeps giving since the cycle of excrement and birth create generations of more worms and castings. On the other hand, fertilizer needs to be store-bought. 

When choosing between fertilizers and worm castings, here are considerations to keep in mind.

  • Fertilizers are a great way to get nutrients into your soil, but worm castings are better for plants. 
  • Fertilizers are made from chemicals, while worm castings come from worms. This means that fertilizers may not be as natural or good for the environment.
  • Vermicomposting or working with worm castings may require extra care, such as feeding your vermicompost, having a worm casting bin or ecosystem, etc. 
  • Vermicomposting may attract birds to your garden if they’re looking for food. On the other hand, worm castings are also good for protecting against pests that may harm your plants. 

As I mentioned earlier, if you want to use both at the same time, the best option is to apply fertilizer to the ground followed by the worm castings several weeks later, as the effects of the fertilizer will have worn off by that time. This is especially important if you use a chemically strong fertilizer. 


You can use both worm castings and fertilizer, but not at the same time. It’s considered best practice to fertilize the soil for several weeks before adding vermicompost. You can do it the other way around, but using the fertilizer first will have a better effect on your plants.

Just like you wouldn’t use oil and butter to grease a pan (or maybe you would!) or a hose and a watering can to water the same plant, you don’t need to nurture your soil with fertilizer and vermicomposting.

Alexander Picot

Alexander Picot is the principal creator of, a website dedicated to gardening tips. Inspired by his mother’s love of gardening, Alex has a passion for taking care of plants and turning backyards into feel-good places and loves to share his experience with the rest of the world.

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