Compost vs Worm Castings - The Key Differences & Which One to Use in Your Garden

If you're looking to give your plants or garden a boost you have probably considered adding compost or worm castings to your soil or plantings. 

Both are packed with nutrients and minerals beneficial to plants, and it can be confusing to know which one you should use.

Since they are both excellent options, let's take a closer look at the unique characteristics of each to help you decide how to use them in your garden or soil.

Compost vs Worm Castings

Worm castings and compost are different organic materials that offer similar benefits to soil and plant growth. Both compost and castings are rich in nutrients, minerals, and microbial activity that help plants thrive, but worm castings also include beneficial bacteria and enzymes that come from worms’ highly specialized digestive process. Quality compost and worm castings are both great additions to soils, plants, and gardens.

Here’s a quick summary of the benefits of worm castings and compost:

Benefit	Worm Castings	Compost
Nutrient-rich	Yes	Yes
Easy for plants to absorb	Yes	Yes
Highly Concentrated Nutrients, Minerals, & Microbes	Yes	No
Promotes plant growth	Yes	Yes
Reduces need for synthetic fertilizers	Yes	Yes
Water-soluble Nutrients	Yes	Some
Good for Soil Aeration & Water Retention	Yes	Yes
Good Source of nitrogen, phosphorus, and potassium	Yes	Usually
Good Source of Micronutrients like calcium, magnesium, and sulfur which are important for plant	Usually	Usually
Source of trace elements like copper, zinc, and iron?	Usually	Usually
Beneficial Bacteria and Enzymes from Worms	Yes	No
Reduces Plant Pathogens	Yes	Yes

 

Both worm castings and compost have a variety of benefits for plants and soil, but worm castings are generally considered to be more nutrient-rich and concentrated. This makes castings an excellent choice for gardeners who want to give their soil and plants an added boost for plant growth, disease-prevention, and water retention.

What is Compost?

Compost is a type of organic matter made by heating up and decomposing organic materials such as food scraps, yard waste, and manure. Compost is commonly used as a fertilizer and soil amendment.

Mostly finished compost ready for worms (or plants!)

Compost is typically made by combining various types of organic material, such as leaves, grass clippings, and food waste, and allowing it to break down naturally over time. The resulting material is a rich, crumbly substance that can be added to soil to improve its structure, water-holding capacity, and nutrient content.

As the organic materials break down during decomposition, 2 key things happen:

• The organic materials release a variety of nutrients, minerals, and elements that are beneficial for plants and soil
• Harmful bacteria like e coli and salmonella are killed off, while beneficial microbes and bacteria survive and multiply.

As the organic materials break down further during the composting process, they provide a source of food for bacteria and other microorganisms. These bacteria play a key role in the decomposition of the materials, converting them into a form that is more easily absorbed by plants.

It's important to note that the exact composition of compost can vary depending on the materials used to make it and the conditions under which it was produced. So not all compost is created equal, and some will be more nutrient-rich and effective than others. 

What are Earthworm Castings?

Worm castings, also known as vermicast, are the nutrient-rich excrement of earthworms. These tiny, squirmy powerhouses eat organic materials like food and yard waste, process it through a highly specialized digestive tract, and convert the waste into a form that is easily absorbed by plants.

Mostly processed worm castings aka "vermicompost"

This process is the result of 500 million years of evolution, so it’s safe to say worms and their castings have a symbiotic relationship that can help your plants thrive!

Worm castings are often called a fertilizer but they are really a natural soil amendment. Due to their rich, concentrated levels of nutrients, minerals, and beneficial microbes like fungi and bacteria, castings add essential elements for a healthy soil and also create conditions that can lead to more beneficial microbial activity in the areas surrounding a plant’s root system.

Benefits of Compost and Worm Castings

Worm castings and compost benefit soil and plants in similar ways. 

Both add nutrients, minerals, and beneficial microbes that feed root systems and create healthy conditions for plant growth.

As mentioned above, both compost and worm castings have nitrogen, phosphorus, and potassium (the "big three" macronutrients), as well as a range of micronutrients such as calcium, magnesium, and sulfur, and trace elements like copper, zinc, and iron (which are needed in small amounts by plants).

Worm castings and compost also contain beneficial microorganisms such as bacteria, fungi and protozoa. These microorganisms feed on harmful bacteria and also can help to break down organic materials and release nutrients into the soil, making it easier for plants to uptake nutrients.

In addition, both castings and compost can help with soil aeration, soil porosity, and water retention in the areas around plant root systems.

Differences Between Worm Castings and Compost

Although worm castings and compost offer many of the same benefits to plants, they differ in several ways.

Compost is broken down organic matter while castings are the excrement (aka poop) of worms. 

Due to worms’ highly specialized digestive process, castings have specialized bacteria and enzymes that are not present in compost. Castings are also considered to be a more concentrated source of nutrients and microbes that compost.

Some of the key nutrients found in worm castings include nitrogen, phosphorus, and potassium, which are known as the "big three" macronutrients. 

Worm castings also contain a range of micronutrients such as calcium, magnesium, and sulfur, which are important for plant health and development. Worm castings may contain trace elements such as copper, zinc, and iron, which are needed in small amounts by plants.

Overall, the exact composition of worm castings can vary depending on the materials that the worms have been fed and the conditions under which they were raised. And it is important to note that worm castings (and compost) will be either bacterial or fungal:

• Bacterial-dominant castings and compost are best-suited for annuals and grasses
• Fungal-dominated castings and compost provide the most value to perennials and woody plants (e.g., trees and vines).

One other difference between castings and compost? Worms love eating compost!

Are Worm Castings Compost?

Worm castings are not the same as compost since castings are the waste by-product from worms and compost is broken down organic material. Although compost and worm castings offer many of the same benefits to plants and soil, castings are a different organic matter and contain bacteria and enzymes not found in compost.

Are Worm Castings the Same as Worm Compost?

Yes - worm castings often go by the names castings, worm humus, worm compost, vermicompost, and vermicast. All of these terms refer to organic matter that has been processed through worms’ highly specialized digestive tract. More specifically, vermicast refers to "pure" worm castings while vermicompost and the other terms refer to worm castings mixed with organic matter not fully processed by worms. 

The concept of vermicast or “pure” worm castings is very difficult to obtain (or prove!). Virtually all worm castings products will have some level of organic matter not yet processed by worms (hence all castings products are really vermicompost).

Are Worm Castings Better than Compost?

Although worm castings and compost provide similar benefits to soils and plants, worm castings are often seen as a better soil amendment because they offer a higher density and concentration or nutrients as well as specialized bacteria and enzymes that come from the worms’ specialized digestive tract.

The reason worm castings are more nutrient-rich than compost is because as worms break down organic matter, worms are concentrating the nutrients in their bodies and adding important biology to the castings that is not present in compost. 

Worm castings also tend to be more finely textured than compost and offer nutrients in a better water-soluble form that is easier for plants to take up. 

Finally, worm castings are often free of weed seeds and other contaminants that can be present in compost, making them a more consistent and reliable source of plant nutrition. 

Disadvantages of Worm Castings

Worm castings require time and patience to make on your own and can be more expensive or harder to find than other types of compost or fertilizer.

The reason for this is the production of worm castings requires a specialized set of conditions, including the presence of worms, moisture, and the right mix of organic material.

This can make the process of producing worm castings labor-intensive and, like compost, quality can suffer if the process is not managed correctly.

Disadvantages of Compost

Composting requires organic material to break down naturally over several weeks or even months, and this can be a slow and labor-intensive process.

Composting also needs a certain balance of moisture, temperature, and air flow to be done properly, and this can be difficult to achieve (especially for beginners!).

As a result, composting can be frustrating and time-consuming, and it may not always produce the desired results.

Another disadvantage of compost is that it can attract insects, rodents and animals, which can be a nuisance and or result in contamination of the compost pile.

Compost piles may also produce unpleasant odors, especially if the compost pile is not properly managed or if it becomes too wet or too dry. This can be a problem (especially for people who live in urban areas where space is limited).

Finally, like worm castings, the nutrient content of compost can vary widely depending on the materials that were used to make it, and the quality of the compost can be difficult to predict.

The quality of commercially-bought compost can also be very unpredictable with respect to pH and nutrient availability, so it’s important to test these products on a small scale before using them across all of your plants or garden areas.

 How to Use Worm Castings

Because of their nutrient density, worm castings are usually applied at a rate of 15%-20% castings to 80%-85% planting or grow medium (i.e., a ratio of 1 part castings to 4 parts planting medium).

Worm castings can be used with existing potted plants, gardens, and lawns, or with new plantings and seedlings. The application and use will differ, though, with castings being mixed into the potting medium for new plantings and placed on the surface and gently kneaded into the soil for existing plants.

Here’s a breakdown for using castings for virtually any home horticulture or garden project:

 

Type	Application & Amount to Use	Frequency
Existing Vegetables, Perennials, & Annual Flowers	Spread 1-2 inches of castings on the soil above the plants’ root systems. Gently work the castings into the soil and water lightly.	2-3 times per year in early spring, early summer, & early fall
Existing Potted Plants, House Plants, & Hanging baskets	Sprinkle ½ to 1 inch of castings on the top of the soil. Gently knead the castings into the soil and water lightly.	Every 2-3 months
Existing Trees, Shrubs, Vines, & Roses	Apply a 2-3 inch layer of castings around the base of the plant and root system. Gently knead or rake into soil and water lightly.	2-3 times per year in early spring, summer, and fall
Existing Gardens & Flower Beds	Apply a 1-2 inch layer of castings (~5 pounds per 10 square feet) to the garden or flower area. Knead or rake into the soil where possible and water gently.	2-3 times per year in early spring, summer, and fall
New Vegetables, Perennials, & Annual Flowers	Line each seedling hole with ½ to 1 inch of castings, or each plant hole with 1 to 2 inches of castings, insert seed or plant, and cover with soil or your grow mix. If you are planting a new garden or raised bed, you can instead mix 15% to 20% (1:5 ratio) castings with your soil or favorite planting medium before seeding or planting.	Once at planting, then top dress with ½ inch of castings every 2-3 months from spring to fall.
New Potted Plants, House Plants, and Hanging Baskets	Use 15% to 20% (1:5 ratio) castings with your soil or favorite planting medium.	At planting, then top dress with ½ inch of castings every 2-3 months
New Trees, Shrubs, Vines, & Roses	Mix 15% to 20% (1:5 ratio) castings with your soil or favorite planting medium before planting. Line plant hole with mixture, insert plant roots into the mixture and cover with additional 1:5 castings to soil mixture.	At Planting
New Gardens & Flower Beds	Mix 15% to 20% (1:5 ratio) castings with your soil or favorite planting medium before planting.	At planting, then top dress with 1-2 inches of castings every 2-3 months
New & Established Lawns	Apply 20-25 pounds of castings for 100 square feet of lawn space. Water lightly.	2-3 times per year (spring and fall)

 

Can I Replace Worm Castings with Compost?

Because worm castings contain specialized enzymes and bacteria that come from a worm’s digestive process, compost can’t be used to fully replace worm castings. Worm castings and compost both offer similar nutrients, minerals, and microbes to soil and plants, but castings offer additional biology that compost does not have.

Is There a Worm Castings Substitute?

Worms have evolved over 500 million years to have a specialized digestive process that inoculates their poop with enzymes, bacteria, and microbes that are not found in other forms of organic matter or fertilizer. So finding a substitute that exactly replicates worm castings isn’t possible.

Can You Mix Compost with Worm Castings

Yes - worm castings can be mixed with compost at a rate of 15%-20% castings to 80%-85% compost. This mixture can then be spread on the surface of a garden or planting area, or it can be used to line the edges of seedling holes where the plants root systems will be.

If you have the option, we recommend getting the benefits of both worm castings and compost by mixing the 2 for use during seeding or on existing gardens and flower beds.