How Much Worm Castings to Add to Soil

Adding worm castings to your soil is an excellent way to enrich it with nutrients and improve its structure. Knowing how much worm castings can be challenging.

I’m here to tell you it’s an easy solution.

Worm castings, also known as vermicompost, is a natural fertilizer that is rich in beneficial microorganisms and essential plant nutrients.

You can think of them as a slow-release food source for your plants, providing a steady supply of organic matter.

Whether you’re tending to a small container garden or managing a large vegetable patch, understanding the right amount of worm castings to add can make a difference in the health and productivity of your plants.

My introduction to worm castings started after I learnt about composting. Compost made such a huge difference in my acidic garden, I started looking into how to turn over even more compost.

Worm castings seem to be the answer.

The quantity of worm castings you should apply depends on several factors, like the type of plants you’re growing and the current condition of your soil.

My husband and I bought our first homestead in 2014.

It was part of an organic farmers field. Organic sounds great, until you realize that improper management of organic farming can really decrease soil life, just in a different way than conventional farming does.

We moved into our new home in 2015, and a big garden was a must.

It did very poorly. And that started me on my journey of learning about soil health. A few books from the library opened my eyes to a whole new world of gardening.

Worm composting was one of them. In 2021, I finally found a source of red wigglers (they weren’t common in our small town).

At this point we had a tumbling composter, and were using cover crops to build our garden soil. Worm compost was next.

A general guideline is to mix worm castings into the top few inches of soil at a ratio of 1/4 cup per plant for small plants, such as herbs, or around 1 to 2 cups for larger plants, like vegetables.

For an entire garden bed, you might aim for a ratio of about 10 percent worm castings to the total volume of soil. This is a lot of castings if you want to treat your entire garden bed.

Therefore, I recommend just adding 1/4 cup per plant. The amount of microbiology in 1/4 cups of worm compost is mind boggling.

Worm compost helps your plants have access to the nutrients they need without the risk of nutrient burn, which can happen with over-fertilization. This gives you a good ballpark amount.

Understanding Worm Castings

Worm castings are an organic form of fertilizer produced by earthworms (or in my case, red wigglers).

Benefits of Worm Castings

  • Soil structure improvement: Worm castings enhance soil aeration, which is essential for root growth.
  • Nutrient availability: They slowly release essential nutrients, making them readily available to plants over time.
  • Enhancement of microbial life: Castings are teeming with beneficial microorganisms that support plant health.

Worm Castings Composition

  • NPK values: Worm castings contain nitrogen (N), phosphorus (P), and potassium (K), albeit in lower concentrations than chemical fertilizers.
  • Trace elements: Essential trace minerals such as iron, sulfur, and magnesium are present, supporting overall plant growth.
  • Organic matter: They enrich the soil with decomposed organic material, which retains moisture and improves root environment.

Application Guidelines

Worm castings are a nutrient-rich, organic fertilizer and soil conditioner. Here’s how you can apply worm castings to your soil effectively.

General Application Rates

When incorporating worm castings into your garden, the general rate is 1/4 to 1/2 inch layer. You should apply the castings evenly across your beds.

PurposeApplication Rate
Vegetable gardens1 cup per square foot
Flower beds1/2 cup per square foot
New lawns (before seeding)10 pounds per 100 sq ft
Established lawns5 pounds per 100 sq ft

Mixing with Potting Soil

When preparing potting mixes, an ideal mix consists of 10-20% worm castings. This proportion balances nutrient availability with soil structure.

  • Seed starting mix: 10% worm castings
  • Potting mix for houseplants: 20% worm castings

Top Dressing

Top dressing is when you add a layer of compost to the soil at the top. It’s a great way to nourish plants during the growing season. Gently work in a 1/4-inch layer of castings around the base of your plants without disturbing the roots.

  • During plant growth: Apply castings once every 2 months
  • For potted plants: Apply 1 tablespoon per inch of pot diameter

Remember to water your plants after applying castings to help with nutrient absorption.

Factors Affecting Usage

When adding worm castings into your soil, take into consideration the needs of your plants, the existing soil condition, and the specific growth stage.

Plant Type

  • Vegetables: Use 1 cup of worm castings per square foot of soil. Mix thoroughly.
  • Flowers: Sprinkle 1/2 cup of worm castings around the base of each plant.

Soil Condition

  • Nutrient-rich soil: Less worm castings are required. Use about 1/4 cup per square foot.
  • Poor soil: Increase worm castings to 1-1/2 cups per square foot to improve fertility.

Growth Stage

  • Seedlings: Lightly dress with 1 tablespoon of worm castings to avoid overwhelming young roots.
  • Mature plants: Apply up to 2 cups per square foot as they can handle more nutrients.

This gives you a rough guideline on how much worm castings to add to soil.

There is no precise measurement. You won’t mess up your plants.

These guidelines are only to help you use your worm compost efficiently.

Worm castings are a nutrient rich fertilizer that will help you grow big and beautiful plant. The more nutrient rich the soil, the more nutrient rich the food, which is a big reason to feed your plants worm castings.