Worm castings, often called vermicast, are the nutrient-rich, microbially active excrement of earthworms produced during vermicomposting. These castings are a highly valued soil amendment providing beneficial microbes, organic matter, and slow-release nutrients to plants. Unlike synthetic fertilizers, castings will not “burn” roots or foliage, making them safe for direct application to sensitive seedlings and established plants. Utilizing this natural product effectively requires understanding the specific methods for incorporation, from blending into bulk soil to creating a liquid application.
Preparing Soil and Seed Starting Mixes
Incorporating worm castings directly into growing media provides a foundation of fertility and microbial activity. When preparing a bulk potting mix for containers or raised beds, a common incorporation rate is between 15% and 20% castings by volume. For example, blending 1 gallon of worm castings into 5 gallons of soil ensures even distribution. This ratio introduces a diverse microbial community that helps make existing soil nutrients more available to plant roots.
For seed starting, where young roots are delicate, a lower concentration is recommended, often aiming for 10% to 20% of the total volume. The inclusion of castings in a seed mix promotes faster germination and strengthens the developing root structure without the risk of over-fertilization. When making a mix for large garden beds, spread a 1- to 2-inch layer of castings over the soil surface and gently till it into the top 6 inches before planting.
Applying to Established Plants
For plants already growing in the garden, applying castings directly to the soil surface is an effective way to deliver nutrients and microbes to the root zone. This method is known as top dressing, where a thin layer is spread across the soil around the base of the plant. A typical top dressing application involves spreading a 1/2-inch to 1-inch thick layer of castings within the plant’s drip line.
An alternative, targeted method is side-dressing, useful for row vegetables or individual, well-established plants. This involves creating a shallow trench or ring a few inches away from the plant stem, near the active root zone. Incorporate about 1 to 2 cups of castings per established plant into this area. After applying the dry castings, lightly scratch them into the topsoil and water thoroughly. This action prevents UV light from degrading the beneficial microbes and carries soluble nutrients down to the roots.
Creating and Using Worm Casting Tea
Worm casting tea is a liquid extract used to deliver a concentrated boost of soluble nutrients and beneficial microorganisms directly to the plant.
Passive Brewing
The simplest method is a passive, non-aerated soak. Mix one cup of castings with one gallon of non-chlorinated water. Allow this mixture to steep for 24 to 48 hours, with occasional stirring, before straining for immediate use. While easy, this method primarily extracts soluble nutrients and fewer microbes compared to active brewing.
Active Brewing
The most potent method is active, or aerated, brewing, which uses an aquarium air pump and air stone to continuously oxygenate the mixture. Aeration encourages a rapid multiplication of the beneficial aerobic bacteria and fungi. To brew actively, suspend 1 to 2 cups of castings in a mesh bag within one to five gallons of non-chlorinated water. Add a food source for the microbes, such as 1 to 2 tablespoons of unsulfured molasses, to fuel their growth during the brewing process.
The tea should be brewed for 24 to 36 hours and used immediately, ideally within 12 hours of turning off the air pump. The finished, strained tea can be used as a soil drench to feed the roots or as a foliar spray to coat the leaves. Foliar application should be done in the early morning or late evening when the sun is low, which prevents leaf burn and allows for maximum absorption.
Recommended Application Rates and Timing
The frequency and quantity of worm casting applications depend on the plant’s growth stage and the chosen method. For established plants in the garden, re-apply castings every 4 to 6 weeks throughout the active growing season, from early spring to late summer. This consistent timing ensures a steady, slow supply of nutrients and sustained microbial activity in the soil.
When top dressing individual plants, use 2 to 3 tablespoons of castings per small potted plant or half a cup per established garden plant every two months. For larger applications, such as amending a lawn, use 10 to 15 pounds of castings per 1,000 square feet for maintenance in the spring and fall. Liquid worm tea is applied more frequently than dry castings, with a soil drench recommended every 30 days. The greatest benefit is realized at an incorporation rate of about 10% by volume, with diminishing returns above 20%.