Earthworm castings (vermicompost) are the refined organic matter produced after earthworms digest organic materials. This dark, soil-like material is a powerful, natural soil amendment that will not burn plants. Castings contain water-soluble nutrients (nitrates, phosphorus, and calcium) readily available for immediate plant uptake. They are rich in beneficial microorganisms, such as Bacillus and Azotobacter species, which promote plant growth and improve soil biology. Castings enhance soil structure, increase water retention, and offer a gentle, slow-release source of nourishment for all plant life.
Preparing Castings for Application
Earthworm castings can be used in their raw, granular form or transformed into a liquid feed for rapid delivery. When creating a custom potting mix, incorporate granular castings directly into the soil medium. For general soil enrichment, use 10 to 20% castings by volume (about 1/4 to 1/2 cup per gallon of soil). Heavy-feeding plants can use up to 25% castings without the risk of nutrient burn.
The beneficial microbes can be extracted to create liquid casting tea. The simplest method involves steeping 1 cup of castings in 1 gallon of non-chlorinated water overnight. For a greater microbial boost, aerated tea is preferable, requiring castings in a porous bag submerged in water while bubbling air through it for 24 to 48 hours using an aquarium pump. Adding 1 to 2 tablespoons of unsulfured molasses acts as a food source, promoting the rapid multiplication of beneficial bacteria and fungi.
Direct Application Techniques for Gardening
Application depends on the plant’s stage of growth, ensuring nutrients and beneficial biology reach the roots. For starting seeds, mix castings into the seed-starting medium at a 1:4 ratio to provide gentle nutrients and improve moisture retention. Alternatively, cover newly sown seeds with a thin layer (about 1 centimeter) of pure castings before watering.
When transplanting seedlings or new plants, targeted application is highly effective. Place 1/2 to 1 cup of castings directly into the planting hole and mix with the surrounding soil before setting the root ball. This method ensures the roots immediately encounter the active biology and available nutrients.
For established plants, castings are typically applied as a top dressing or side dressing. To top dress containers or garden beds, spread a layer 1/2 to 1 inch deep over the soil surface around the plant’s drip line. Gently scratch the castings into the top few inches of soil to protect microbes from UV light and help them filter down with irrigation. Casting tea can be applied as a foliar spray, but it must be strained through a fine mesh or cloth to prevent solid particles from clogging sprayers.
Optimal Timing and Dosage Guidelines
Application timing should focus on periods of active growth, beginning with pre-planting soil preparation and continuing with periodic boosts. Incorporating castings into the soil before planting provides a foundational amendment that lasts for many months due to its slow-release nature. For established garden beds, a general dosage is about 10 to 15 pounds of castings per 100 square feet, worked into the soil once at the start of the growing season.
Established vegetable plants benefit from a side dressing of 1/2 to 1 cup of granular castings applied around the base every two months. Container plants can be side-dressed with 2 to 3 tablespoons of castings every 6 to 8 weeks. Liquid casting tea offers a faster-acting boost and can be applied every two to four weeks as a soil drench or foliar spray. For soil drenching, the tea can often be used undiluted, but a 1:1 dilution is recommended when feeding young seedlings.