All You Need to Know About Earthworm Castings

Earthworms benefit gardens in many ways. When they move through soil, they create spaces that help air, water and nutrients reach plant roots. But earthworms also leave behind something else: earthworm castings.

Castings, the end product of worm digestion, can improve soil and plant health for indoor and outdoor plants. They're so beneficial, in fact, that some gardeners raise their own worms. But there's a much easier way to bring the benefits of earthworm castings to your garden and home.

How Earthworm Castings Improve Soil

Earthworm castings are naturally full of organic matter and desirable microorganisms that yield multiple benefits. Nutrient-rich, biologically active earthworm castings provide essential plant nutrients, including iron, but they also work to improve the structure of soil — from potting soil mixes and indoor plants to outdoor garden soil.

The humus in earthworm castings helps increase soil's water retention, improve soil aeration and anchor plant nutrients that would otherwise leach away with water. Castings also feed beneficial soil microorganisms that produce, store and slowly release plant nutrients into your soil to feed plants.

Pennington Ultra Grow Potting Soil Mix is the easy way to bring all the benefits of earthworm castings to indoor houseplants, container gardens or in-ground gardens — without any mess or goo. Plus, this premium potting soil mix delivers other benefits, including mycorrhizae that improve your plants' ability to take nutrients from your soil, indoors or out.

And don't worry — the casting granules in Pennington Ultra Grow Potting Soil mix don't hint at their origins. Their rich, earthy texture is similar to fine garden compost or dry coffee grounds.

How Earthworm Castings Benefit Plants

On top of their soil benefits, earthworm castings provide substances that directly influence healthy plant growth. Research conducted over several years at The Ohio State University Soil Ecology Laboratory found that worm castings enhance seed germination, plant growth, flowering and fruit production. Castings also curb certain plant diseases, including root and crown rots and wilt disease, and inhibit some insect pests, including mites, aphids and mealy bugs.1

In addition, researchers at the Cornell University Department of Plant Pathology and Plant Microbe Biology demonstrated that worm castings suppress damping-off disease in seedlings. They also found that castings naturally degrade the protective covering of some insect pests, regulate plant nutrient release and stimulate the cycle of nutrients from soil to plants.2

The impact of earthworm castings increases when they're worked into soil before planting or potting. Pennington Ultra Grow Potting Soil Mix has already done the work for you. Castings are incorporated into this premium mix — along with other beneficial, sustainably sourced ingredients. You and your plants enjoy the benefits without the need to raise worms, harvest worm castings or mix your own potting soil.

How to Bring Castings to Indoor and Outdoor Gardens

Bringing earthworm castings to indoor plants, containers or in-ground gardens is as simple as using Pennington Ultra Grow Potting Soil Mix indoors and out. This premium mix combines the benefits of earthworm castings with mycorrhizae that work like root extensions to enhance nutrient uptake. Plus, your soil and plants benefit from sustainably sourced Canadian sphagnum peat moss, processed softwood bark, water-holding crystals and a wetting agent that helps optimize your water use indoors or out.


  • Check your container size against the guidelines on the product label to make sure you have enough potting soil mix for the job. This applies to hanging baskets, window boxes, large planters and raised beds, too.
  • Take your plant from its existing pot and gently brush the old soil from its roots with your hand. If your plant is rootbound, gently loosen or untangle the roots.
  • Add a layer of Pennington Ultra Grow Potting Soil Mix in the bottom of your new container. Place your plant so the base of its stem will sit slightly above the surrounding soil surface when you're through.
  • Fill in around your plant with the potting soil mix until you cover the roots completely and the base of the plant's stem is at the same level it was growing at in its old pot. Firm the soil gently with your hands.
  • Water your newly potted plant thoroughly, allowing excess water to drain out the bottom. Then add more Pennington Ultra Grow Potting Soil Mix, as needed, to get the correct soil height in your pot.


  • Measure the length and width of the garden area where you want to improve your soil and plant health.
  • Multiply the length by the width to get the total square feet. For example, a garden 4 feet wide and 4 feet long equals 16 square feet.
  • Cover the area with a 3-inch layer of Pennington Ultra Grow Potting Soil Mix. For each 16 square feet of garden, you'll need four bags or 4 cubic feet of potting soil mix.
  • Incorporate the layer of potting soil mix into the top few inches of your garden soil to enhance the benefits of its sustainably sourced ingredients even more.

With Pennington Ultra Grow Potting Soil Mix, you can enjoy the goodness of nutrient-rich, biologically active earthworm castings and create a living sustainable soil ecosystem for your plants — whether indoors, in containers or in gardens outside.

At Pennington, we've been helping gardeners grow and care for plants since 1945. We work hard to bring you the finest plant, lawn and garden products to help you grow bigger, better, happier plants. From premium potting soil mix with earthworm castings to specialty plant fertilizers, we bring you our best so you can grow your best.


1. The Ohio State University Soil Ecology Laboratory, Publications on Earthworms and Vermiculture, 2003-2008.

2. Cornell University Department of Plant Pathology and Plant Microbe Biology, Vermicompost Research.