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  • Writer's pictureAndrew Hemmer

Using Worm Castings to Grow Healthy Plants

Updated: Mar 2, 2023

You may have heard about using worm castings from other gardeners or maybe here or there on the internet. With so much information out there, its hard to know what to trust and what to use and how to use it. Luckily, worm castings have been used for some time now and its nutrient rich for all of your plants. Worm castings are used widely in gardens and container grows and are easier to apply and use than you thought. In this article, we will go over the benefits and advantages of using worm castings. Along with that, we will go over application methods and best practices.

First, lets dive into what worm castings actually are and where they come from.

What are Worm Castings?

Gosh, there really is no other way to put this. Worm castings are a gardeners term for worm poop. Worm castings, as nutrient rich as they are, are nothing more than the droppings of earthworms. You may have heard of gardeners having a worm farm. This literally means they are making bins that they will fill with vegetable scraps, fruit scraps, and many other things. They then place specific types of worms that eat those trimmings, waste, and scraps. What happens is the worm will eat the contents and then poop it out. That poop is nutrient rich and SUPER healthy for the soil and plants. Who knew something that gross would be so beneficial for the soil!? So what are these benefits anyway?

Benefits of Worm Castings on Soil & Plants

Worm castings have magical benefits to both plants and soil. But not all worm castings are the same. Differences in what is fed to the worms, climate, and proper storage and handling can all affect the benefits – or lack thereof – that worm castings can provide. So let me start with that there is no guarantee that simply adding castings will give your garden fantastic results. But lots of academic literature make a convincing case that worm castings have substantial benefits to your plants and soil.

Soil Benefits Using Worm Castings

Increased Soil Matter

The addition of worm castings, like the addition of other manures, gives the soil a boost of the organic matter that serves as a source of food for bacteria, fungi, and other soil microbes. Healthy soil should test at 5% or more organic matter. Worm castings are an excellent source of organic matter!

But modern farming techniques have helped rob the soil of this precious resource, reducing that percentage to 1% or less in some cases, resulting in worm-free, mostly lifeless soil. Not only does it deplete organic matter but many other nutrients and fungi that aid the root web. Adding worm castings is an excellent way to increase organic matter in the soil. In other words, adding worm castings actually feeds the living and breathing ecosystem underneath your feet!

Better Soil Structure & Water Retention

Worm castings aid in soil aggregation, acting as a binder to help soil particles like sand, silt, and clay stick together creating a better space between them to help store water. In fact, just a 1% increase in soil organic matter will allow a single acre of soil to hold and additional 25,000 gallons of water. This is such a staggering statistic to us! By making soil more absorbent, you help decrease runoff, topsoil erosion, and algal blooms as synthetic fertilizers will remain in soil rather than ending up in our waterways.

Plant Benefits Using Worm Castings

Better Plant Germination Rate

While conventional wisdom says the seed should germinate with only its own nutrient reserves, the addition of worm castings to a seed starting mix is shown to improve seed germination. Studies suggest that no more than 20% worm castings by volume should be used in seed starting mixes. After a quicker germination, lots of academic research suggests that seedling growth is also much faster in soil that has been amended with biologically-active worm castings.

Worm Castings Grow Strong Roots

A plant’s root structure, responsible for pumping nutrients and water into the plant, can be much healthier in soil amended with worm castings. Roots are thicker and will fan further out into the surrounding soil. Stronger roots mean better delivery of nutrients to your plants. Stronger roots also make a plant more hardy and more resistant to disease, temperature fluctuations, and pests.

Applying Worm Castings

Worm poop are an incredible addition to soil, but there are no defined application rate as you might find on synthetic fertilizer labels. Most of the academic literature we’ve seen suggests that the majority share of benefits are captured when worm castings make up roughly 10% of the medium. In other words, if your planting medium is 10 gallons, then 1 gallon of worm castings is likely enough.

See below for a chart of application depending on what you are growing in:

Container Growing

10% of volume. Ex. 10 gallons of soil would require 1 gallon of worm castings

Lawn & Garden Soil

1 pound per 10 square feet

Germinating Seeds

1/4 teaspoon per seed

Young & Mature Transplants

Young: 1 TBSP per hole / Mature: 2 cups per hole

Side Dressing Established Plants

​2 cups per plant, scratch into soil

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