Learn everything there is to know about worm castings. We look at what they are and how good they are for your plants.

Reading time: 3 mins

You may have heard fellow gardeners talking about worm castings, worm tea, or black gold with great enthusiasm.

If you’re asking yourself what exactly are worm castings, and what all the fuss is about, don’t worry, we’ve got a quick rundown to help you out. 

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What are worm castings? 

One of the greatest by-products of vermiculture is worm castings. As the worms in your bin consume and break down the organic matter we add as their food source, they expel waste. This waste is called castings. 

The biological evolution of worms has resulted in a cycle where worms consuming and digesting organic matter results in a waste product that is rich in nutrients, enzymes, and bacteria that plants love. Plants then grow and thrive and produce more organic matter that falls to the ground as worm food and the cycle repeats. 

Worm castings can be collected as a soil-like product that is made up of small individual castings or as a liquid ‘tea’ which has all the good stuff from the castings suspended in a liquid. 

How are they made?

Worm castings are the by-product of the digestion of organic matter by worms.

Any worm farm or compost heap with worms will have castings within, but they might be hard to find. Good vermiculture setups will allow the owner to collect and harvest the castings with little difficulty. 

In a good pile, the location you place your food should be the area with the richest concentration of castings. 

To harvest worm castings, simply expose your pile to the sunlight. As worms don’t like the light, they will retreat deeper into the pile and bedding material leaving you free to scoop out the castings as you need to. 

To make ‘worm tea’ or liquid castings, you’ll want to wrap some fresh worm castings in a piece of cloth and “steep” them overnight in a bucket of water. As the castings brew like real tea, the water will collect lots of nutrients, enzymes, and bacteria from the castings. You can even add molasses to feed the bacteria, and a small air pump to the bucket to aerate the mixture to produce a high-quality liquid fertilizer.

 What makes castings so good?

The digestive process of worms means that the nutrients in their waste are in an available and usable form for plants to absorb straight away, so they make great instant impact fertilizers. 

Castings can contain the essential nitrogen, phosphorus, and potassium (NPK) trio that plants need to survive but they also contain trace element nutrients like copper, iron, magnesium, and calcium. 

The bacteria and enzymes that come along with the castings are also beneficial to plants by aiding and increasing micro-organism activity in your soil, and the final product comes out at a neutral pH of 7. 

Worms also have the ability to filter and clean soils of contaminants like heavy metals, so mixing in quantities of contaminated soil and replacing it with castings can slowly rehabilitate a contaminated area. 

Because worm castings are a natural product, they can be applied to most plants without any issues. Worm castings also won’t burn plants when they’re applied, as other fertilizers can. 

As well as nutrients, worm castings help improve the structure of your garden soil or potting mix by improving drainage and moisture retention. 

How do worm castings compare with commercial fertilizers? 

Worm castings are an excellent natural product for your garden.

They contain all the nutrients your plants need, and the enzymes and micro-organisms within them are great for building soil structures.

Worm castings won’t burn your plants as synthetic fertilizers can, and they’re pH neutral, so don’t drastically change your soil pH levels when applied. 

…But I wouldn’t classify them as a fertilizer.

While they have a lot of available nutrients, they don’t have the same amount of nutrients that you would get from commercial fertilizer.

Commercial fertilizers are made to have high levels of specific nutrients and you can purchase products that suit your needs as close as possible, even down to plant-specific nutrient blends.  

Weight for weight, you will get far more impact from synthetic fertilizer than you would from worm castings.

It’s better to think of worm castings as soil conditioners rather than fertilizers. Worm castings compare incredibly well against commercial soil improvers, plus have the added benefit of being a free by-product of your waste. 

In summary

Worm castings, worm tea, and black gold are all names for the waste product produced by worms as they consume organic matter in the soil. 

You can harvest worm castings from your vermicompost pile by scooping them out or pouring water through the layers of your worm farm and collecting it at the bottom. 

Worm castings are rich in available plant nutrients and beneficial enzymes and micro-organisms. Adding them to your garden will provide a nutrient boost and improve your soil health and structure. 

Worm castings as fertilizer aren’t as good as commercially available products because they don’t have the same volume of nutrients weight for weight, but they’re still worth using.

Worm castings are better thought of as soil conditioners and are excellent when spread through garden beds, over your lawn, or mixed into old potting mix. 

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