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Can You Use Worm Castings Instead of Fertilizer?

I’ve had my vermicompost bin for a few months now, and I’ve had amazing results in my garden by simply applying 1-2 inches of worm castings per plant. The results are so good that I was wondering if I can use it to replace fertilizer entirely. So, I did some research to find out more. Here’s what I found.

Worm castings can’t replace fertilizer completely as it’s missing the volume of nutrients that plants need (such as nitrogen). However, worm castings are an extremely valuable nutrient source and should still be used if possible. To apply, mix in 15-25% worm castings (or 1-2 inches) with soil and fertilizer.

So, while worm castings are a proven nutrient-rich resource for plants, they’re not a complete fertilizer. But what nutrients do they have and are they better than fertilizer and compost?

Can Worm Castings Replace Fertilizer?

Tylers vermicomposting bin
My vermicomposting bin

Even though worm castings provide high-quality nutrients that can be absorbed by the plant immediately, they simply lack the volume of nutrients that’s in fertilizer. Worm castings also have a neutral pH of 7, so acid-loving plants, such as citrus, will likely need some acidic-amendments mixed in.

Let’s take a further look into what worm castings are and how we can use them.

Are Worm Castings a Fertilizer?

Tyler holding worms over his Meyer lemon tree

Worm castings aren’t a fertilizer—they’re a soil amendment. Fertilizers are often synthetic and contain specific amounts of nutrients in high amounts. However, worm castings are often touted as a better amendment to soil than synthetic fertilizers due to their many soil-promoting properties.

Ever since I saw the benefits of worm castings first-hand, I’ve become a big promoter. While, compared to fertilizer, worm castings might be lacking in their quantity of nutrients, their quality should not be ignored.

Here’s a list of the nutrients in worm castings, almost all of which can be immediately absorbed by the plant:

  • Nitrogen
  • Phosphorus
  • Potassium
  • Magnesium
  • Manganese
  • Zinc
  • Iron
  • Copper
  • Calcium
  • Cobalt
  • Carbon

These nutrients are favored by plants to the point where they dramatically increase germination rates. A study done by Mississippi State University found that worm castings not only increased the germination rates of cucumbers by 20% but also the seedling and root length.

Source: MS State

Even with these benefits, fertilizer still has its place as many plants, especially large and mature ones, need a larger amount of nutrients that worm castings can’t offer on their own.

So, while we can clearly see that using worm castings alongside fertilizer is a good idea, are they good for all plants?

Are Worm Castings Good for All Plants?

Worm castings are good for almost all plants as they contain a variety of nutrients that are ready to absorb by the plants. The only time not to use worm castings you’re using it on plants that prefer acidic soil. However, this can be easily fixed by adding some acidic amendment or fertilizer to the soil.

If you didn’t already know, worm castings are the worm’s poop. But don’t worry—it smells like earthy soil and looks like organic black soil.

One of the main benefits of worm castings compared to synthetic fertilizers or manure is that the nutrients are water-soluble and can be used immediately by plants.

There are also many other benefits worm castings can offer the soil, and therefore—plants:

  • Maintains a balanced, neutral pH in soil
  • Increases nitrogen in the soil
  • Reduces an overabundance of carbon
  • Filters out heavy metals
  • Reduces disease in the soil
  • Promotes beneficial soil bacteria
  • Improves the plant’s natural defenses to fight pests
  • Retains water and minerals in the soil in the form of humic acid
  • Insulates plant roots from extreme temperatures
  • Reduces soil erosion

We’ve already covered that worm castings have a neutral pH of 7, which means they can effectively balance overly acidic or alkaline soil, but they can also balance carbon and nitrogen.

Worms can reduce the oversupply of carbon and add more nitrogen into the mix. Leaves and branches are mostly made up of carbon and can make up a lot of the composition in soil. With nitrogen being one of the three primary nutrients for plants (the other two being phosphorus and potassium, together making NPK), worm castings are a great amendment in soil.

When worms consume organic material in the soil, they produce a rich supply of enzymes and beneficial bacteria. This is super useful for the plant as these microbes in the soil help to break down the nutrients even further than the worms can. So, just by having worm castings, you not only have the worms working for your plants, but the bacteria working alongside them.

One of the biggest reasons worm castings benefits the soil is due to the humic acid in them. Humic acid benefits the soil by holding onto nutrients and water and prevents it from leaching further into the soil, where it’s too deep to be used by the plant’s roots. Unfortunately, humic acid isn’t discussed enough in the gardening world. Conventional agriculture often ignores humic acid as one of the most important aspects of healthy soil and promotes synthetic fertilizer instead.

Humic acid is primarily found in organic matter such as compost and worm castings. Here’s what Eco Farming Daily has to say about the use of humic acid for soil and plants.

“Applying organic matter is certainly an excellent way to remineralize a soil that has been leached or has no chemical reactions, such as with some sands. Sand with a low cation exchange capacity (CEC) has difficulty holding onto the cations of nutrients, and these cations can easily leach deep into the soil and become unavailable for plant uptake.”

Additionally, due to the physical structure of worm castings, they’re very effective in helping the soil retain water and minerals. Because of their added sponge-like quality in the soil, the plant’s roots are also better insulated from extreme temperatures. Their sponginess, along with the potential for the plant to grow larger and stronger roots, also means that soil erosion is reduced.

So, because worm castings are transformative for almost any type of soil, they’re beneficial for just about any kind of plant. Just make sure to tailor the pH to the plant if needed. Many plants prefer slightly acidic soil (between 5.0-7.0). To make your soil more acidic, you can add some peat moss, sand, or pine needles.

One of the easiest ways to measure the pH of your plant’s soil is to use a pH meter. To see which pH meter I recommend, you can check out my recommended tools page.

Are Worm Castings Better Than Compost?

Worm CastingsCompost
Cost$0.10-$5 per pound$0.015-$0.05 per pound
How long it lasts in soilup to 6 monthsup to 12 months
Time to make3-6 months3-24 months

In small volumes, worm castings are better than compost because their nutrients can be immediately used by plants and won’t burn them like some composts. Worm castings can also be made in less than half the time. However, due to the higher cost of worm castings, compost is the better choice if you need larger volumes.

Depending on the quality of worm castings, and if you buy in bulk, the cost can range between $0.10-$5 per pound. The price of compost also depends on the quality and if you’re purchasing in bulk. Compost costs $25-$50 per cubic yard, and a cubic yard of compost is about 1000-1600 lbs. As you can see, if you’re planning on using a large volume of these organic materials to amend your soil, compost is much cheaper and will get the job done (assuming you use one that’s high quality).

Another factor to consider is how long it takes to make the worm castings and compost. Worm castings can a fraction of the time as worms can consume half their weight per day. This might not sound like a lot, but even in the smallest worm bins, having 1 lb of worms is fairly normal. So, processing 1/2 lb of food per day is a good deal of volume. On the other hand, compost can take 3-24 months to make, depending on the type of compost (hot, cold, etc) and the ingredients used.

For example, if you’re using manure in compost, you’ll have to wait for this to breakdown before the nutrients can be used by plants. Manure typically takes at least 6 months to breakdown enough to be used by plants. Many manures are also “hot”, meaning they can’t be directly applied to a plant’s soil anyways as it will chemically burn them with the higher concentration of nutrients (like some synthetic fertilizers). On the other hand, worm castings can be used immediately and won’t burn plants.

Speaking of manure, worms are so great at breaking down nutrients, that they’re even incredibly effective in waste management. So, regardless of if you’re using manure or not in your compost bin, consider adding some worms and making it a vermicompost bin to speed up the process.

How To Apply Worm Castings to Plants

The best way to apply worm castings is by either adding them to the initial potting mix or by using them as a mulch. If you already potted your plant, or are going to use the worm castings for plants in your garden, mulch is the best way to go. This is because mulching doesn’t disturb the often ignored soil ecosystem and beneficial microbes.

If you’re using worm castings in your potting mix, a good amount to aim for is at least 15% of the soil. Like compost, having more worm castings in the soil won’t hurt your plants. The more the better.

When applying worm castings or compost, the two biggest rules to remember are to check the soil’s pH and to not let these soil amendments touch the plant’s base as they can introduce mold.

If you’re applying worm castings in a garden, 15% might not be a useful number for you. In this case, apply 1-2 inches as a mulch in your garden. If you don’t have enough castings to cover the entire garden, focus them near the plants you’d like to support or the area with the weakest soil health. The best place to do this is underneath the canopy, or drip-line, of the plant.

Here are some steps to apply worm castings as a mulch:

  1. Measure the worm castings you’re applying (either at least 15% or 1-2 inches)
  2. Apply evenly over your garden or potted plants
  3. Avoid the trunks or base of plants
  4. Water well to help the castings and nutrients work their way into the soil
  5. Apply every 2 months for best results

Although applying worm castings every two months is beneficial, this can be expensive depending on the amount you’re using. Because castings can stay in the soil for up to 6 months, applying twice a year in addition to fertilizer is a good approach as well.

If you’re planning on continually using worm castings, the best way is to get your own vermicompost bin. I made mine in 10 minutes for under $50, so there’s not much of a barrier here. If you’d like a quick visual of how to do this, you can check out this video I made here:

You don’t need many worms, just a dozen will do (get red wigglers if you can). They’ll reproduce quickly and you’ll have plenty in your soil in no time.

Remember, don’t till your soil as it will disturb and kill off the beneficial bacteria by exposing them to the elements. Your plants use these bacteria as a way to process and absorb more nutrients in the soil. Simply apply the worm castings on top of the soil or in the initial potting mix (the same goes if you’re using compost).