STOP! Does Fertilizer Kill Worms and Helpful Insects?

Are you concerned about the impact of products used in your garden?

If not – then you should be.

It’s all well and good trying to create a stunning lawn, beautiful flower bed, or bountiful vegetable patch with the plethora of treatments available.

But you need to be aware of the “side effects” of such chemicals and formulas.

This includes all fertilizers and pesticides – as collateral damage can be potentially harmful.

Does fertilizer kill worms, for example?

It depends on the type of fertilizer, how often it’s applied, and other key factors.

Let’s explore what they are in more detail, so we don’t harm the organisms that are beneficial to our gardens.

Does Fertilizer Kill Bugs? The Short Version

In order to get you back to working in the garden as soon as possible, here’s the abridged version of this article:

For the most part, the answer is ‘no.’ Fertilizers generally do not kill worms or other helpful insects and bugs. In fact, most fertilizers – particularly of the organic variety, can actually boost worm population.

small worm on green plant

However – there’s always a caveat.

Ammonia and ammonia-based fertilizers can have a negative impact on earthworms, and if using such products, you will likely see a reduction in their numbers. AKA – it can kill worms.

Read on for more information – including why this might be happening, if the type of fertilizer you use has any effect, and more useful fertilizer/worm information.

The Benefits of Worms

Earthworms are the gardener’s best friend.

These little guys are so cool, they offer multiple benefits when it comes to creating and maintaining a beautiful, healthy, lawn and garden.

This includes (but is not limited to):

  • Helping to balance the pH level of the soil.
  • Stimulating microbial activity.
  • Providing channels for root growth.
  • Mixing and aggregating the soil.
  • Controlling pests.
  • Aerating the lawn to improve the flow of nutrients, and drainage.

And speaking of aeration, sometimes, you can’t rely on earthworms to do the job for you, and they need a bit of help. Follow this link for our ultimate lawn aeration guide.

Worms are masters at producing an all-natural fertilizer known as vermicompost – and there’s more on this process coming up.

So, before you start dishing out potentially harmful products, formulas, and/or chemicals, you should be aware if it’s going to harm our little buddies who are hard at work beneath the surface.

Read on to find out.

And if you’d like to know more about other friendly garden creatures and critters, you can take a look at this article on the benefits of sharing your yard with snails.

However, it is possible to have too much of a good thing, and the overpopulation of earthworms can be a bad sign.

This article explains how and why worms can become a problem – and what you can do about it should it occur. If you need to know what kills earthworms in lawns – then that’s your best bet.

Fertilizer vs Pesticide vs Herbicide

Does fertilizer kill bugs? Before we provide the definitive answer, we should bust some jargon.

Crystal clear to some, a little confusing to others – the terminology for certain garden chemicals and formulas is worth explaining, so we’re all on the same page.


Plants need macro and micro-nutrients in order to survive and thrive, and fertilizer can be used to provide them.

hand with granual fertilizer over plant

It should be noted that most fertilizers are designed to nourish all plants, and unless you’re using a weed and feed product, it will also give a boost to undesirable vegetation.

For more information, take a look at this article – does fertilizer kill weeds?


There are commonly three types of pesticide – insecticide, rodenticide, and herbicide.

Insecticides are designed to kill, repel, and/or control unwanted bugs and insects that are harmful to our plants and gardens – such as aphids, caterpillars, grubs, and certain beetles.

Rodenticides are used for the extermination of rodents, such as mice, rats, beavers, and squirrels.

And for the destruction, removal, and control of plant life – we use herbicides for killing weeds – which are, contrary to the belief of some, considered to be part of the pesticide family.

Check out this article for an in-depth guide to the types of weed killers available.

Many pesticides will have a negative effect on worms, insects, and other bugs – so care should be taken when laying this type of product down.

If you have a grub problem, for example, you might prefer to check out this article on how to get rid of grubs naturally, if you’d like to avoid harsh chemicals on your lawn.

And while herbicides are not designed to kill animals and insects, they certainly can do – particularly if you’re using a harsh chemical like glyphosate – the main ingredient in Roundup.

Does grass grow back after Roundup? Follow that link to find out.

Now that’s all cleared up, let’s turn our attention to fertilizers, and see if they have any impact on bug and worm populations in our gardens.

Synthetic or Organic Fertilizer vs Worms

There are many types of lawn fertilizer – and you can follow that link for a detailed guide. But they can generally be divided into two groups – synthetic and organic.

Synthetic fertilizers are man-made products designed to boost/balance nitrogen, phosphorus, and potassium levels in the soil – essential nutrients to plant life.

Often referred to as NPK, after their chemical symbols, achieving the correct balance is vital to healthy plant life in your garden – be it lawns, flowers, and/or vegetables.

person using fertilizer

Organic fertilizers are better suited to providing micronutrients (which synthetic options can’t do), and finely tuning the soil with all-natural, time-release formulas.

But which one, if any, is more harmful to worms?

It’s not as black and white as you might initially think. Actually, more white and white.

Organic fertilizers are very beneficial to the earthworm population, and using such products can result in an increase of worm presence in your soil.

The United States Department of Agriculture estimates that using organic fertilizer can “double or triple earthworm numbers in a single year.”

But synthetic fertilizers – if used correctly and at the right amount – should actually have a similar effect.

Although this is more indirect, as worms benefit from the increase in plant life that results from such action, “synthetic bad – organic good” is not the case.

Worms can and should be thriving in adequately fertilized soil – regardless of whether it’s organic or synthetic.

The problems start when you lower the soil’s pH level – which is done by using ammonia-based products, sulfur, and/or sulfuric acid.

Our earthworm friends prefer to make their homes in neutral pH soil, so lowering or even raising the acidity/alkaline level into a range that makes them uncomfortable isn’t going to do them any favors.

You should always check the ingredients of anything you use in your garden, so you know exactly what you’re laying down, and you can research any potential unwanted effects.

That said, the worm population might take a dip at first, but they should bounce back, once the soil has balanced out, and new biomass gives them a rich food and nutrient supply.

Liquid or Granular Fertilizer vs Worms

Like herbicides and pesticides, fertilizer is available in liquid and granular form. But does that make a difference when it comes to harming worms or other insects?

If you’re using a liquid fertilizer, a dip in earthworm numbers may result, largely thanks to the salt and ammonia content.

This is only temporary, however, and the worm population should bounce back in due course, as mentioned above.

At the time of writing, there’s no evidence that granular fertilizer will have a negative effect on earthworms, at least, not because it’s in pellet form.

And for more general information on when it comes to choosing liquid or granular fertilizer – follow that link.

Fertilizers and Pollinators

Bees have been in the news a lot recently because it’s thought that their numbers are dwindling – which would cause a knock-on effect that basically spells doom for every living thing.

Bees are vital to every living organism, and should be nurtured, protected, and assisted at every available opportunity.

So, does fertilizer kill bees?

Thankfully, there’s no evidence to suggest using synthetic or organic fertilizer in your yard is harmful to the pollinator population.

granual lawn fertilizer

Again, I would urge caution when using any type of pesticides, though. And if you are removing dandelions from your lawn, consider planting a bee-friendly desirable to balance out the loss.

Final Thoughts

Not being a biologist/scientist/horticultural/chemical boffin, I can only use my research skills and experience to find out if fertilizer kills worms.

Here are my two cents:

Lowering or raising the pH level in your soil is going to adversely affect the earthworm population, and potentially that of any other useful bugs and insects in the process.

But the resulting plant life and new biomass should help numbers return to normal, and you will potentially see an increase in worm activity in due course.

However, just to cover my own butt (and as a general word of caution and common sense), you should always take care when dispensing any kind of synthetic product/chemical/formula in your lawn.

When in doubt – go organic. And you can follow this link for some excellent organic gardening tips to get you started.

Organic fertilizer is going to attract helpful bugs to your yard, such as dragonflies, ladybugs, and bees.

Ladybugs (or ladybirds if you’re not from North America), are particularly useful as a natural predator for aphids. In fact, did you know they were introduced in the US in the mid-1900s to control the aphid population?!

Working in balance with Mother Nature should always be preferred over using harsh chemicals – and will inevitably yield better results.

Try building your own worm farm to produce something called vermicompost – the end-product of worm waste, and an unbeatable nutrient-rich soil fertilizer and conditioner.

This article on compost worms vs earthworms will show you how.

Either way, if you don’t have a good kitchen compost bin on the go already, then follow that link to get one, ASAP – and stop throwing money away!

person in yellow gloves holding granual fertilizer

Used in conjunction with a  compost tumbler in the yard, it will help you create the best all-natural fertilizer, and save you a stack of cash at the same time!


How Often Should a Garden be Fertilized?

It is possible to overfeed a garden, and damage just about everything in it – including worms and insects.

If this occurs, then even a product that is supposed to be beneficial can actually do harm.

As such, it’s important to understand how and when a garden should be fertilized. That link will tell you everything you need to know, so you don’t end up “burning” your plants, flowers, and vegetables.

Or harming our industrious invertebrates!

Does Miracle Gro hurt worms?

This particular product contains – among other things – ammonium phosphate, which, as explained above, can have an initial negative impact on the earthworm population by reducing the pH level of the soil.

However, this should only be temporary, and you should see a resurgence in earthworm activity with new plant life.

Just take care when using anything toxic in your garden.

Does fertilizer go bad?

Good question – and it’s something we hear often at Yardthyme. Can you use a fertilizer indefinitely, or does it have a use-by-date? Will it have an impact on plants and animals if left for a long time?

This article on fertilizer shelf life will tell you all you need to know.

Do worms eat fertilizer?

Not directly, no. They will, however, eat the resulting organic matter that the fertilizer boosted, but you don’t have to worry about them crawling around, hoovering up any granules or anything like that!


Does fertilizer kill worms? Not right out of the box/packet/bottle, and nor is it designed to.

But certain ingredients might have an effect on the worm population in your lawn, flower beds, and vegetable patches, as well as overfertilizing and applying too much product.

I hope this article has provided you with enough information for you to treat your garden confidently, as you see fit.

Reach out if you’ve had any negative experiences with fertilizers harming anything they’re not supposed to. Let the community know if we should steer clear of a particular product.

Stay safe out there, and happy gardening!

Andy Gibson

My name's Gibson. Andy Gibson. I like to think of myself as the Bond of the backyard, that is if yard work ever became sexy. I write about everything about indoor and outdoor gardening and the dread-it-but-still-need-to-do-it chores around the yard, like cleaning out the gutter guards.

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