Can Too Much Manure Kill Plants? (Only If You Do This)


A little bit of manure can go a long way in helping your plants to thrive.  Manure adds structure to soil and provides nutrients for plant growth, including nitrogen, phosphorus, and potassium.

However, I started to wonder if it is possible to “kill your plants with kindness” by giving them too much of a good thing (that is, excessive manure).  After all, it is possible over water plants, so perhaps over manuring is a problem as well!

So, can too much manure kill plants?  Yes, too much manure can kill plants.  This is especially true if the manure is fresh (not composted), which means that it will contain high levels of nitrogen in the form of ammonia.  Manure from cows and horses may also contain herbicides that can harm your plants.

This doesn’t mean that we should stop using manure entirely.  We just need to be careful about where we get our manure, how long we let it decompose, and how much of it we use.

Let’s take a closer look at the ways that too much manure can harm your plants.  We’ll also get into how long manure takes to break down, and how to tell if manure is ready to add to the garden.

Can Too Much Manure Kill Plants?

Yes, too much manure can kill your plants.  There are a couple of different ways this can happen.

One way is by excessive nitrogen and salt, which can happen with “hot” (high nitrogen) manure or “fresh” (not composted) manure.  The other way is a little less obvious, and very sneaky: herbicides (weed killers).

Nitrogen and Salt in Manure

If you use fresh manure in your garden, you run the risk of exposing your plants to high levels of nitrogen (in the form of ammonia) and salts.

manure
Manure that is too fresh, or not composted completely, may contain too much nitrogen (ammonia) or salt, which can harm your plants.

This is much more likely with “hot” manure that contains lots of nitrogen.  Chicken manure stands out as having the highest nitrogen content, while cow manure has the lowest nitrogen content.  Of course, this will vary depending on the diet of the animals in question.

The ammonia in animal waste helps to explain the strong foul smell that we associate with manure.  In concentrated amounts, ammonia may be more than your plants can handle all at once, leading to “fertilizer burn”.

One sign of fertilizer burn on plants is leaf scorch.  Scorched leaves will turn yellow or brown, and may eventually become dry and brittle or fall off the plant.

Manure may also contain high amounts of salt.  According to the University of Arizona, cow manure can contain 5% to 10% salt.  If applied fresh to your garden, this is enough salt to damage or kill your plants.

Adding too much salt-heavy manure to your garden at once can prevent plants from absorbing water, even if you are providing plenty of irrigation.  Too much salt can also interfere with a plant’s abilities to absorb other nutrients in the soil.

For more information, check out my article on over fertilizing your plants.

Herbicides in Manure

Herbicides are often used by gardeners and farmers to prevent the growth of weeds (which are basically undesirable plants).

Unfortunately, these herbicides can also kill some of the plants that we do want around, such as the ones in your garden.

weeds
Herbicides are meant to kill weeds, but they can kill some of the plants we want to grow too!

Animals such as cows and horses sometimes end up consuming herbicides when they eat grass, hay, or straw from areas treated with herbicides.

These herbicides will sometimes pass through an animal’s digestive system and end up in manure.  If the manure is used in a garden, it will still contain some of the herbicide.

This can kill your plants, especially when they are young and just getting started (as with direct seeding).  To avoid this, make sure that you know and trust the source of the manure you are using.

Ask what the animals eat, and if the food is grown in areas treated with herbicides.  If you cannot verify that manure comes from herbicide-free animals, you should find another source.

If you raise your own chickens (or if you know someone who does), you can use chicken manure instead.  Just remember what we mentioned before about chicken manure – it is hot, meaning that it contains the most nitrogen out of any type of manure.

If you want to find more sourcing ideas, check out my article on where to find manure for your garden.

How Much Manure is Too Much?

To reduce the risk of killing your plants, apply a reasonable amount of manure.  This will depend on your soil type and conditions.

Usually, a thin layer of composted manure (0.25 to 0.5 inches deep) is all you need to provide some nutrients and organic matter to your soil.  You may want to use more manure (a layer up to 1 inch deep) if your soil is low in organic matter.

For instance, sandy soils can benefit from a deeper layer manure or compost.  In that case, you may want to work the manure into the soil instead of using it to cover the soil.

sandy soil
Sandy soil can benefit from the addition of more manure to provide organic material and allow for better water retention.

Don’t apply manure in heavy amounts year after year without testing your soil.  For more information, check out my article on how to test your soil.

Even more important is that you allow the manure to decompose (break down) completely before using it in your garden.

How Long Does Manure Take to Break Down?

Manure takes 3 to 12 months to break down completely.  Manure will take longer to break down if it contains lots of animal bedding, such as sawdust or straw, since these have high carbon content.

sawdust
Manure that contains sawdust has a higher carbon content and takes longer to break down.

For more information, check out this article from the cooperative extension on aging manure.

If you want composted manure for this year’s garden, you might not have time to wait a full year.  In that case, there are several ways to make manure decompose faster.

First, you can add “greens” to the manure and mix them together.  This provides extra nitrogen, which balances out the carbon in sawdust or straw.

Greens that are rich in nitrogen include:

  • Grass clippings
  • Leaves and other cuttings from plants
  • Weeds
  • Tea bags
  • Coffee grounds
  • Seaweed or kelp
  • Vegetable and fruit scraps from your kitchen

By adding greens, you are creating more of a compost pile than a manure pile – but it will still decompose and provide nutrients to your soil when it is ready.

coffee grounds
Adding coffee grounds and other greens to your pile will provide nitrogen and speed up decomposition.

You can also add worms to your manure or compost pile to speed up the decomposition process.  Worms will often find their own way into manure or compost, but you can also “transplant” them from elsewhere, or buy some online or at a local bait shop to use for this purpose.

In addition, you can mix the pile periodically to aerate it and provide oxygen to the bacteria that break down the manure.  Do this by turning the manure pile over with a pitchfork or shovel.

The manure should get hot (130 to 160 degrees Fahrenheit) if all is going well.  It may cool down temporarily, but it will heat back up several times if you turn the pile.

Finally, you may need to add water to your manure or compost pile if it is dry.  A good time to add water is when you are turning the manure pile as mentioned above.

For more information, check out this article from Oregon State University on turning your manure into compost for the garden.

How to Tell If Manure is Composted (Well-Rotted Manure)

As mentioned earlier, it is important to make sure that manure is well-rotted (composted) before adding it to your garden.  That raises the question of how to tell if manure is composted completely.

When it is ready for your garden, composted manure will look like soil.  There should be no visible trace of sawdust, wood shavings, straw, or other animal bedding.

compost bin
When manure is well-rotted, it will look like soil rather than manure.

The composted manure will be dark and crumbly.  It should not have any foul smell.  Instead, it should smell like fresh earth.

Another way to tell if your manure is composted is to let the pile itself tell you.  First, turn the pile over with a pitchfork or shovel to aerate it, as mentioned above.

Wait a few days, and if the pile is hot, then the bacteria are still working.  In that case, you can allow them to keep processing the manure.  If the manure pile stays cool a few days after turning, then it is probably done.

You may want to turn the manure pile about 6 times in total to aerate it and allow it to get hot in between turnings.  The high temperature in the pile (130 to 160 degrees Fahrenheit) will usually kill weed seeds and pathogens.

After the bacteria have worked the compost, allow it to “cure” for an equal length of time.  For example, let’s say that you turn and aerate the manure pile once a week for 6 weeks.

Then you should let it cure for 6 more weeks.  This means it will take a total of 12 weeks, or about 3 months, for the entire process.

For more information, check out this article on manure from the Michigan State University Extension.

Conclusion

By now, you have a good idea of how too much manure can kill plants.  You also know how to prevent the problems of excessive nitrogen or salt and herbicides in manure.

I hope you found this article helpful – if so, please share it with someone who can use the information.  If you have any questions about manure, please leave a comment below.

jonathon.david.madore

Hi, I'm Jonathon. I’m the gardening guy (not guru!) who is encouraging everyone to spend more time in the garden. I try to help solve common gardening problems so that you can get the best harvest every year!

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