Baking Soda in the Garden – Hacks that Work and Don’t Work

Baking soda in the garden seems to be the next fad. Numerous videos and blog posts tout the many ways you can use baking soda, but do any of these hacks work?

It’s a pesticide, a herbicide, a fertilizer and it makes tomatoes taste sweeter. You can even use it to measure the pH of your soil. It’s a wonder product for the garden. You might think that is all nonsense, but there is some truth in some of the claims. In this post I will look at the claims and separate fact from manure.

What is Baking Soda?

A little chemistry goes a long way in understanding baking soda and what it does in the garden.

Baking soda is sodium bicarbonate with a chemical formula of NaHCO₃. This is a salt and in water it separates into sodium ions and bicarbonate ions. The latter is completely harmless, is found in all living systems, and easily converts to CO2 and water.

Sodium is an essential  plant nutrient, but they need very small amounts of it. Too much sodium in soil is toxic to plants and will kill them.

Baking soda in water is alkaline with a pH of about 8.3.

How is Baking Soda Used in Gardens?

There are numerous claims for baking soda in the garden. Here are some of them.

• Kills insects
• Cures fungal diseases
• Cleans plants
• Stops the foul smell in compost piles
• Increases the number of blooms on plants
• Makes tomatoes sweeter
• Kills weeds
• Good fertilizer
• Kills ants
• Can be used to measure pH of soil

It is also good for cleaning hands and garden tools, but why would you want to clean tools? And when my hands are dirty from gardening, I just use the soap which is already by the sink. If I went to the kitchen and grabbed the baking soda with dirty hands, I would no longer be married. 🙂

Our skin has a protective coating on it which is slightly acidic. Baking soda removes this resulting in dried, cracked skin.

Does Baking Soda Kill Insects?

When you read the fine print, you notice that DIY baking soda insecticides include some neem oil, or horticultural oil, or even cooking oil. It is quite possible that these concoctions kill some insects, but neem and other oils do this all by themselves. You don’t have to add baking soda to make them work.

One recipe used 1 tablespoon of baking soda in a gallon of water. That is quite dilute but it will make the solution slightly alkaline which could harm insects.

Although baking soda may work as an insecticide, it would need to be sprayed right on the insect. Spraying a plant to prevent insect damage won’t work. I found no government agency that recommends baking soda as an insecticide, while many do suggest neem.

Does Baking Soda Cure Fungal Diseases

This is a good example of taking a little bit of science and turning it into a cure-all.

Sodium bicarbonate has been shown to limit the growth of powdery mildew. I have discussed this before in Baking Soda, a Home Remedy Fungicide – the Cornell Formula. Note that this can prevent the growth of mildew, but it does not cure a plant once it has mildew. It does not kill the fungus, it only makes the leaf surface alkaline so that the fungus doesn’t grow.

Most scientific testing has focused on potassium bicarbonate instead of sodium carbonate. Potassium is less toxic to plants and is a macronutrient they need. When combined with various oils it has been shown to be effective at controlling some fungal diseases.

You can source potassium bicarbonate from Amazon (this is an affiliate link and if you buy using the link I may get a fee, at no extra cost to you).

The limited testing of sodium bicarbonate as a fungicide has shown some control in the lab, but when tested in the field the results were poor, even when baking soda is mixed with horticultural oil.

It is frequently claimed that baking soda stops black spot on roses, – it doesn’t.

Baking Soda Can Be Used to Clean Plants

Mix some baking soda in water and use this to wipe down indoor plants. It gives them a nice shine, or so it is claimed. I think this suggestion stems from the fact that baking soda is a good cleaning agent. If it can clean your hands, and pruner, why not the plant.

The Ancient Egyptians used baking soda as a cleaning agent as far back as 3500 BCE, but they also used it to mummify bodies. Is that why mummies have dry skin?

I don’t know if this actually works, but I do know sodium can be toxic to plants, and exposing leaves to a high pH just to clean them, does not seem like a smart thing to do.

Stop Smelly Compost With Baking Soda

I am sure that this idea came from the fact that a box of baking soda in the fridge absorbs odors. If it works there, why would it not work in the compost pile?

Since baking soda is alkaline, it can react with volatile acids (gases), and neutralize them. The level of some acids, like butyric acid from rancid butter, are reduced by baking soda. However, it does not absorb many other odors. This is especially true if the baking soda is sitting in a box with very limited surface exposed. It works better if you dump it out on a plate – but who has room for that in a fridge?

The idea that baking soda keeps your fridge free if odors is mostly a myth.

As your kitchen waste starts to rot it can form some nasty odors, and if these are acids, baking soda can neutralize them. So baking soda in the compost may help a bit. But here is a much simpler solution. Add some dead leaves or straw instead. This will absorb the liquids and once absorbed, the smell is dramatically reduced. Or mix the kitchen waste into the compost pile and the smell stops almost instantly.

This seems like a solution searching for a problem.

Does Baking Soda Increase Blooms

Back to some chemistry. Adding sodium to plants will NOT make them produce more blooms unless the plant is starved of sodium, which is rare.

What about the “bicarbonate”? It is made up of carbon, oxygen and hydrogen and these are important nutrients for plants, but a healthy plant has access to plenty of these through CO2 and water. Adding bicarbonate to the soil does nothing to make the plant healthier.

One video making this claim said, “hydrangeas like alkaline soil” and then showed a blue-flowered Hydrangea macrophylla as an example. What rubbish! This hydrangea grows best in acidic soil, and in alkaline soil the flower color is pink, not blue. Clearly, the people promoting these ideas have very little gardening knowledge.

A plant, that needs a higher pH, growing is acidic soil might benefit a bit, but changing soil pH is very difficult and short term. It certainly won’t increase the number of flowers short term. If you do need to raise the pH of soil, baking soda is a poor and expensive option.

Baking Soda Makes Tomatoes Sweeter

This is another case of using a little science to confuse gardeners.

Baking soda is alkaline and adding it to soil will reduce the acidity of soil. This less acidic soil produces less acidic tomatoes, which taste sweeter.

The only truth in the above paragraph is that baking soda is alkaline. The rest are incorrect conclusions with no scientific basis.

First, the change in soil pH would be insignificant given the amount of baking soda added.

Second, growing tomatoes in less acidic soil does not mean the fruit will have less acid. Flavor in fruits is mostly determined by the environment (temperature and light) and genetics.

A study done to measure the acidity of tomatoes found that all tomato varieties have about the same amount of acid. The sweetness of tomatoes is determined by other chemicals in the fruit.

Does Baking Soda Kill Weeds?

The advice goes like this, “dump handfuls of baking soda over the top of weeds and they die”.

Baking soda contains sodium and sodium is toxic to plants. So yes, if you dump enough sodium onto a plant, it will die.

However, sodium is very soluble in water. So when it rains, the sodium is washed into the rest of the soil, where it might kill plants that are not weeds. Even if this does not happen the sodium is washed into rivers and lakes causing damage there.

It always amazes me that people who will not use chemicals and who want to be organic, have no problem harming the environment with household products.

Even if you want to kill weeds with sodium, table salt is cheaper and more effective, since each ounce contains more sodium. But this is a dumb idea since sodium harms the environment.

Baking Soda is a Good Fertilizer

The headlines read, “Baking Soda is a Great Fertilizer“, or “Baking Soda Gives Plants a Boost“. When you read the fine print you find out that they also add Epsom salt and ammonia, so what they are really doing is adding some magnesium, which probably does very little, and some nitrogen which is the nutrient that is most likely in short supply.

Sodium, carbon, oxygen and hydrogen i.e. baking soda, is NOT a good fertilizer.

Most of these mixtures are quite dilute so they probably do very little unless you use a lot. A little nitrogen can help plants. Just leave out the baking soda and Epsom salt.

Does Baking Soda Kill Ants?

What happens when you add baking soda to an acid? You get lots of bubbles as CO2 is released (think, volcano science project).

If we feed baking soda to ants, and it mixes with the acid in their stomach, what happens? They explode, what else?

The secret is to mix baking soda with sugar in a 50/50 ratio. The ants eat the sugar and ingest the baking soda. This sounds just like the recipe for borax and sugar! Maybe the inventor of this myth ran out of borax and figured any box of white stuff from the kitchen would work as well?

The reality is that adult ants do not eat solid food, so they would not eat the baking soda and therefore would not explode. Baking soda is also not very toxic, although high levels of sodium can probably kill ants.

There is no evidence that baking soda kill ants.

Baking Soda Can be Used to Measure pH of Soil

If your soil is alkaline, adding vinegar will result in bubbles as the acid is neutralized. Adding baking soda to acidic soil also produces bubbles. By using these two tests you can determine if your soil is acidic or alkaline, or at least that is what is claimed.

This topic is a bit more complicated and I’ll do a separate post about it.

I suspect that this test works some of the time, but not always. Even when it does work, it does not tell you how acidic or how alkaline your soil is, and without a number, it is not of much use to a gardener.

What Does Baking Soda Do in the Garden?

Mixed with horticultural oil it can prevent some fungal diseases, but it needs to be applied before you see the disease. Potassiumbi carbonate is a much better choice.

It will kill weeds along with your special plants, and it harms the environment.

That seems to be it. Keep it in the kitchen and bake some cookies instead.