Does Lemon Juice Lower pH? (How it Can Hurt Plants)


The other day, I was wondering if there is a natural way to lower the pH of water, without using harsh chemicals.  I decided to do some research, and I came across the idea of using lemon juice.

So, can you use lemon juice to lower pH?  Yes, adding lemon juice to water will lower pH.  However, doing so may harm your plants in a hydroponic or traditional gardening system.

Remember that lemon juice acts as an antimicrobial agent, which means that it kills bacteria and fungi.  This includes beneficial bacteria in your water or soil that help your plants to grow!

Let’s take a look at the science behind why you should not use lemon juice in a hydroponic system.  Then, we’ll get into the way you should lower pH, what to look out for, and how to make your pH calculations.

Why Not Use Lemon Juice to Lower pH?

Lemon juice has a pH between 2.0 and 3.0, so when added to water, it will certainly lower the pH.  However, the acidity of lemons is not what will kill bacteria.

Lemons contain flavonoids, which are a specific type of antioxidant.  These flavonoids give lemons their antimicrobial properties.  When you add lemon juice to your hydroponic system, you run the risk of the flavonoids killing the bacteria that live there.

(For more information about the study on antimicrobial activities of citrus juice concentrates, including lemon, check out this article on the NCBI site.)

Furthermore, some of the bacteria living in your system are nitrogen-fixing.  This means that they take nitrogen gas from the air and convert it into ammonia. The ammonia is then converted into nitrites, and finally into nitrates.

This is a critical function of bacteria, because plants cannot use nitrogen directly from the air.  Instead, they need nitrates in order to obtain nitrogen for growth.

Nitrogen is one of the most important nutrients for plant growth.  It is found in the compound chlorophyll, which is what makes plants green.  Chlorophyll also helps plants to create energy (sugar) from water, carbon dioxide, and light.

lettuce
Hydroponic lettuce – looks like this plant got enough nitrogen!

Now we’ve uncovered why you should not use lemon juice to lower pH in a hydroponic system.  For the same reason, you should not use the juice of citrus fruits, such as lime, orange, or tangerine, to lower pH.

These substances also contain flavonoids that have antimicrobial properties.  Again, this would be bad news for the nitrogen-fixing bacteria in your system.

So, if you shouldn’t use citrus juices to lower pH, what should you use?  It’s time to talk about the right way to lower pH in your system.

What Should You Use to Lower pH in a Hydroponic System?

To lower the pH in your hydroponic system, you should use something called pH DOWN.  As the name suggests, you simply add pH DOWN to a hydroponic solution to lower the pH.  As an example, the General Hydroponics pH DOWN solution has a pH of 1.2 (more acidic than lemon juice).

Generally, pH DOWN contains food grade phosphoric acid, along with citric acid and mono ammonium phosphate.  Food grade simply means that pH DOWN is safe to use for growing hydroponic plants intended for human consumption.

You can purchase pH down at hydroponic supply stores, either online or in-person.  You can save money by purchasing in bulk quantities.  However, if you are just starting hydroponics as a hobby, it might be better to start with a small bottle until you get your bearings.

What to Watch Out for When Adjusting pH

Remember that pH DOWN contains a strong acid, so you need to be careful when handling it.  Don’t get it on your skin, in your eyes, or in your mouth.  Also, be careful not to splash it on the leaves of your plants, or you will burn them.

In addition, the pH DOWN solution is buffered, meaning that it resists changes in pH.  This helps to keep the pH of your system stable.

However, from the planet natural website, you should be careful “when pH unstable media – such as rock wool or gravel products – are used, or when high plant growth rates destabilize the mix”. (I will go into more detail about how high plant-growth rates can change pH later in the article).

When you add pH DOWN to your nutrient solution, make sure to do it gradually.  A large drop in pH in a short time can stress and damage your plants.

Start off by adding a small amount to your water – less than you think you need, since pH DOWN is very acidic.  I am talking about a fraction of a teaspoon to start with.  Mix thoroughly, wait a bit (perhaps 30 minutes), and see how much the pH changes before you add more.

If you happen to overshoot with pH DOWN and the pH becomes too low, you can always add pH UP to counter the effect.  However, this is a waste of both products, and it takes time to seesaw back and forth.  Also, your plants might not like the swings in pH, so try to get it right the first time!

Finally, remember that the ideal pH range will vary by plant, but a good general range is 5.5 to 6.5 (slightly acidic).

What Else Can Affect pH?

There are some other factors that can affect the pH of your system, so keep these things in mind as you monitor and adjust your pH from day-to-day.

Adding Nutrients to Your Solution

The first factor affecting pH is the concentration of nutrients in your solution.  The nutrients are acidic, so when you add them to your system, they will lower the pH.  To counter this, make sure to add nutrients before you try to balance pH, or you will end up with a solution that is too acidic.

Plants Using Nutrients from the Solution

The second factor affecting pH is the uptake of water and nutrients by your plants.  When plants use dissolved nutrients from the water, the pH of the nutrient solution can move up or down.  This is known as pH drift.

As your plants grow, they will use both water and nutrients from the solution.   Ideally, the plants will use water and nutrients in such a way that the pH and concentration of nutrients stays roughly constant.  However, you will often see plants using one resource (water or nutrients) at a faster rate than the other.

hydroponics bay
Hydroponic growing at NASA

When plants use water faster than they use nutrients, the solution becomes more concentrated with nutrients, leading to a higher TDS (total dissolved solids) or EC (electrical conductivity).

When plants use nutrients faster than they use water, the solution becomes less concentrated, leading to lower TDS or EC.  In this case, you will need to add nutrients to the system.

In both cases, you will need to add water to the system to replenish what the plants have used for growth.  Distilled water or reverse osmosis water is the best choice, since it has few dissolved solids in it.

Also, distilled water does not contain chlorine, which is often found in municipal tap water.  As you might guess, cities and towns use chlorine to kill microbes in the water supply.  While this is good for public health, it is not good for the bacteria in your system!

Of course, you can leave your tap water sitting in a bucket for a day or so to allow the chlorine to evaporate out.  However, tap water can still contain other solids, including iron, copper, or various trace minerals.  Make sure to test the PPM (parts per million) of the water to get an idea before you use it in your system.

Temperature and Carbon Dioxide

The third factor affecting pH is temperature and carbon dioxide in the air.  Carbon dioxide gas will dissolve in water to form carbonic acid, which will lower the pH of your system.  Remember that as the water gets colder, more carbon dioxide will dissolve in the water, leading to more carbonic acid and a lower pH.

On the other hand, keeping your water warm will result in less dissolved carbon dioxide, which means less carbonic acid, and a higher pH.  Before adding pH DOWN, make sure to check the temperature of your system, and adjust downward if appropriate.  Just make sure that the new temperature is within the range that your plants can tolerate.

If you find that the temperature of your water is consistently too high, make sure that your grow lights are not giving off too much heat.  Another option is to use a water chiller for your system.

Other Questions About pH

There are a couple of other questions about pH that we should address.  We already talked about lowering pH, so let’s briefly touch on raising pH.

Can I Use Baking Soda to Raise pH?

Baking soda will raise pH when dissolved in water.  However, this will also increase the sodium content of your water.  This is not a recipe for success when growing plants!  Sodium can restrict the uptake of calcium, which a necessary nutrient for plant growth.

baking soda
Baking Soda, or sodium bicarbonate

The right way to raise pH is to use pH UP, which contains potassium hydroxide and potassium carbonate.  The pH of the General Hydroponics brand is 11.4 to 11.9.  You can check the price of pH UP on the planet natural website.

As with pH DOWN, you should avoid getting pH UP on your skin, in your eyes, or in your mouth.  A strong base can burn you just as badly as a strong acid!

In addition, be careful not to splash it on the leaves of your plants, or you will burn them.

Finally, be sure to add only a small amount of pH UP at a time, to gradually raise the pH of your solution to the desired level.

What Does pH Mean?

The term pH means potential of hydrogen, or the concentration of H+ (positively charged hydrogen ions) in a solution.  A higher concentration of hydrogen ions means a lower pH, and a more acidic solution.

A pH has a range from 0 to 14.  A pH of 7 is considered chemically neutral.  Anything below 7 is an acid, and anything above 7 is a base.

One key thing to remember about pH is that it is based on an exponential (or logarithmic) scale.  So, a solution with a pH of 6 has ten times the hydrogen ion concentration of a solution with a pH of 7.

Similarly, a solution with a pH of 5 has one hundred times the hydrogen ion concentration of a solution with a pH of 7.

You can imagine how quickly this grows: a solution with a pH of 1 has one million times the hydrogen ion concentration of a solution with a pH of 7.

Conclusion

Lemon juice or other citrus juices are not suitable for lowering pH in your system, due to the potential to harm bacteria in your system.  Similarly, baking soda is not suitable for raising pH, due to the sodium content.

The key takeaway is to start slowly, using a little pH DOWN or pH UP to gradually change your pH.  Otherwise, you run the risk of overshooting the target pH range and damaging your plants.

I hope this article was helpful in explaining a bit about pH, and the factors to consider when adjusting your system.  Please leave any questions in the comments 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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