Does watering indoor plants with carbonated water boost growth?
Don't throw out your Soda Stream! It turns out carbonated water can actually benefit our house plants, but there are a few things to watch out for before you pick up the bubbly next time you water.
How does carbonated water benefit indoor plants?
Fizzy water. Sparkling water. Soda water. It's the added carbon dioxide gas, or CO2, that makes the bubbles. That's one way it could benefit your plants. Not for the bubbles, but for the higher carbon levels and increased mineral uptake, which have been shown to increase growth rate and make green foliage greener.
Plants are known for taking CO2 from the air through their leaves, however roots do also take up CO2. Multiple studies have shown plants can derive carbon from the CO2 in carbonated water. A few studies also reported watering with carbonated water increased levels of calcium, magnesium and zinc present in the leaves, compared to the control plants watered with plain water.
One often cited study by the University of Colorado Boulder in 2002, found that plants watered with carbonated water grew more than twice as fast and developed healthier shades of green over a 10 day period. That study was performed using Baby Tears (Soleirolia soleirolii). However other studies have found carbonated water did not change the growth rate, or in some cases, stunted growth.
Depending on the type and source, carbonated water may also give your plants a mineral boost. This is thought to be due to a combination of the minerals in sparkling water itself, and carbonated water being more acidic than plain water, which can potentially increase nutrient availability in the soil.
Some sources of carbonated water include extra phosphorus, potassium and sulphur. Sparkling mineral water is what you ideally want, as mineral water includes extra magnesium and calcium also. Not exactly budget-friendly however compared to tap water or rain water!
4 things to watch out for when watering house plants with carbonated water...
#1 Less minerals if you use Sodastream
If you have a Sodastream at home and are feeling pretty excited right now about its new found use, hold your horses. Note that Sodastream themselves say that the process of carbonation with a Sodastream means...
"...any minerals originally present in the water are often removed during processing"
...that doesn't take away the benefit of the CO2 however. Sodastream water will still add carbon dioxide, and still help soil become more acidic.
#2 Carbonated water doesn't replace fertiliser
If you don't fertilise, or you do but you're not using a complete and balanced fertiliser, the added minerals and acidity may temporarily boost absorption of certain minerals like calcium.
However carbonated water doesn't provide all the minerals plants need, or at the levels they need them. Plus the acidity of sparkling water (see #3 below), means while intake of some minerals may be increased, others are likely decreased, it all depends on the current pH of your soil.
#3 Acidic pH
You may have heard carbonated water is bad for your teeth. That's because it's is below 'critical pH' (that's levels below 5.5, which is when our teeth start to demineralise).
I tested my tap water (I'm in Auckland, New Zealand), and it was neutral, around pH 7 (if you want to test yours I use my aquarium pH test kits). Yes, carbonated water is acidic. It averages around 4 to 5 pH. Compare that to Coke which is even more acidic, around 2.7 pH.
Keep in mind also that pH is logarithmic, which means a pH of 4 is 10 times more acidic than a pH of 5, and 100 times more acidic than a pH of 6, so the change in pH levels are a much bigger deal than they sound.
Soil pH below 4.6 is too acid for most plants. A pH range of around 5.5 to 6 is ideal for most indoor plants. Above and below that can cause problems, reducing availability of some nutrients, and turning other nutrients toxic.
For example, at a pH of 5 only 40% of nitrogen, 35% of phosphorus and 50% of potassium are available. The jump to a pH of 5.5 means nitrogen and potassium both go up to 70% available, but phosphorus is still low at 45%. Shift again to a pH of 6 and all 3 major essential nutrients are fully available. What we also don't know is how much carbonated water drops soil pH, and for how long it impacts pH, but it certainly has been shown to change the pH of the soil.
In saying all that, keep in mind if your soil pH is too alkaline, then the acidity of carbonated water is a very good thing! It has been shown help bring alkaline soil pH down into a healthier range, in which case it would increase nutrient uptake. In studies where the soil pH was neutral or alkaline, carbonated water performed best to increase growth.
Tip: Half and half is better
One recommendation is to mix half plain water, half carbonated, to help bring the pH of the carbonated water up to a less acidic level.
Tip: Warmer's better
The good news is carbonated water does become less acidic when warm, with the average shifting from 4 to 5 pH, to around 4.5 to 5.5 pH when tested at 21 degrees. Still acidic, and still below critical pH (for teeth), however not quite as bad when you consider how big even a small change in acidity is.
Sparkling water is often stored in the fridge, but carbonated or not, always avoid watering indoor plants with cold water
Too cold and you can cause root shock which can lead to irreversible root damage as well as leaf drop. Best taken out of the fridge and left sitting out for a while to warm up to room temperature before watering. Also better not left with the lid off as the Co2 will 'bubble off' and lose its fizz.
#3 Not all types of carbonated water are okay for plants
Avoid anything with added sugar or colour. You want to use plain carbonated water only, ideally sparkling mineral water if budget allows. Don't use Tonic water ether. Although it is a form of carbonated water, it also contains quinine and added sugar. Sugar can cause reverse osmosis, making a plant lose water and eventually die.
#4 Fertiliser and carbonated water (probably) don't mix
If you do fertilise your plants, but also want to try carbonated water, it's likely best to alternate between fertiliser for one water, then carbonated water for the next, rather than combining the two together.
Three reasons. One is because the carbonated water (depending on the source), has some minerals in it already. The second is because adding fertiliser may cause the carbonated water to fizz up, reducing the Co2. The third reason is I doubt fertiliser companies have tested what carbonated water does to the fertiliser itself!
Sparkling water may interfere with the nutrients in the fertiliser. The acidic pH may also inhibit the plant's ability to absorb the nutrients anyway. Some fertilisers (like GT), are pH buffered to maximise nutrient availability, and carbonated water would no doubt play havoc with that, somewhat defeating the purpose of fertilising in the first place.
Should you water your indoor plants with carbonated water?
Taking everything into account, although I think it is worth a try as an experiment, it isn't something I'm going to do on the regular myself. Done occasionally, and keeping these tips in mind below, I think we can call this as a plant hack that does work with a few caveats.
If you do want to give it a go, my tips would be...
- Unflavoured, sparking mineral water will likely offer the most benefits, but don't throw away your Sodastream as it will work too.
- Let it warm to room temperature or around 21 degrees before watering.
- Mix half and half with plain water to help counteract the acidic pH (unless you have alkaline soil, in which case best used undiluted).
- Alternate one water with carbonated, one with fertiliser and plain water. Don't mix fertiliser and carbonated water together.
In an ideal world you'd test your soil pH first though, as a key reason carbonated water appears to benefit plants is when your soil pH is too alkaline. If you're already in the ideal range, or already too acidic, it's less likely to help, and may even harm growth and nutrient availability.
For me personally, I prefer providing the minerals my plants need, in the correct amounts, at the correct pH, by fertilising. I use the 'weakly weekly' method, which means fertilising lightly, every time you water, to avoid both deficiencies and excess.
If you're wanting to use sparkling water to boost your calcium or magnesium levels in particular - which carbonated water has been shown to do (and considering most fertilisers do not provide calcium) - I'd either change to a fertiliser with calcium in it, like GT or Dyna-Gro, or sprinkle a teaspoon of Dolomite Lime over the surface of your indoor plants once a season.
However what I definitely will be doing, is saving this one for when my son has to do his first science experiment!