#3 Total Dissolved Solids

Addison
Jul 13, 2024
5 min read

Updated: Apr 13

Total Dissolved Solids

Total Dissolved Solids (TDS) is the quantity of dissolved material in a given volume of water. Similar terms are Total Dissolved Salts and Salinity. [1] TDS includes salts as well as small organic particles that are suspended in the water whereas the other two terms only measure salts in the water. Depending on your tap water and how many small organic particles are suspended in it, TDS can be the same or different from Total Dissolved Salts/Salinity. TDS can be a useful measure for us in bonsai because it can tell us roughly how much stuff is making its way to our bonsai tree from our tap water.

Salts

When you hear salt, you probably think of the kind you eat (chemical formula = NaCl), but that’s only one example of a Salt in chemistry. Salts are ionic compounds formed of a cation (+ charge) and anion (- charge). In water, the ionic bonds holding the two charged particles together break and each ion separates and forms connections with the water molecules instead. When you water your plants, these ions enter the soil and are able to bind to exchange sites on your soil particles or are brought by mass flow to the root surface by water that is entering the roots directly. Once they have proximity to the roots, they can be absorbed by the plant. In general, nutrients have to be in a water soluble form (split into ions) before they are available to the plant! [2]

There are many types of organic and inorganic salts and not all of them are useful for plants. Common salts used in fertilizers to deliver elements like N, P and K are things like ammonium nitrate (NH4NO3), Potassium Chloride (KCl), Phosphorus Pentoxide (P2O5) , etc. As an example, KCl forms K+ and Cl- ions in water, and the K+ is absorbed into the plant to provide for its Potassium needs.

How do we measure TDS?

To truly measure TDS, you need to evaporate all the water in a solution and weigh the leftover material. As a hobbyist the easier and faster way to measure TDS is to use a TDS meter which measures specific conductance (eC) as a proxy for TDS. Pure water has almost no electrical conductance. [3] When salts are dissolved in water, however, it can raise the conductivity precipitously. A TDS meter measures the conductivity of the water by emitting an electrical charge into the water and seeing how strong the charge is in order to correlate that with the number of dissolved ions in the water. [4] A stronger charge means there are more ions and a weaker charge means there are fewer. The meter measures in units of eC (electrical conductance) and so an empirical formula is used to estimate the TDS based on this electrical conductance measurement. There are multiple different constants used to do the conversion, but we will use TDS = Electrical Conductance (mS/cm) * 640. [5] It is important to note that TDS doesn’t tell us exactly what molecules are in the water, just that the electrical conductivity of the water is high enough that we’re guessing there are about x milligrams of dissolved substances per liter of water also expressed as parts per million (ppm).

What is High or Low TDS?

Let’s put TDS readings in the context of common liquid fertilizer package instructions. I have a package of 20-20-20 inorganic fertilizer that you mix with water. After mixing to the every two weeks recommended dose for my fairly aggressive 20-20-20 fertilizer, my water goes from 33 ppm TDS to around 2000 ppm TDS. This means I went from 33 milligrams of dissolved substances per liter of water to 2000 milligrams of substances per liter of water. If I was watering with this daily instead of biweekly, I’d have an average of 142 ppm TDS per day of watering to hit the same amount of fertilizer in a two week period. We know that fertilizing impacts our trees and we can see that the TDS is relatively low for the package instructions on an average daily basis. Many hobbyists water with tap water greater than 142 ppm TDS. Tap water is typically under 500 TDS as a standard for drinking water for humans, but based on the above calculation, we can see that plants are impacted by compounds in water even at much lower TDS values like 142 ppm TDS.

If you’re fertilizing with a more mild fertilizer, then you could be fertilizing in the 200-1000 ppm TDS range. If you’re fertilizing once a week, but watering every day, then your plants could be getting way more exposure to ions in your tap water than actual fertilizer. You can see how not being cognizant of what’s in your water could have a big impact on what your plants are exposed to. In bonsai, since we have a semi closed ecosystem, the plant is really relying on any solid fertilizers you put in the pot, liquid fertilizers you add to your water, or elements in your tap water for nutrients.

Conclusions for Bonsai

TDS is not a specific enough measurement for us to make a blanket statement that High TDS is bad since TDS just says there is “stuff” in our water without giving details. Your water could have high TDS because it is sea water and there are a lot of toxic NaCl ions in it, or it could be filled with a bunch of plant fertilizer components and be great for your plants.

That being said, tap water isn’t optimized for plant nutrition. We know this from the first article where we saw the low overlap between the 17 essential elements and common substances in tap water. This underscores why having high TDS tap water should at minimum lead to more thorough investigation. We’ll need to keep studying the elements in tap water to see which ones are harmful, which ones are helpful, and which ones are a distraction. Watering with high TDS tap water does mean that more of the cation exchange sites that roots grab nutrients from in the soil are being taken up by cations in your water and leading to fewer interactions between beneficial cations (fertilizer) and the roots. This could lead to slower growth and nutrient imbalances.

There is no excuse to not have a TDS meter as they only cost $9 on Amazon. I recommend all bonsai enthusiasts have a TDS meter to test their tap water and their fertilizer mixtures so they can ensure consistency in fertilizing and understand the balance of elements reaching their plants.