pH & TDS Explained

Few things in Hydroponics are as important as water. To get control of your water you’ll need an understanding of pH and TDS. This article will explain the tools and techniques used in testing, adjusting and controlling the levels of a nutrient solution.


One of the largest advantages to hydroponics is control, and one of the key factors to control is your water. Both the pH value and TDS (total dissolved solids) play a pivotal role in the health of your garden. If the water’s pH value is too low or too high the plants have trouble metabolizing a whole array of different nutrients. If your nutrient strength is too low it will likely cause deficiencies, too high and you get nutrient burn or lock out. This article will review the techniques and tools that will help you stay in control of your water.

pH Defined

pH stands for potential hydrogen, in chemistry terms it is the measure of the activity of the solvated hydrogen ion in a solution. In laymen’s terms it is the unit of measure indicating whether a solution is acidic, neutral or alkaline, and to what degree. The pH scale goes from 0 to 14. A pH value of 7 is neutral, any values below 7 are acidic, and any values above 7 are alkaline.

The pH scale is logarithmic and as a result, each whole pH value below 7 is ten times more acidic than the next higher value. For example, pH 4 is ten times more acidic than pH 5 and 100 times (10 times 10) more acidic than pH 6. The same holds true for pH values above 7, each of which is ten times more alkaline than the next lower whole value. For example, pH 10 is ten times more alkaline than pH 9 and 100 times (10 times 10) more alkaline than pH 8.

Plants growing in soil typically like a pH value between 6.0 – 7.0. In hydroponics, plants like it slightly more acidic, and pH value between 5.5 – 6.5 is usually best. When outside of this range nutrients become less accessible for plants as shown in the graph below.

So now you understand the importance of properly balancing your water’s pH level, and the role it plays in the overall health of your garden. Now we should review the different ways to test and adjust pH.

Testing pH levels

In order to change or adjust the pH of a solution you must first have a way to accurately test it. You can use manual test kits or electronic meters to get your base level pH to determine if it needs adjustment, and if so, which way to adjust it, up or down.

Manual testing methods include Litmus papers or liquid test kits. Litmus papers are very inexpensive but can be quite hard to decipher. They are orange slips of paper that change color when dipped into a liquid solution. Each color represents a different pH value. The trouble is that the values we test at for gardening are often very close to the natural orange color of the litmus, making it difficult to pin point a result. Liquid test kits, on the other hand, will give much better results for only a slightly higher price. Both of these tests will only get you in the ball park of results, for more detailed testing you may want to look into a digital testing meter.

Liquid pH Test kit by General Hydroponics. Click on the image to open the product page for this item.

pH meters

The vast majority of hobbyist level pH meters use a combined glass electrode for measuring the pH of a liquid solution. A combined glass electrode has an in-built reference electrode inside of a porous glass ball. For long probe life it is crucial to keep the probe hydrated with a storage solution to keep the pores on the glass electrode from drying out. A periodic use of pH electrode cleaning solution will also help in keeping sediment from clogging the pores of the glass electrode.

Frequent calibration is also a necessity with pH meters, as their readings can drift over time. Most meters will calibrate to a standard buffer solution of 7.0. Some meters will allow for a two point calibration calibrating to 7.0 and 4.0. A two point calibration is more accurate, as it will accommodate the fact that the “slope” may differ slightly from ideal.

The Milwaukee pH600 is a economical meter with a low price. It us not waterproof and uses a trimmer screw for calibration which can be a little finicky. Click on the image to open the product page for this item.

The Oakton Eco-Testr pH1 is a mid range meter that features a waterproof body and easy push button calibration.

The Hanna PHEP 4 is a high end pH meter that features: waterproof body, easy push button calibration, two point calibration, and a replaceable probe.

pH Adjustment

Now that you can test your pH levels you can determine whether it needs to be adjusted. You want to be certain all your nutrients are added in as they themselves can alter the pH of the solution. pH adjustment should be your last step in making a nutrient solution. If you need to lower the pH of you water the best ingredient to use is phosphoric acid, the plants can just absorb it as another form of phosphorus. To raise the pH of a nutrient solution the best ingredient is potassium hydroxide that the plants can just take up as potassium.

Our in-store pH adjusters are made from phosphoric acid and potassium hydroxide. Unfortunately, since they are classified as an acid and base they have strict shipping regulations and we cannot offer them online. If you can’t find a local source for these adjusters you can use some household items in a pinch. Lemon Juice can work to lower the pH and baking soda can be to used to raise the pH.

TDS defined

TDS stand for total dissolved solids and refers to the measure of all substances contained in a liquid. It is the measurement used to determine nutrient strength. TDS in hydroponics has two different readings; Conductivity and Parts per Million (PPM). Conductivity is the more scientific of the two readings, it can be represented as Electro-Conductivity (EC), Conductivity Factor (CF) or Micro Siemens per square centimeter (MS/cm). PPM is not quite as accurate a representation of TDS but much more common within the United States. If you talk with a gardening friend or read information on a forum chances are the growers there will talk in PPM terms and not Conductivity.

PPM is directly calculated from Conductivity and there are two different scales: NaCl (1EC = 500ppm) and the 442 scale (1EC = 700ppm). Different meters offer different conversion factors and some offer a sliding scale or the option to change the scale to your choosing.

Testing your TDS

There are not many manual testing options for TDS and the ones we have seen use to large of a value range to be useful for hydroponics. Digital Meters are you best option for monitoring your nutrient strength.

All TDS meters are, in reality, conductivity meters, the PPM meter still measure conductivity but use a conversion formula to translate its reading to ppm. TDS meters work by applying a voltage between two or more electrodes. Positively charged ions will move toward the negatively charged electrode, and negatively charged ions will move toward the positively charged electrode. Because these ions are charged and moving, they constitute an electrical current. The meter then monitors how much current is passing between the electrodes as a gauge of how many ions are in solution.

TDS meters will only detect mobile charged ions. They will not detect any neutral (uncharged) compounds. Such compounds include sugar, alcohol, many organics (including many pesticides and their residues), and unionized forms of silica, ammonia, and carbon dioxide. These meters also do not detect macroscopic particulates, as those are too large to move in the electric fields applied. Bacteria and viruses also won’t be detected.

The HM Digital TDS-3 is an economy scale TDS meter. It is not waterproof but still feature a push button calibration. (this meter calibrates to the NaCl scale of ppm)

 

The HM Digital COM-100 is a mid range TDS meter with a lot of functionality. It can read/calibrate EC and PPM (NaCL or 442 scale). The Com-100 has a 1 year warranty and offers a replaceable probe.

 

The BlueLab Truncheon TDS is a high end meter with an unbeatable 5 year warranty. The Truncheon shows four scales: EC, CF, ppm(442) & ppm (NaCl). Also it never needs to be calibrated.

Combo Meters

You can get the best of both worlds in one device with a combo meter. A combo meter will have the electrodes and components needed to test both the pH and TDS of a solution. Many people like this option for ease of use and less equipment to deal with.

The Hanna Combo HI-98129 is a compact handheld combo meter. It reads pH, MS/cm or ppm (with and adjustable scale). It is waterproof and the pH probe is replaceable.

The Nutridip Tri Meter is a wall mounted continuous combo meter. It reads pH, ppm (NaCl scale), as well as temperature.

The Hanna HI-9813-6 is a portable combo meter with all the bells & whistles. It measures pH, MS/cm, and ppm (sliding scale). It also offers a cal-check feature along with simple turn dial calibration.

 

Thank you for reading our article on “pH and TDS Explained”. If you have an questions or would like us to expand on some information in the article please post a response below. You could also contact us directly at 1-888-833-4769 (toll-free) or write to us at sales@urbansunshine.com. Now that you understand pH and TDS you ready to start growing hydroponically!

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4 thoughts on “pH & TDS Explained

    • Discussing pH in terms of hydrogen ions (H+) and/or hydroxide ions (HO-) can get confusing without a chemistry background. Which is why we only touched on it briefly in this article. The pH scale, (0 – 14), is the full set of pH numbers which indicate the concentration of H+ and OH-ions in water.

      pH Scale Principle:
      H+ ion concentration and pH relate inversely.
      OH- ion concentration and pH relate directly.

      The following statements may be made about the pH scale
      numbers.

      a. Increasing pH means the H+ ions are decreasing.
      b. Decreasing pH means H+ ions are increasing.
      c. Increasing pH means OH- ions are increasing.
      d. Decreasing pH means OH- ions are decreasing.

  1. I was trying to figure out why the pH of my solution drifted upwards over a day or two, even when it was just sitting there and not recirculating. As an experiment, I took a glass ball quart jar, and filled it with water adjusted to pH of 5.5. I put a ceramic air stone in it and left it run for two days. After the two days, the pH had risen to 6.5.

    I read on another web site that CO2 and carbonates (i.e. hard water) may cause this, I believe in two different ways:

    1) When you add an acid to water to lower the pH, and the water has carbonates (a form of water hardness?) in it , the acid combines with the carbonates to form free carbon dioxide and water. ( HCO3 (carbonates) + H (acid) = CO2 + H2O)

    The free carbon dioxide (CO2) then reacts with water to form carbonic acid, further lowering the pH: CO2 + H2O = H2CO3 (carbonic acid)

    The problem is that higher levels of CO2 cannot be maintained in the water, and as that CO2 leaves the water over time as a gas, there is less carbonic acid and the same amount of carbonates available, and thus the pH drifts upward.

    2) Your water right out of the ground or water supply may already have a higher content of CO2 in it from the limestone aquifer, potentially decreasing your pH initially but increasing over time as the CO2 off-gases.

    I suspect the air stones we use in hydroponics just makes the CO2 off-gas faster.

    Does this sound right, do I understand this correctly, or, am I missing something?

    I suspect that it would be good (if possible) in such a situation, to have a larger reservoir of water on hand that is aerated for a few days prior to use, in order to stabilize the pH a bit more prior to adding nutrients and pH down.

    • Hi Richard,

      Your chemistry looks correct. You are also correct that the best way to alleviate this would be to aerate the reservoir before hand. If you don’t have the time to do that just keep a closer eye on the pH levels during the first few days after a fill, and keep it aerated.

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