Hydroponics Part IV: Defining and Measuring pH, EC, and TDS

The beauty of a hydroponic system is that plants can be grown in the absence of soil. Additionally the water needed to grow plants is about 90% less than traditional gardening. Since soil is not used, the success of your system is truly dependent on the proper nutrient solutions. Just as soil types directly affect the growth of plants, the same is true for nutrient solutions. 

That said, once your nutrients are mixed in proper ratios that reflect the needs of your plants, transpiration and differences in plants means that nutrient levels can drift away from optimum levels. This simply means that your nutrient solutions need to be properly managed. This is accomplished by measuring certain parameters such as pH, EC (electrical conductivity), and TDS/PPM (total dissolved solids/parts per million).

The Importance of Proper pH

The nutrient solution pH level is crucial in hydroponics because it affects nutrient availability to growing plants.When plants are grown without soil, they do not get the benefits of exposure to microorganisms, organic matter, and the interactions between water and minerals that help to regulate pH levels. Consequently, the hydroponics gardener must monitor and artificially regulate the pH of the system. 

Specific nutrients require certain pH levels for plant to uptake and utilize the nutrient. The wrong pH level can result in too little or too much of certain nutrients. For example, when the pH level drops below 5.0, plants can develop either magnesium and calcium deficiencies or copper and iron toxicity. A pH level above 6 or 6.5, however, can cause iron deficiency.

With some exceptions, the optimal pH range for plants that are grown hydroponically is generally between 5.5 and 6. Many fruits and vegetables, as well as melons, apples, beans, squash, and tomatoes prefer that range. Some plants, such as  blueberries, need a lower, more acidic pH between 4.0 and 5.0. Kale, onions, and peas prefer and pH between 6.0 and 7.0. Mints plants prefer a pH of 7.0 to 8.0. Other crops, such as pumpkin, will thrive in a pH between 5.5 and 7.5

The pH range of hydroponic nutrient products depends on the specific formulation. However, most products typically start with pH levels between 5.5 and 6.0, which is the optimal level for most crops. For example, ammonium nitrate has a more acidifying effect than just nitrate and will cause a drop in pH. Calcium salts, on the other hand, cause a rise in pH, resulting in a more alkaline solution.

Why pH Levels Change in a Hydroponics System 

In the natural environment, the soil acts as a buffer to help regulate pH levels. The contents of the soil makes a difference and this has a lot to do with why plants prefer one soil type over another. However, due to the artificial environment in a hydroponics system, there several things that can cause pH levels to widely fluctuate. 

• As plants absorb nutrients and the solution in the reservoir drops below one gallon, the solution becomes more concentrated. Consequently, you must monitor solutions levels, keep the reservoir full and regularly test the pH. 
• The presence of inorganic and organic material can alter the pH. For example, gravel, hydration, and other inorganic material used in media based systems act as a buffer and cause pH level to rise (become more alkaline). To get an accurate pH reading in media based systems, test the pH in the reservoir as well as that of the solution that drains from the beds or other containers that hold the plants. 
• Algae and bacteria are the principle organic material that affect pH. If you observe pH levels rise in the morning and then fall by evening, mostly likely algae is the cause. Algae consume acidic CO2 during the day which causes pH to rise and then it falls by evening. On the other hand, bacteria from root disease will cause pH levels to dramatically drop because the bacteria release acids in to the hydroponic solutions. 

How to Maintain the Correct pH

The best way to properly maintain optimum pH levels is regular testing. You should test the pH regularly not matter what type of system you choose. If you are new to hydroponics you may even want to do this daily especially when adjusting the nutrient solutions.  

This can be done in a variety of ways. Test strips and liquid test kits can be obtained from pool supply stores and even garden centers. This is likely the least expensive method but is not going to give you repeatable results. To get repeatable, ore accurate results, use a meter. There are a variety of products on the market. Just be sure to purchase a good quality meter. 

If using a recirculating system, pH levels need to be adjusted base on samples from the supply reservoir. In a media based system, the pH needs to be adjusted based on testing the leachate (the solution that drains from the grow beds) This is because the pH changes as the nutrient solution travels from the supply reservoir and out through the grow base. 

The best way to adjust pH levels is to use a commercially prepared “pH up” and “pH down” products. These are available in dry and liquid form. Just be sure to purchase products that are designed for hydroponics systems. 

Electrical Conductivity (EC)

By now you already know that water conducts electricity. But the reason it conducts electricity is because of the presence of all kinds of minerals and other impurities. For example, calcium, iron, potassium, and magnesium. It is because of the presence of these particles that water is able to conduct electricity. In fact, purified water is a very poor conductor of electricity. The same goes for distilled water, de-mineralized water, or rainwater. 

Hydroponic nutrient solutions contain many dissolved particles but it is the dissolved salts, ie potassium, calcium, phosphate, nitrate and sulfate, that are responsible for the conductivity. When salts dissolve in a solution, they dissociate into positively and negatively charged ions, both of which effectively carry a current. 

Electrical conductivity is the ability of a solution to carry an electrical current. The higher the current obtained the higher the conductivity. Consequently, the EC value of your nutrient solution is representative of the amount of salts (aka nutrients) dissolved in your solution.  The more nutrients and impurities that are dissolved in water into the ionic form, the higher the EC value. This means it is representative of the strength of your solution. 

In general, the EC of nutrient solutions should be kept between 1.2 and 2.0

Adjusting EC Values

The key to EC is dilution. If the EC value is too high, you can easily lower it by diluting your system with fresh water. Just take measurements periodically while adding water until the solution reaches the appropriate EC value. You can of course raise the EC value by adding more nutrients.

If EC values rise over time, it is an indication that the plants are unable to take up nutrients. This may be an indication of the wrong nutrients or the wrong pH.   

EC and TDS (total dissolved solids)

As stated above, the conductivity of a nutrient solution is related to the concentration of dissolved solids, also known as salts. Consequently, the conductivity of a nutrient solution can also be expressed in total dissolved solids (TDS). The measurement of TDS indicates the amount of ions dissolved in a solution. In terms of hydroponics, TDS is another way to express and measure the salt concentration and strength of a nutrient solution. A TDS measurement with a meter is not entirely accurate. In order to accurately measure TDS a gravimetric analysis needs to be performed.  However, for the purposes of hydroponics, a reasonably accurate TDS measurement can easily be extrapolated from an EC measurement.

Conductivity measurements are often expressed in units of microsiemens per centimeter (µS/cm) or millisiemens per centimeter (mS/cm). Typical EC values for hydroponic nutrient solutions can range from 500 to 2000 µS/cm, or 0.5 to 2 mS/cm.

A simple conversion easily relates the two units of measurement; 1,000 µS/cm is equal to 1.0 mS/cm.

TDS measurements are typically expressed in units of parts per million (ppm) or parts per thousand (ppt). In terms of conversion, a reading in µS/cm will be expressed in units of parts per million (ppm), while a reading in mS/cm will be expressed as parts per thousand (ppt).

Because TDS is not a direct measurement, the conversion factor used is based on the type of solution being measured. The conversion factors of 0.5 and 0.65 are most commonly used in TDS meters available in the market. The 0.5 conversion factor is based on a solution of sodium chloride since sodium chloride will completely disassociate in water into sodium ions and chloride ions. However, other salts such as magnesium sulfate will not completely disassociate into solution. Consequently, not all of the magnesium sulfate will be detected by an EC or TDS meter. For this reason, a higher conversion factor is needed to compensate for the salt that does not fully disassociate in water. A conversion factor between 0.65 and 0.70 is the best estimate of the salts present in fertilizer that do not disassociate.

In order to relate EC and TDS readings, simply multiply the EC value (µS/cm) by the appropriate conversion factor to get the TDS value (ppm). For example, a 1000 µS/cm reading on an EC meter will read 500 ppm on a TDS meter with a 0.5 conversion factor. It will read 650 ppm on a TDS meter with a 0.65 conversion factor.

To make things less complicated, many TDS meters are either pre-programmed with the conversion factor or available with an adjustable conversion factor. In either case, it is important to ensure the conversion factor of the meter matches the factor recommended by the manufacturer of the nutrients.

EC and TDS measurements are both affected by temperature. As the temperature of a solution increases, the movement of ions increases which results in increased conductance. The reverse is also true. When the temperature of a solution decreases, the movement of ions decrease which results in decreased conductance. Consequently,  it is important to compensate for temperature in order to obtain accurate measurements.This is most easily done with a TDS meter that has a built in automatic temperature compensation (ATC).

Accuracy of Measurements is Absolutely Vital

EC and TDS readings are only going to be as precise as the standards used for calibration. Therefore, it is imperative to  calibrate to a known standard on a periodic basis.  Prior to calibration, it is critical that the sensor is clean and no salts are present. The probe should be rinsed in water that has been distilled or purified by reverse osmosis.

TDS or EC standard solutions can be easily ordered online. The solutions should always be fresh since the smallest amount of contamination will change the value. It is also important to take measurements with a meter and has automatic temperature compensation because EC and TDS measurements are highly variable based on temperature. 

Also, having a meter that has a built in and adjustable conversion factor that relates EC and TDS is advantageous. This allows the grower to adjust parameters based on what the nutrient manufacturer reports about their products.

Importance of Understanding Nutrient Solutions 

Especially if you are new to hydroponics, taking the guess work out of your nutrient solutions is critical to properly feeding your plants and promoting proper growth in your operation. It is important to understand proper nutrient mixing parameters and how to measure them. Additionally, you must have an accurate measurement of base nutrient levels in a solution before adding more nutrients. Otherwise your plants may have stunted growth or worse yet they will wilt and die.  

For several reasons it is important to properly balance your nutrient solutions.  If there is an excess amount of nutrients in solution compared to what is naturally present in the plant root, 

plants can experience fertilizer burn. This can result in the browning and wilting of plant leaves, and can ultimately lead to plant death. The reason for this is a process called osmosis.

The principle of osmosis is related to the natural spontaneous movement of solutions across a permeable membrane in order to equalize the concentration of salts on each side of the membrane. In this case the permeable membrane is the root of the plant. For example, if the nutrient solution is more concentrated than the inside of the plant root, the natural tendency is for fluid to move out of the plant root and into the nutrient solution in an attempt to balance the concentration on each side of the membrane. Again, the permeable membrane being the plant root. Consequently, if the nutrient solution is too concentrated, your plant will dry and wilt, and experience fertilizer burn. A good example of this can be seen when someone applies too much fertilizer to their lawn. It causes the grass to wilt and die because the excess fertilizer depletes the grass of much needed water. 

You should add nutrients until your meter shows a reading of between 600-1000 TDS. I suggest using a TDS reading of 800. One of the exceptions to this rule is Pac-Choi, which prefers a TDS reading closer to 1,200-1,400.

Every hydroponic grower should have an EC/TDS meter. This takes the guesswork out of the critical process of feeding your plants. It is also important to purchase a good quality meter. They are generally inexpensive and can easily be found online. Less expensive versions can be found for around $20 for both a pH and EC/TDS meter. Better quality meters will cost around $150.  

If you are serious about being a hydroponics grower, then learn all the basics about nutrients solution and of course purchase a good quality meter.  Not only this this enhance your success but before long you will being enjoying good fresh garden produce.  

Go off grid and live well, 

Patrick