In hydroponics, plants uptake nutrients that are dissolved in water rather than nutrients available in soil. Most of the time, these nutrients are provided to the plant in an ionic or inorganic form. In order to grow healthy hydroponic crops, it is essential that growers maintain proper electrical conductivity (EC) and pH levels.
For most hydroponic crops, the ideal range of EC for most crops is between 1.5 and 2.5 dS/m. A higher EC could prevent the plant from absorbing nutrients due to increased (more negative) osmotic pressure, and EC levels that are too low could adversely impact yield.
The rest of this article will further describe why EC levels are important for your hydroponic system and how to maintain appropriate levels of EC. In addition, it will present optimum EC values for several common hydroponic crops.
What is EC?
In hydroponics, electrical conductivity (EC) is a measure of how easily electricity can pass through your nutrient solution. Similarly, the conductivity factor (CF) is a measure of EC with units of millisiemens per centimeter (mS/cm).
Most plant nutrients are available as soluble ionic compounds. Ions are charged particles. Adding more of these charged particles will increase the EC of a nutrient solution.
Units of EC
In order to be able to read and interpret EC readings taken from your nutrient solution, it is important to understand the units used to measure EC.
EC is typically expressed as siemens per unit area (e.g., mS/cm, dS/m, S/m). For a nutrient with a conductivity of one S/m, the electric current through a nutrient solution will increase by one ampere for every increase of one volt of electric potential across one meter of solution.
Various metric prefixes are used when expressing EC readings. The table below lists some of the common prefixes used for EC.
| Symbol | Prefix | Multiplier |
|---|---|---|
| d | deci | 0.1 |
| c | centi | 0.01 |
| m | milli | 0.001 |
| µ | micro | 0.000001 |
Sometimes the unit mho (℧), which is the reciprocal of one ohms is sometimes used in place of siemens. One mho is equal to one siemen.
Converting EC to ppm
As previously discussed, EC is a direct measurement of the electric conductivity of a substance. Because the nutrients dissolved in the water used for hydroponics are ions, EC can also be used as an indirect measurement of total dissolved solids (TDS). TDS is measured in parts per millions (ppm) and represents the concentration of nutrients in the solution.
There are several standards used worldwide to convert EC to TDS. This is important to understand because different EC meter manufacturers use different conversion factors. Always read the literature that comes with your ppm meter to determine which conversion factor is being applied. A table of some of the most common factors is presented below.
| Country | Manufacturer | EC (mS/cm) | TDS (ppm) |
|---|---|---|---|
| USA | Hanna, Milwaukee | 1 | 500 |
| European | Eutech | 1 | 640 |
| Australian | Truncheon | 1 | 700 |
It is also important to keep these standards in mind when interpreting advice presented in various blogs and books because growers from different countries may be using different factors.
EC Ranges By Hydroponic Crop
Not every plant has the same optimum EC value. There are a number of reasons for this. First, different plant species have different nutritional needs. The amount of nutrients in the nutrient solution impacts EC levels.
The optimal EC also depends on the plants’ stage of growth. Seedlings prefer nutrient solutions with a lower concentration of nutrients (lower EC) because they can burn easily. In contrast, more mature plants should be grown in nutrient solution with a higher EC.
In addition, environmental factors such as the ambient air temperature are important in determining the optimum EC. For example, when the ambient air temperature is above 86°F (30°C), EC levels should be reduced to account for the reduction in the plant’s rate of photosynthesis.
The optimum range of EC values for different hydroponic crops is listed in the following table.
| Crops | EC (dS/m) | TDS (ppm) |
|---|---|---|
| Asparagus | 1.4 to 1.8 | 896 to 1,152 |
| African Violet | 1.2 to 1.5 | 768 to 960 |
| Basil | 1.0 to 1.6 | 640 to 1,024 |
| Beans | 2.0 to 4.0 | 1,280 to 2,560 |
| Bananas | 1.8 to 2.2 | 1,152 to 1,408 |
| Broccoli | 2.8 to 3.5 | 1,792 to 2,240 |
| Cabbage | 2.5 to 3.0 | 1,600 to 1,920 |
| Celery | 1.8 to 2.4 | 1,152 to 1,536 |
| Carnation | 2.0 to 3.5 | 1,280 to 2,240 |
| Cucumber | 1.7 to 2.0 | 1,088 to 1,280 |
| Eggplant | 2.5 to 3.5 | 1,600 to 2,240 |
| Ficus | 1.6 to 2.4 | 1,024 to 1,536 |
| Leek | 1.4 to 1.8 | 896 to 1,152 |
| Lettuce | 1.2 to 1.8 | 768 to 1,152 |
| Pak Choi | 1.5 to 2.0 | 960 to 1,280 |
| Peppers | 0.8 to 1.8 | 512 to 1,152 |
| Parsley | 1.8 to 2.2 | 1,152 to 1,408 |
| Rhubarb | 1.6 to 2.0 | 1,024 to 1,280 |
| Rose | 1.5 to 2.5 | 960 to 1,600 |
| Sage | 1.0 to 1.6 | 640 to 1,024 |
| Spinach | 1.8 to 2.3 | 1,152 to 1,472 |
| Strawberry | 1.8 to 2.2 | 1,152 to 1,408 |
| Tomato | 2.0 to 4.0 | 1,280 to 2,560 |
| Zucchini | 1.8 to 2.4 | 1,152 to 1,536 |
Source: Sharma, Nisha & Acharya, Somen & Kumar, Kaushal & Singh, Narendra & Chaurasia, Om. (2019). Hydroponics as an advanced technique for vegetable production: An overview. Journal of Soil and Water Conservation. 17. 364-371. 10.5958/2455-7145.2018.00056.5.
The EC ranges listed above were presented in dS/m in the literature. However, many EC meters that hydroponic growers use provide readings in parts per million (ppm). For this reason, I used the following relationships, in accordance with recommendations outlined by the University of California, to convert EC to ppm.
TDS (ppm) = EC (dS/m) x 640 (EC from 0.1 to 5.0 dS/m)
TDS (ppm) = EC (dS/m) x 800 (EC > 5 dS/m)
The Impact of EC On Plant Yield
So what is the impact of growing plants in a nutrient solution with EC levels that are above or below the optimum range?
A research study that examined the effects of EC on tomato yield found that yield increased as the EC of the nutrient solution increased from 0 to 3 dS/m. However, the yield decreased when the EC was further increased from 3 to 5 dS/m. Researchers found that EC levels between 1 and 3 dS/m, depending on the stage of growth, resulted in higher tomato yields (Zhang et al. 2016).
Symptoms of Excessive EC
Ironically, signs of excess nutrients are similar to the symptoms of nutrient deficiencies in conventional growing systems. For example, excess magnesium in a hydroponic system looks just like calcium/magnesium deficiencies in growing media.
In general, here are some signs that the EC of your nutrient solution is too high.
- Leaf and stem wilting
- Tip burn
- Stunted growth
- Dropping leaves
Note that many of these symptoms may also indicate other problems such as disease, lack of water, too much heat, or excessive light.
If you suspect that the EC of your nutrient solution is too high, dilute it with distilled water. As you are adding distilled water, take measurements intermittently until the EC is back to appropriate levels.
Measuring EC
The average daily water loss in a hydroponics system ranges from 5% to 30% depending on the size of the system and the type of plants you are growing (Resh, 2006). This water loss results in a nutrient solution with a high EC. This means you should be measuring the EC of your system on a regular basis.
EC Meters
There are a variety of EC meters available at different price ranges. So what should you look for when searching for an EC meter? Depending on your budget, you will want an EC meter that is waterproof, has a battery life indicator, is easy to use, has an automatic shutoff, is shockproof and is easy to calibrate.
Some of the most popular brands of EC meters are Bluelab, Hanna, Essentials, and Apera.
There are several TDS meters available on Amazon that automatically EC to TDS (ppm) for you. These meters tend to be very affordable ($15-$30) and often come with a pH meter as well. If you are just starting out in hydroponics, an inexpensive TDS meter might be a good option for you.
Keep in mind that cheap TDS meters tend to be less accurate than some of the more expensive EC meters. In addition, they will apply different conversion factors to convert EC measurements to ppm. This can cause an extra layer of confusion when you are comparing your measurements with recommendations outlined in journal articles or blogs.
A very popular, and somewhat more expensive ($70), pocket tester is the Bluelab PENCON conductivity pen. It allows you to choose a conversion factor and it automatically accounts for temperature changes, making readings more accurate. You can find this item on Amazon or eBay.
Larger grow rooms require higher-quality EC meters. An example of such meter is the Bluelab BLU27100 Guardian Monitor. This monitor costs between $300 and $400 and has alarms that alert you if your EC gets too high or too low.
Calibration and Cleaning EC Meters
If you consistently use your EC meter, salts will build up on the meter. This salt buildup can skew your readings and shorten the life of the EC meter. For this reason, it is important to calibrate your EC meter before taking readings. You should also clean your meter after every use.