Electrical conductivity test (EC) is a quick and inexpensive way to determine the salt concentration of a solution. For growers, it provides a reliable method of nutrient monitoring. But what exactly is EC? How does temperature affect it? How does fertilizer application correlate to EC values? And why does EC even matter to a grower? To answer these questions, first we must discuss four things:
- The formula for electrical conductivity and electrical current, what each component means and how an EC probe works
- What is ionization and what ions are present in water
- Fertilizers and how they contribute to EC
- Nutrient deficiencies and the effect of high and low EC values
Electrical conductivity is the measure of a material’s ability to conduct an electrical charge, measured in Siemens per meter. An electrical current (measured in amperes) is the movement of electrons over time across a medium such as water. Put simply, EC gauges how a current moves in solution. The link between EC and fertilizers will be discussed later—first, let’s look briefly at how an EC probe works and how temperature affects EC.
An EC probe is comprised of two electrodes to which voltage is applied. The voltage reading is the resistance (ρ) of the solution. The instrument calculates the reciprocal of this value, allowing electrical conductivity to be calculated. The resistance is calculated based on the distance between the two electrodes.
The relationship between temperature and EC is direct, in that with a one degree Celsius increase (33.8°F) there is a two per cent increase in electrical conductivity—therefore, EC readings must be adjusted relative a standard of 25°C or 77°F. Most EC probes that also measure temperature should have a built-in adjustment so that no correction is necessary—if in doubt, be sure to check the specifications of the EC probe. Next, the elements and compounds that act as electrical onductors—ions—and the action of ionization will be examined.
An ion is an element that has gained or lost an electron. This gain or loss of electrons occurs because water breaks the ionic bond of certain compounds in a process known as ionization. For example, let us examine a compound such as magnesium chloride (MgCl2). When this compound is added to water, the affinity of water for electrons breaks the bond between the magnesium and the chloride, forming Mg2+ and Cl-.
Since these are charged ions, they are now able to act as electrical conductors and will contribute to electrical conductivity. What is important about this chemical process for growers to understand is the relationship between EC and fertilizers.
Synthetic fertilizers are made from (among other things) soluble salts of nitrates or ammonia, phosphates, potassium, calcium, magnesium or sulfates. Organic fertilizers are not high in salts and will often have a very low EC, so proper nutrient monitoring using standard guidelines is problematic.
Fertilizer salts will ionize (in water) into individual components; for calcium nitrate, for example, into a cation (Ca2+) and an anion (NO3-1). Since ionized water sources will have an EC value due to the rock-derived minerals surrounding the watershed, the water will contain a variable amount of cations (Mg+2, Ca2+, K+ and so on) and anions (CO3-2, Cl-, SO4-2 and so on).
This will be an important consideration when attempting to mix appropriate fertilizer solutions for plants that will account for the ions in the water. An EC reading will provide not only a measurement of the fertilizer content prior to incorporation with the plant, but also the salt content in a saturated substrate—a high EC value indicates high electrical conductivity and thus a high level of salt.
EC measurement does not differentiate between individual nutrients (nitrogen, phosphorous, potassium and so on), but simply provides the sum total of all salt content. Also, EC measurements cannot determine whether one macro or micronutrient is being absorbed at a higher rate than another. Measuring the EC of the saturated rooting substrate allows the grower to gauge the nutrient needs of the plant.
For instance, if the EC value is high in the substrate, there is no need for further fertilization—if it is too high, then flushing with water might be necessary. Likewise, if the reading is low, this is an indicator that the plant needs some supplementation of nutrients. Make sure that when using an EC probe the substrate is wet as there must be a solution for the current to travel through.
One final consideration with EC monitoring is the relationship between salt and water content. As the substrate dries out, the nutrient (salt) content increases—at this point, the salt concentration might be high enough to damage the roots of the plant. Likewise, if the substrate is constantly flushed with water, the nutrients will be removed completely. Next, we’ll discuss the concept of watering regimes and the ideal watering temperature.
Consider the analogy of having boiling hot or freezing cold water poured onto your skin. Not a pleasant thought! Plants too respond unfavorably to extremes in temperature—that’s why it is recommended that all watering should occur at a temperature range of 70 to 80°F. Low water temperature—from 40 to 60°F—leads to a decrease in water and nutrient absorption via a decrease in root permeability (the passage of materials in and through the roots). This is especially true with tropical or warm-season plants whose roots are not acclimated to colder temperatures.
While watering with cold temperatures might not kill your plants, it could cause root stress and will reduce the absorption of water and nutrients, leading to a slow decline in health. With regard to watering regimes, avoid the pitfall of establishing a daily routine. Let the plant—via the rooting substrate—tell you when watering is necessary. Monitor moisture by touching the top third of the substrate surface—if it feels moist, delay watering; if it is dry, watering is appropriate.
It is important to note that plants will only use enough water to meet their physiological demands. Generally, plants will not utilize excess water and too much moisture in the substrate forces air out of the interstitial spaces, leading to anaerobic conditions. Over time, this will lead to roots rotting due to insufficient oxygenation. While there are some plants adapted to these conditions, most common plants do not benefit from ‘waterlogging.’
Finally, let’s discuss the plant response to substrates having low or high EC values and the ideal EC range. Properly monitoring EC levels can contribute significantly to sustainable or ‘green’ practices. Essentially, maintaining the proper EC levels prevents overfertilization.
Excessive amounts of nutrient runoff from lawns, greenhouses and backyard gardens can enable algal populations to grow exponentially, drastically changing the ecosystem of a waterway. For example, surface algae can cover the top of a body of water, blocking the path of light to the benthic (bottom) plants and eventually killing them.
Algae will also impact dissolved oxygen levels at night when they respire and—more dramatically—when crashes occur due to the bacterial count being so high it causes hypoxia (no oxygen). This can kill off the fish and other aquatic life that are dependent on varying levels of dissolved oxygen, which in turn impacts terrestrial predators that rely on that aquatic food source.
There is a direct and critical correlation between EC and plant growth performance. The response of plants to either low levels of fertilizer salts (EC <1) or high fertilizer salts (EC >1) will ultimately result in stunted growth and poor health. This means that for most plants an ideal EC range should be between one and three milli Siemens per centimeter. Plants subjected to low nutrient levels (low EC) will present with nutrient deficiencies.
Nutrient deficiencies are caused by poor watering regimes, improper fertilizer rates and improper pH levels. Some nutrient deficiencies (a lack of nitrogen, for example) can result in the yellowing of leaves—especially older leaves—and a very pale green coloration to newer growth.
Other signs of a nutrient deficiency include the yellowing of leaf margins and veins, burnt leaf tips and irregular leaf shape. Fertilizer solutions with a high EC (above three) can cause burning of the roots due to excessive salt buildup in the substrate. In addition, this accumulation of salts in the substrate and subsequent uptake by the plant roots can result in salt stress.
A plant’s sensitivity to salt is highly variable—some are very sensitive, while others are very tolerant of salt. Symptoms of salt stress include necrosis (death) of the roots and yellowing and wilting of the leaves. Thus, even though nutrient levels might be high the plant might show signs of nutrient deficiencies and drought stress.
Depending on the type of plant, the salt concentration and the duration of exposure, a very high EC can quickly lead to plant death. If overfertilization occurs and the EC is too high, you should immediately flush the substrate with copious amounts of water to remove the salt. It is important to note that signs of salt stress and nutrient deficiencies can be very similar, so proper monitoring of your substrate salt content and moisture is essential for optimal plant health.
In conclusion, electrical conductivity (EC) is an effective way to estimate the fertilizer content via salts in your growing substrate. Monitoring your EC will remove most of the guesswork in meeting the nutritional needs of your plants, resulting in a happier, healthier garden.
The recommendations set forth in this article are by no means set in stone—personal research will give you the most complete understanding of the ideal growing conditions (temperature, lighting, nutrient and water requirements, salt tolerance and so on) of each particular plant species.
Hopefully this article has provided enough basic information for you to appreciate the importance electrical conductivity has on monitoring the nutritional needs of your plants.