A recurring series focusing on plant cultivation by university researchers
All plants require certain macro- and micronutrients to grow properly and complete their lifecycles. If one of these elements is lacking or overapplied, plant growth problems and tissue damage will occur, both above and below ground. When applying these necessary elements, it is important to hit the “sweet spot”—the optimal level for each nutrient. But looking at the overall nutrient levels in your crop is also important. This is where electrical conductivity (EC) comes in.
As we explained in the article, “Optimizing Electrical Conductivity (EC) in Cannabis Cultivation,” in Cannabis Business Times’ April 2019 issue, “EC is the electrical charge that moves through a solution. The higher the salt concentration, the greater the electrical reading. The higher the concentration of fertilizer salts in the solution or in the substrate, the higher the EC reading will be.” That article detailed how to monitor and manage EC. While that is essential, your plants can also tell you when EC levels are too high, and knowing the warning signs can help you diagnose high EC.
How to Test EC Levels
Most water-soluble fertilizers are comprised of a cation (ions that carry a positive charge in their natural state) and an anion (negatively charged ions). These ions will dissolve and disassociate in water, so to measure the EC level with an EC meter, an electrical charge from two diodes must pass through a solution. Like drawing lines from dot to dot, the electrical charge will “connect” fertilizer ion to fertilizer ion until it reaches the other diode.
While an EC reading will tell you how many dissolved ions are in solution, it does not identify each individual element or the concentration of each. For all plants, including cannabis, excessively high EC will result in plant damage (more on this later).
Additionally, fertilization and irrigation methods can impact the distribution and accumulation of the applied fertilizer ions (Fig. 1).
With overhead or top-down irrigation, water movement in the pot will flow downward to the bottom of the container due to gravity, root uptake and evaporation occurring at the intersection of the substrate and air through the drainage holes. These three factors will cause salt accumulation at the bottom of the container, given this is where water is evaporating and being taken up in its greatest quantities. Thus, the root damage will be greatest in the lower portion of the substrate in top-down irrigation.
A simple visual inspection of the roots can help you determine where the greatest root damage has occurred. Roots that are clear or transparent are healthy, while roots that appear tan or brown may be damaged. This rule typically applies to cannabis plants in their younger and vegetative states. Larger cannabis plants and some cultivars have darker roots, and larger roots will often be tan or brown due to lignification, or a natural hardening of cell walls that occurs in older plants. Root rot will also result in discolored roots.
The same applies when irrigating from the bottom or with sub irrigation, as well, but is inverted. In sub irrigation or flood irrigation, the bottom of the containers are flooded with a small quantity of water. This water is then absorbed up into the pot through adhesion, cohesion, root uptake, evaporation and capillary action. Thus, fertilizer salts will accumulate in the upper third of the root ball or below the media surface.
When EC damage is suspected when using sub or flood irrigation, examine the roots in the upper third of the pot for discoloration.
Visual Symptoms of High EC
Symptoms of toxicity vary depending on crops’ age. We have described symptoms often observed by age below.
Initial Symptoms. High EC damage symptoms will first appear in the lower leaves and in the new and expanding growing tips (Fig. 2, top of this page). Damage starts there due to the way nutrients move within the plant. The ions with the greatest quantities in a high-EC solution are typically nitrogen (N), phosphorus (P), and potassium (K). As the plant takes up more water, it also takes up more ions, which are dissolved in that solution. This is a beneficial process when nutrients are in balance; however, if the EC is too high (i.e., there are too many dissolved ions), severe tissue death, or necrosis, will occur due to excessive uptake.
Younger and Expanding Leaves. As noted earlier, symptoms of high EC will first develop and appear in the new and expanding leaves. These leaves are often found on new growth, whether on the tips, side shoots, or interior branches, and at any stage. These areas are undergoing rapid cell division and are absorbing a lot of water to fully develop and expand. Hence, these areas will be the first to show high EC symptoms. Symptoms that may surface among new growth include light-green coloration, and the margin (outer edges) of the expanding leaflets will turn brown (Figs. 2 & 3). This browning and leaf margin death will continue based on the severity of the toxicity or concentration of accumulated fertilizer salts (high EC). If the EC is very high, then these symptoms will be very severe, and the entire leaf growing tip may turn brown and die.
Leaf Margin. The older and expanded leaves will also develop symptoms based on high EC stress. These symptoms may appear in conjunction with the new and expanding leaves, or they may appear slightly after. Much like the symptomology in the new and expanding leaves described on p. 26, the initial symptoms of high EC burn will appear on the leaflet margins. Browning and tissue death will first appear on the leaflet margins and will progress inward toward the middle of the leaf (midrib) and down toward the leaf base (toward the petiole) (Figs. 3 & 4). Cannabis plants take up a lot of water when actively growing. The older leaves are very photosynthetically active and thus will use more water resources and accumulate the ions in the high EC solution faster.
Advanced Symptoms. If high EC solutions are not mitigated, then more salts will accumulate in the plant, especially in the older, lower leaves (Figs. 5 & 6). As the leaves continue to accumulate the ions, the leaflet margin will begin to take on a wilted appearance and will become brittle and brown (necrotic). Eventually, the entire leaflet will die, and only the middle of the leaf along the midrib will be green. If conditions are not corrected, the entire plant will die. Additionally, because the plant is not taking up as much water, a layer of salt crusting may appear on the substrate surface due to evaporation with any irrigation method (Fig. 7).
Correction and Prevention
The best way to ensure that high EC conditions are avoided is to set up a monitoring program so you can identify accumulating salts or fertility miscalculations early. Other articles we have written for CBT address how to set up a monitoring program and how to perform an in-house test on substrate pH and EC, and will guide you in this process. (For more on this, read the previously mentioned “Optimizing Electrical Conductivity (EC) in Cannabis Cultivation,” “Troubleshoot Cannabis Nutrient Problems Before They Occur,” and “New Research Results: Optimal pH for Cannabis.” Such tests include the PourThru, 1:2 dilution, or saturated media extraction (SME). Checking the nutrient solution at the hose end or drip emitter will help determine if injector calibration is required, if injector failure has occurred, or if the fertilizer was miscalculated.
Submit Samples for Lab Testing
In-house monitoring of the EC offers a quick check of your crop’s nutrient status. Another useful tool is lab testing. Consider submitting periodic fertilizer solution samples to a commercial testing lab to determine your fertilizer solution’s nutritional composition. This is a great way to determine the nutrient levels provided by an organic fertilizer. Periodic water tests will provide insight into your water quality. Finally, periodic substrate samples will help you correlate your PourThru EC readings with the actual individual element contribution of your fertilizer program.
Corrective Procedures for Modifying Substrate EC
When the EC drifts into unwanted territory (Table 1), adjustments must be made. Below are the standard corrective procedures used to modify the substrate EC for greenhouse or indoor plants grown in soilless substrates and adapted for cannabis.
1. High Substrate EC Correction. If the EC is increasing over time, this indicates the fertilizer is accumulating in the pot and the rate is higher than the demand by the plant. The first step would be reducing the fertilizer rate to moderate the accumulation of fertilizer salts. (This also can help save money by not wasting fertilizer solution.)
The second option is to irrigate the substrate twice with clear water to leach out the accumulated fertilizer salts. This method is used when EC levels are extremely high to help avoid plant burn. (It also flushes away your fertilizer investment.) Monitoring the crop during production will help to avoid these elevated levels in the first place and help you save money.
2. Low Substrate EC Correction. On the opposite end of the spectrum, if the levels are lower than the recommended range (Table 1), this implies the nutrient demand of the plant is higher than what is being supplied. In this case, increase the fertilizer rate. One or two applications of a higher N rate in the 250 to 400 ppm range will boost the EC.
Excessively high EC levels lead to stunted growth, leaf damage and eventually plant death. It is best to avoid this situation by implementing in-house monitoring of the EC in your standard operating procedures (SOPs), thereby establishing a regular quick check of the nutrient status of your crop.