A Dichotomous Key for Understanding Nutrient Deficiencies
One of the most important skills any grower can have is the ability to visually identify a deficiency or issue with their crop. Proper scouting and early identification of nutrient or IPM issues is critical to staying on target with your harvest goals. This can be confusing for newer growers and can take decades to learn, requiring constant repetition. This is especially true for cannabis growers with few scientifically backed representative images available due to prohibition. We are all familiar with the old nutrient deficiency posters that did little despite their best intentions to help growers determine nutrient issues. Where many vegetable and ornamental cultivators can simply consult the venerable Nutrient Deficiencies In Bedding Plants to visually search for an answer to their question it still becomes a game of memorization. That has always made it very hard for me to teach others and at best I had to hope that experience and time would guide them.
What was needed was a dichotomous key. Rather than relying on memorization, utilizing a dichotomous key gives a clear and simplified path to determining the nutrient deficiency at hand. The staff at KIS has built this off of existing models from more traditional agriculture. Our hope is that it will be of value for all growers. Pulling from traditional agricultural tools we have crafted a useful key for all growers but also specifically for cannabis growers. KIS is here to support all forms of cultivation in an effort to move towards sustainable practices. Proper utilization of the appropriate nutrients is key to managing these strategies. Before we teach you the process for determining nutrient deficiencies we have a very important decision to make. Is this really a nutrient issue?
BIOTIC OR ABIOTIC
Experienced growers who perform regular scouting can easily recognize patterns in their canopy. With multiple cultivars, as most cannabis growers tend to run, this can get tricky. However, if you pay attention to each cultivar as its own project it is greatly simplified. Growers should be looking for areas of chlorosis, yellowing of the plants, and necrosis, brown dead tissue that cannot regenerate. Wilting and leaf angles are more important as environmental signalers rather than as indicators for nutrient issues and we will discuss that separately. The first step we need to take in diagnosing a plant issue is to determine the cause of the visual cues we are evaluating. That first step is to determine if the cause of your plant's stress is biotic (think pests and pathogens) or abiotic (think nutrients and environment).
A biotic symptom is almost always displayed randomly throughout the crop, unless you wait far too long and then it will affect the entire crop. Pests and pathogens tend to jump around from area to area and plant to plant, skipping some plants entirely and going after others. Be sure to include a sufficiently strong scope in your scouting tool kit. Examine the underside of leaves with a strong handheld scope. The crucial importance of proper scouting can not be over emphasized! A real life example would be a grower showing signs of K deficiency that after scoping realized he had been fighting russet mites for months with K inputs! You can't act appropriately without a proper identification, regardless of whether you are dealing with a biotic or an abiotic issue. If you are looking at an individual plant for symptoms ask yourself if there is a discernible pattern on the leaves? If you were to fold a leaf over would it line up with damage on the other side fairly consistently? If your response is yes and you are seeing patterns; entire benches or quadrants within your canopy, then you are most likely dealing with an abiotic stress. Abiotic stresses are caused by nutrient issues and environmental issues. If you find that you are dealing with a pest issue you need to contact a licensed, qualified professional. While trying to remain brand agnostic you can not do better than contacting Suzanne Wainwright Evans, The Bug Lady at "https://bugladyconsulting.
Now the fun part. Let's get started with nutrient symptoms! Once we have recognized a pattern within the canopy or single plant, and eliminated pests and pathogens as the culprit, we are ready to dive into our dichotomous key. (NOTE:Assessing crops for patterns is a good indicator that you can eliminate IPM issues as the culprit for the most part. However, you can never be sure without testing. KIS offers pathogen test kits if you are concerned about Hop Latent Virus or other pathogens common to cannabis) Most of you will be familiar with dichotomous keys from high school biology or field guides. A dichotomous key offers clearly defined options for the investigator. To begin our key we first need to determine where on the plant we first started seeing issues. Again, this is another reason that regular scouting should be performed by the same individuals. If scouting is delayed or skipped altogether it will be next to impossible to determine where the issue first started showing on your plant. This is incredibly important as the location of the initial deficiency clue will tell us whether the nutrient is mobile or immobile within the plant.
Obviously all nutrients are "mobile" as they have to move into the plant. What we are discussing is whether they are able to be translocated from one location in the plant to another. If they are able to translocate then they are mobile. The apical or growing tips of the plant are the most important to the plant for reproduction so they will get the benefit of moving mobile elements from the lower leaves to the top leaves if sufficiency is not maintained throughout the growing cycle. That is why you will see mobile element symptoms at the bottom of the plant. Those nutrients are being sacrificed in lower tissues and reallocated to the important growing, apical tips.
If an element is immobile it cannot be translocated. Therefore, if there are insufficient mineral nutrients for an immobile element it will show in the growing tips first. If a nutrient is partially mobile then you will see it throughout the entirety of the plant.
MOBILE NUTRIENT SYMPTOMS - BOTTOM OF PLANT
Let's deal with lower leaf symptoms first. Then we can work our way up the plant. These are mobile elements, as explained previously and there are only four. (We are assuming you are already aware of the nutrient elements that are required for plant growth. Environmental nutrients are C,H and O and are supplied by natural processes through water, carbon dioxide and O2 in the rhizosphere. The following macronutrients are needed in large quantities: N,P, K, Ca, S, and MG. Micronutrients are just as crucial but are usually recycled in plant metabolism processes rather than complexed into plant tissues meaning less is needed.The micronutrients required for growth that we are concerned with are: Cu, Z, Fe, B, and Mo.
If the first symptoms you are seeing are on the lower leaves we need to decide if the nutrient in question is N, P, K or Mg. These are the mobile elements. If the first symptom you are seeing is chlorosis, yellowing, on the lower leaves then you can narrow your target to two mobile elements; N or P. While both show lower leaf yellowing, chlorosis, that can run to necrosis if left far too long we will concentrate on the differences not the similarities to determine which nutrient is insufficient. While both show chlorosis of the entire leaf N has a tell tale clue. If the lower leaf comes off cleanly with a firm pull or snap then this is leaf abscission and you are in need of N! The leaves will drop off the plant where the petiole or leaf stem attaches to the main stem if left to itself. In ornamentals you will see early flowering. We already know that excess N will slow flower production so that is an easy connection to make. If tugging on a chlorotic lower leaf with a P deficiency the leaf doesn't pop off easily. P deficiency will distinguish itself by displaying a deep dark foliage aside from the lower yellowing and they will have shorter and fewer roots. This makes for a clear and easy distinction between N and P. Yes, you need to inspect your plant's roots. Pests and pathogens as well as nutrients and watering practices can affect root growth and without strong roots no amount of sufficiency will promote vigorous growth or fruitful harvests. At the very least be sure to inspect the roots of "problem" plants and all plants at time of harvest. P deficiency is also commonly associated with purpling of stems, leaves, and petioles. This is not always the best indicator, however, as temperature and cultivar can also be heavy influencers of the accumulation of red and purple pigments in the plant.
Nitrogen: Lower leaf chlorosis, Leaf abscission
Phosphorus: Lower leaf chlorosis, Dark foliage, Purpling, Shorter and fewer roots
If the first symptom you are seeing is interveinal chlorosis, yellowing between the veins, then you are dealing with Mg. Tiger striping of lower leaves is a dead giveaway that you are dealing with Mg. Keep in mind that Mg is the carrier molecule for P so a Mg deficiency will often show as a P deficiency as well. Once Mg is sufficient the P symptoms will also disappear, assuming adequate P in the soil solution.
Magnesium: Lower leaf interveinal chlorosis, Tiger striping, Phosphorus symptoms
If the first symptom is rapid necrosis, dead tissue, along the outer leaf margin you have a K issue. K issues begin as a chlorotic band or speckling specifically along the leaf edge that moves rapidly to necrosis. You rarely notice the chlorotic phase as it will turn to necrosis within a day or two, progressing very quickly but restricted to the margins. Be sure you are eliminating pest and pathogen issues with a scope and or testing.
Potassium: Lower leaf marginal chlorosis rapidly going to necrosis
PARTIALLY MOBILE SYMPTOMS-THROUGHOUT THE PLANT
Partially mobile elements are even easier because there are only two to worry about; Sulfur and Molybdenum. Sulfur excesses are almost never a problem in cannabis. I have seen ridiculously high amounts of sulfur with no issues but it can be too low sometimes. I always hedge on the heavy side as sulfur is needed for all of the volatiles that we want in a finished flower. If you are seeing uniform chlorosis across the entire plant then you have a sulfur deficiency. Remember it's where it starts not where it ends up that is important, That's why we want you to do daily scouting. Severely advanced sulfur deficiency will advance to necrosis on the tips but again it will be uniform throughout the plant, not just lower leaves or growing tips. A plant that is pale all over is most likely suffering from a sulfur deficiency.
Sulfur: Overall chlorosis
Mo deficiency is never seen in cannabis but it does show in poinsettias and some leafy greens as a pale chlorotic band around the margins of all leaves (again not at the top or bottom of the plant). Rather than going necrotic quickly as seen in a K deficiency it will stay light yellow to pale white along the margin. Again, we never see this in cannabis (yet).
IMMOBILE SYMPTOMS - TOP OF THE PLANT
Finally, we have the immobile elements. As we previously mentioned immobile elements cannot be translocated or moved within the plant once complexed. Because of this their symptoms will always show in the top of the plant, at apical points. This is the most complicated simply becasue we have six mineral nutrient elements to discuss but don't be intimidated. By following the dichotomous key you will be able to quickly and confidently determine your nutrient issue. The nonmobile elements we are concerned with primarily will be Ca, Cu, Fe, B, Mn and Z. Any of these elements can show deficiency symptoms at the top of your plant.
Let's look at the primary diagnosis clues.
- chlorosis at the top is Fe or Mn
- necrosis or distortion at the top is Ca or B
- young and most recently expanded leaves is Cu or Z
We will look at chlorosis first. If you are seeing yellowing at the top of the plant your plant is deficient in either Fe or Mn. Chlorosis at the top of a plant can show as interveinal, uniform or marginal chlorosis. If the color transitions from pale yellow to white you are dealing with iron. Iron is one of the most commonly seen deficiency symptoms across all fields of agronomy. Management of pH and chelated forms of iron are often required to get sufficiency levels of iron into the plant but the symptoms are clear. It is a pale to white chlorosis at the top of the plant. In extremely severe cases you may get necrotic tissue. If, on the other hand, the symptom you are seeing at the top of your plant is chlorosis accompanied by tan flecking, irregular spotting with angular shapes, you have a Mn issue. I see this a lot in young plants. Mn drops off quickly as the pH climbs and I've seen many a garden that had a Mn issue early on because the grower was feeding with tap water that was out of range.
Iron: Chlorosis on top of plant, Very pale to white starting at petiole end
Manganese: Chlorosis and interveinal chlorosis on top, Tan flecking
Next let's look at necrosis and distortion. If you are seeing necrosis on the growing tips we are dealing with Ca or B. The incidence of having oversupplied or burning your plants with organic inputs is very rare unless you are adding hot (not fully composted) manures or composts so seeing necrosis on your top leaves is almost always a Ca or B issue. Again we want to look for the differences. If the issue is with Ca you will see necrosis and distortion but you will also see cupping in most crops. In ornamentals you will see incomplete flower formation. Most gardeners are familiar with blossom end rot in tomatoes or cork spot on apples. These are the same symptoms expressed in the firm fruits of those species.
.Calcium: Chlorosis at top of plant, Cupping, Distortion, Blossom end rot, transpiration issues
Boron also shows necrosis and distortion but differs from Ca in that you will get very tight internodal spacing. Often the apical points will be growing so closely between nodes that the plant takes on the appearance of a tightly packed rose flower. Thus, "rosettes" make the diagnosis for B fairly easy. As opposed to the incomplete or twisted shape of a Ca deficient leaf B deficient leaves will pack very closely together.
Boron: Chlorosis and necrosis at top of plant, Rosettes, Puckering
Ca travels in the xylem only and so is very dependent upon transpiration. Experienced growers will recognize leaf burn due to fans pushing too many cfms. Often misdiagnosed, the fast air transfer pulls water through the plant so quickly that the large Ca molecule has a hard time moving through the plant leading to a necrotic burned look along the margins, not to be confused with K issues. This marginal necrosis will only be on plants directly in front of the strong fans. Ca can also be sufficient in the soil solution but not in the plant if the temperature or relative humidity are out of range.
Finally, we will look at Cu and Z. Both will show leaf roll or curl at the top of the plant. Most cannabis growers are familiar with "tacoing" or "canoeing" in heat stressed cannabis but this references a tip to petiole roll as opposed to the closing roll seen in heat issues. Cu also shows as chlorosis and rapid necrosis on the most recently matured leaves. We are looking just below theapical points as opposed to the tips themselves as we would for Fe or Mn. These are rarely seen and most often only diagnosed by soil tests in cannabis.
Copper: Most recently matured leaves, Dark spots and necrosis, Leaf roll or clawing
Zinc: Interveinal chlorosis at top of plant, Distortion, Puckering, Clawing
Now that you know what you are lacking we need to determine why. Is it simply insufficient quantity of a given element or is there an environmental issue (relative humidity, temperature, light or lack of sufficient PPFD) but that is another complex topic too big for this video. Obviously this is a big picture overview and does not discuss many subtle and environmental clues that experienced growers will observe and act on but we hope it gets you started. On that tip be sure to get as much practice as possible. It doesn't even have to be your plant of choice or your plant at all. Take a peek at your neighbors flowers or your grandfather's tomatoes. Practice will engrain the choices this key offers making them easy to recall. If you do find a nutrient deficiency (via soil tests or visually) contact us at KIS and we can advise you on what organic products will work best for your specific environment. We are committed to your success and we hope this helps you to be a more informed and confident sustainable grower! If you have any questions, need clarification or even help with your soil reports please reach out to us at KIS Organics. You can reach me directly by email at firstname.lastname@example.org or call us at 425-588-0990.
Characterization of Nutrient Disorders of Cannabis Sativa