Yellowing: Some Causes, and What it Means for your Grow
Hi Guys, Dr. NPK here. People can be green with envy, red with anger (like when my football teams lose!), but no one is ever…yellow? As growers, that’s the most important color we care about (not) seeing! Yellowing of your leaves is a very vague term that can mean different things to different people. This post is dedicated to helping you understand yellowing: why it occurs, what are some potential causes, and ways to determine the cause.
What Causes Yellowing?
Unfortunately, diagnosing yellowing is an extremely difficult thing to figure out. One reason it is so hard to decisively and quickly diagnose issues in your grow is the sheer number of variables present. To definitively conclude that “X” is causing the problem, two separate trials must be identical except for the variable being tested (“X”). Oftentimes this is not the case when growers are working on solving the issue in a live environment. Below are some of the reasons that this yellowing occurs.
pH Range. I believe incorrect pH is one of the most common mistakes people make in their grows. Of course, many people take the pH of their sumps, but it is also important to test runoff pH at well. If there is a significant difference in pH’s, it could be an indication that something is funky in your medium/soil. Remember, the ideal pH is between 5.8ish and 6.3ish.
Nitrogen Deficiency. In general, there are many nutrient deficiencies that can cause yellowing – by far the most common lacking macronutrient that results in yellowing is nitrogen. Nitrogen is huge in veg because it promotes healthy leaves; because nitrogen is a mobile macronutrient, a nitrogen deficiency results in older leaves turning yellow, curling, and dying off. Nitrogen tolerates pH very well, and is also found in Elite Base Nutrient A and Elite CalMag in concentrated quantities. Be careful: nitrogen toxicity is a thing: symptoms include darker leaves, weak stems, stunted growth, and also a “claw look.” If you’ve ruled out a nitrogen deficiency as your cause of yellowing, it’s time to look at other nutrient deficiencies.
Magnesium Deficiency. One of the common symptoms of a magnesium deficiency is a yellowing of leaves (that’s why it’s in the post, duh!). The key difference in this yellowing is the location: because magnesium is a mobile element (meaning the ion can be shuttled between different cell locations within the plant), the plant will ration out its current supply of lower-than-normal-magnesium to the newer leaves (towards the top, where the new growth is). Thus, if you see yellowing towards the bottom of your plant (aka the older leaves) that are present outside of the veins, absolutely consider a magnesium deficiency as a possible source.
Calcium Deficiency. I’ve mentioned this before, but calcium deficiencies are concerning due to calcium’s role in building the cell wall (weak cell wall = dead plants!). Although close in proximity to the magnesium symbol in the periodic table (Ca vs. Mg), these two secondary nutrients have a significant difference: a calcium deficiency results in yellowing in new or younger leaves. This is because calcium is an immobile element; as a result, the calcium ion cannot move freely between old and new leaves. Thus, leaves will grow in inherently yellow.
Iron/Micronutrient Deficiencies. I chose to lump these together, but basically your cannabis plants require a variety of micronutrients and iron to ensure proper growth. Iron is a vital immobile element, so you will experience yellowing in the newer leaves (like calcium). In general, it is rare to have other micronutrient deficiencies if you are in the right pH and are sure you’re putting in the appropriate micronutrients (such as in Elite Base Nutrient A).
Diseases/Bugs. Another potential cause of yellowing is unfortunately not chemical-related; diseases and pests can be causes of yellowing! Bud rot and fungus gnats, to name a few, are potential causes for yellowing in plants. Ultimately, stress on the plant (even something as simple as too much heat!) will cause yellowing of the plant. If you can eliminate the chemical culprits, it really simplifies your approach to determining the stressor associated with a disease/bug.
Remedies to Yellowing
Once you identify the type of deficiency, that is half the battle! Next step is treating the problem. Obviously, pH adjustment is self-explanatory (use your favorite pH up/down to get to the correct pH). For nutrient deficiencies, especially ones not related to N-P-K, take a peek in your reservoir. Do you see any type of precipitate (solid stuff)? If so, you may be experiencing nutrient lockout. Nutrient lockout essentially means ions in your sump are reacting with one another to make insoluble salts that just chill at the bottom of your res. If you’re yellowing and you see a bunch of solid stuff that you didn’t add initially, chances are, the nutrient lockout is preventing the contents in your sump from being delivered to the plant! Make sure you are mixing individual nutrients slowly and completely.
Lastly, consider supplementing with a CalMag supplement like Elite CalMag to help supply a variety of secondary nutrients. This will likely help clear up a variety of nutrient deficiencies all in one fell swoop. The convenience of using a supplement like this is that it saves you time on “diagnosing” the issue. Instead, you have a blanketed solution.
Yellowing is a difficult issue to remedy in your grow. It is always worthwhile to take the time to ensure that your pH is within the correct range, your plant has plenty of secondary nutrients and that you are mixing you nutrient correctly to avoid making insoluble salts.