Over- and under-fertilizing your plants is usually the second most common problem that happens for new growers. Here, we will be going over how to diagnose and treat this when it happens.
Diagnosing Nutrient Deficiencies
For plant growth and development, the new growth will always need a mix of all the nutrients the plant requires. So, when a plant is deficient in a certain nutrient, it will do one of 2 things: pull the missing nutrients from the oldest part of the plant if possible so that the lower parts of the plant show the symptoms and the new growth does not (mobile nutrients), or if the specific nutrient is not movable, then the new growth will show the symptoms of the deficiency (immobile nutrients).
So, by noticing if the plant is showing symptoms at the top or the bottom, you can already narrow down which group of nutrients are causing the problems with your plants.
Now, onto diagnosing the plant. There two main ways a plant can tell you that something is wrong: yellowing of the leaves, which is called chlorosis, and death of the leaf, which is called necrosis. Based on the location on the plant as well as the location on the leaf that these two problems are appearing in, it is easy to narrow down what is wrong with the plant. So, let us go down the list.
Always check the water pH first! Before trying to correct a nutrient imbalance, check the pH of the water before and after watering the plant by collecting some of the runoff to make sure it is within the 6.0-7.0 range, as this could be the cause of the nutrient deficiency.
Primary Macronutrient Deficiencies
All three of the primary macronutrients of nitrogen, phosphorus, and potassium are mobile nutrients, so the symptoms of each will appear from the bottom and move their way up the plant.
With a nitrogen deficiency, the bottoms leaves will shows signs of chlorosis with no necrosis; the leaves will just keep on yellowing more and more over time and then fall off the plant. The plant's growth will also slow down in height and grow smaller leaves.
With a phosphorus deficiency, there will be a little bit of chlorosis, starting with a lime green color, and over time, this will be accompanied by patches of necrosis near the veins of the leaves.
With a potassium deficiency, there will be chlorosis that starts from the tips of the leaves back to the base of the leaves and, over time, the tips will also show signs of necrosis.
A Lack of Fertilizer
If it looks like all 3 primary macronutrient deficiencies are happening at once, with the bottom leaves yellowing before necrosis kicks in while quickly moving up the plant, then that just means the plant is just malnourished, and you will need to increase the overall fertilizer amount. Otherwise, if it looks like just one nutrient deficiency, then you can increase the amount of just that one nutrient with your fertilizer regimen.
Also note that it is perfectly natural for this to happen slowly over time. As the plant grows significantly larger, the oldest leaves will slowly die and fall off. It is only an issue if the pace of affected leaves is faster than the pace of the new growth.
Secondary Macronutrient Deficiencies
Out of all the secondary macronutrients, the only mobile one is magnesium. The other two, calcium and sulfur, are both immobile nutrients, so the symptoms of each will start to appear from the new growth.
A magnesium deficiency will start with yellowing but only on the fleshy parts of the leaves, also known as interveinal chlorosis, which means that although the leaves will start to yellow, the stem, veins, and edges of the leaves will stay green. On top of the interveinal chlorosis, necrosis spots throughout the leaf will also start to appear as well over time.
A calcium deficiency, on the other hand, will also show necrosis spots on the leaves. However, because calcium is an immobile nutrient, the symptoms will start to appear from the top of the plant, especially on the leaves that are still developing, growing in size and getting the most light. Both calcium and magnesium deficiencies can be cured by just including the nutrient along with your fertilizer regimen, and a lot of times, these two secondary macronutrients will be paired together in a combined supplement labeled as Cal-Mag.
Sulfur deficiency is not very common in soil grows since most soils generally have more than enough for a cannabis plant, but if you notice chlorosis similar to a nitrogen deficiency, except starting from the top of the plant, then you have a sulfur deficiency. This is easy to fix with a teaspoon of epsom salt, which is available everywhere, added per gallon of water when watering your plant.
Since most micronutrients are immobile, if the new growth of your plant shows signs of interveinal chlorosis or just weird stuff happening to the new growth like the color turning copper or the leaves becoming deformed, then you most likely have a micronutrient deficiency.
But because micronutrients are needed by the plant in such tiny trace amounts, a micronutrient deficiency, at least when it comes to cannabis, is almost always linked to a problem with the water pH or a nutrient surplus with one of the macronutrients, which in turn actually locks out some micronutrients. To solve this, you will need to do the following:
Test the pH of your water, and if it is out of the 6−7 range, then pH balancing your water should fix the nutrient lock.
If the pH of the water is not the issue, then flush your soil with water until the runoff turns clear to get rid of all the nutrient and salt buildup in the soil, which will remove any nutrient lock caused by over-fertilizing.
Since the typical plant fertilizer only adds the primary macronutrients of nitrogen, potassium, and phosphorus, we will be covering what happens if you over fertilize with any of these three.
A nutrient surplus of nitrogen will lead to dark green leaves at the base of the plant and will move up to the newer growth over time. With phosphorus and potassium, a surplus of each will both lead to necrosis on the leaf tips, which is commonly called nutrient burn.
All 3 surpluses will also cause growth issues, but the worst part of any nutrient surplus is that it will lock out the plant from being able to take in other nutrients: calcium, magnesium, and a number of micronutrients will not be available to the plant anymore. This is why over-fertilizing is so dangerous. Not only does the plant get sick off a surplus of nutrients, but the surplus also locks out other nutrients, creating multiple symptoms all appearing at the same time.
Now, to cure a surplus of nitrogen, phosphorus, potassium, or most likely a combination of all 3, you will just need to flush your plant's soil with water until the runoff water is clear to get rid of the excess nutrients and salt buildup, and then just decrease the fertilizer amount once the soil is dry again.
A Note about Plant Recovery
Even after a surplus or deficiency has been corrected, whatever necrosis or chlorosis the leaves have will stay on the leaves. This means yellow leaves will stay yellow and dead spots on a leaf will stay dead. So, the only two ways to tell that the plant is now healthy again is that the necrosis and chlorosis are not spreading throughout the rest of the plant and that the new growth on top of the plant looks healthy.
Did this guide help? This article was taken directly from The Absolute Beginner's Guide to Growing Cannabis book, so if you like the advice above you'll love the rest of the book, which includes a lot of exclusive content not found anywhere else. You can find both the e-book and paperback copy on Amazon, and it's free with a Kindle Unlimited subscription!