We all learn from an early age that we have to drink our milk and eat our vegetables.
Milk provides the body with calcium and vitamin D, and vegetables contain a wealth of micronutrients that the body needs to thrive.
If we opt for soda instead of milk and ask for fries instead of veggies, we’re usually in no immediate danger. Our bodies’ nutritional deficiencies won’t affect us for quite a while unless they’re severe.
But weed plants have a much shorter lifespan, and any nutrient deficiencies that they suffer can cause big problems in a hurry. In the best case, a grow will produce a less potent, less bountiful harvest than expected.
In the worst case? The plants may die.
Cannabis nutrient deficiencies are serious and require immediate attention. Here’s what to look for and what to do.
Necessary Nutrients for Cannabis Plants
Plants obtain three crucial elements for their survival from air and water: hydrogen, oxygen, and carbon.
But they also require a supply of nitrogen (N), phosphorus (P), and potassium (K) to survive and thrive. That’s why fertilizers are labeled with their N-P-K ratios by weight.
A fertilizer labeled 10-10-10 contains ten parts nitrogen, ten parts phosphorus, and ten parts potassium. Cannabis fertilizer recommendations considerably, but they should generally suggest a ratio somewhere around 10-4-4 during the vegetative stage and 3-8-7 during flowering.
Those are just the primary macronutrients that plants need. Nitrogen, phosphorus, and potassium are the key ingredients in fertilizer because they’re quickly depleted during a growing season. But most plants need as many as 14 micronutrients other as well.
Many of those micronutrients are added to commercial fertilizers. Of course, they can also be added individually to the plants’ soil or another growing medium. Weed plants require nitrogen, potassium, and potassium to grow, but they also need a regular supply of other micronutrients. The most important are calcium, magnesium, iron, sulfur, copper, manganese, and zinc.
The amounts of micronutrients needed by cannabis plants are relatively small. But a deficiency in any one of them can have adverse — and sometimes fatal — effects.
And the deficiency may not even be caused by the amount of fertilizer or nutrients you’re supplying to your plants.
Soil pH: A Common Culprit
Plants absorb nutrients through the soil (or other growing media) they’re planted in. If there’s a problem with the soil, though, it may not matter whether you increase or decrease the plant’s supply of nutrients.
The problem we’re concerned with here is the soil’s pH (potential hydrogen, measured on a scale of 1-14). pH describes how acidic a substance is, and cannabis plants need a growing medium with a relatively-neutral pH between 6.0 and 7.0 (5.5 to 6.5 for hydroponic grows). That applies to the water you give your plants, too.
If the pH is too low or too high, it can cause a nutrient lockout, preventing your weed plants from getting their nutrients. You can test pH with an inexpensive meter, and that’s the first step to take if you suspect a nutrient deficiency. Additives that increase or decrease pH are available at garden shops or online.
The soil’s pH is fine? It’s time to consider possible cannabis nutrient deficiencies. The most common issues involve the three primary macronutrients.
The number one nutrient deficiency weed plants experience a lack of sufficient nitrogen. That’s because plants use so much of it during their growth.
The primary sign of nitrogen deficiency is mature leaves turning yellow, particularly toward the bottom of the plant. Eventually, the yellowing will move up the plant, and leaves will curl up, wither and fall off.
Nitrogen deficiency is most often seen during the flowering phase because plants store nitrogen in their leaves earlier in their life cycle. If they can’t get enough nitrogen from the soil, they take it from their leaves, and the leaves die.
You can’t make yellow leaves turn green again, but you can add nitrogen to the soil and save the plant. Adding blood meal, bat guano, or fish meal are three quick ways to boost nitrogen levels.
This is another common nutrient deficiency in cannabis plants. Fertilizers usually contain the smallest amount of the macronutrient, even though plants need it for multiple functions, including respiration and disease resistance.
You’ll know your plants are begging for potassium if you see them growing quickly but with their bottom leaves turning brown and dying. Another possible sign is older leaves which take on a dull look, followed by their tips turning brown and their inner growth developing yellow or brown spots. Eventually, younger leaves will become dehydrated and curl if the deficiency isn’t resolved.
Kelp meal, wood ash, potash sulfate, or chemical potassium additive can quickly solve the issue if you catch it in time.
This problem is less common but is still seen, especially in hydroponic grows. It’s often the result of the nutrient lock but may occur when the plants simply aren’t getting enough of the macronutrient.
Plants suffering from a phosphorus deficiency will grow slowly and look somewhat frail. Their older leaves will take on a dark color that usually starts with the stems turning purple and the foliage taking on a bluish hue; the foliage may eventually turn purple, brown, or black or develop rust-colored spots before it falls off. These symptoms are most likely to be seen during cold spells.
Bat guano, bone meal, or crab meal can restore the plants’ health within a week.
Most growers won’t have to deal with micronutrient deficiencies in their grow because the plants need such small amounts of these nutrients to thrive.
Even though these issues are rare, however, they do occur.
- Calcium Deficiency: This is often caused by a lack of sufficient calcium in the water and shows as curled and spotted leaves and cracking or weak branches. Adding lime to the soil will help.
- Sulfur Deficiency: Young foliage yellows, dries, and becomes brittle while bud production slows. Epsom salts or potassium sulfate is the easiest treatment.
- Magnesium: Lower leaves turn yellow, rust spots are seen, and leaves curl. This deficiency must be dealt with quickly before it moves up the plant. Epsom salts added to the soil usually does the trick.
- Copper: The tips and sides of new leaves will become discolored, and they will wilt and twist, eventually becoming limp. Add kelp, compost, or copper fungicides.
- Manganese, Zinc, and Iron: These deficiencies often occur in tandem. You’ll see new leaves starting to yellow, upper leaves turning yellow with the veins remaining green, or the veins in older leaves turning yellow while the foliage stays green and twists. The best solution is usually to flush the plants, check the pH of soil and water, and begin feeding them the proper mix of nutrients.
Cannabis Nutrient Deficiencies: FAQ
Q: Is there any way to prevent nutrient deficiencies other than using the right fertilizer for each stage of plant development?
A: Successful growers often take fertilizing a step further, changing the mix of macronutrients several times during the vegetative and flowering stages to compensate for their plants’ changing nutrient needs. Other smart steps are preemptively checking the pH of soil and water, using living soil as a growth medium, and composting.
Q: Is there a quicker way to deal with cannabis nutrient deficiencies than just altering fertilizer or adding corrective ingredients to the soil?
A: There are “foliar sprays” that are used directly on leaves in the same way you’d apply fungicides. For example, nitrogen, potassium, and phosphorus can all be sprayed on foliage for faster action. Just be sure to take corrective action with your fertilizer as well to prevent future problems.
Bevan, L., Jones, M., & Zheng, Y. (2021). Optimisation of Nitrogen, Phosphorus, and Potassium for Soilless Production of Cannabis sativa in the Flowering Stage Using Response Surface Analysis. Frontiers in plant science, 2587 .
Kidd, P. S., & Proctor, J. (2001). Why plants grow poorly on very acid soils: are ecologists missing the obvious?. Journal of Experimental Botany, 52(357), 791-799 .