BackNutrient deficiencies in cannabis: a guide to detecting the symptoms
30.06.2025
A balanced nutrition is key to the success of a marijuana grow. When your plants don’t get what they need, either because you don’t provide it to them or because they can’t absorb it, symptoms begin to appear.
You could say that plants alert us in their own language when something is not right. For those with an expert eye, knowing how to interpret these signals is essential. If you manage to detect them in time, you’ll be able to correct the deficiency before the problem drastically affects your crop. In this guide, we show you how to recognise the symptoms of the main nutritional deficiencies of marijuana (macronutrients and secondary nutrients), as well as to understand their causes and how to apply effective solutions.
Why do nutritional deficiencies and blockages occur?
There are several reasons that can lead to nutrient deficiencies. These are the most common:
Watering with the wrong pH
Make sure to measure the pH of your nutrient solution or water before watering. When pH levels are outside the ideal range, nutrients are not absorbed properly.
The optimal pH range for hydroponic and coco coir setups is 5.5 - 6.5, and between 6.0 and 7.0 for soil.
Overwatering
A substrate that’s waterlogged for too long can cause root asphyxiation, meaning the roots won’t be able to absorb nutrients.
Excess of salts in the substrate
If the irrigation water is too hard, it may lead to nutrient blockage due to the accumulation of mineral salts in the substrate, which will prevent the roots from properly absorbing the fertiliser.
Using a poor substrate
If you’re going to grow in soil, make sure it is high-quality and specific for marijuana cultivation. It must contain all the necessary nutrients and have the airy and spongy structure that cannabis plants need.
Actual deficiency vs. blockage in marijuana cultivation
When detecting a nutrient deficiency, the first thing you should analyse is whether it is a real deficiency (i.e., if the nutrient is NOT present in the substrate or irrigation water in the amount required), or whether it is more of a blockage for the reasons mentioned above, with the plant being unable to absorb these nutrients.
Macronutrient deficiency in marijuana: symptoms
These are the symptoms that your plant will exhibit when it is suffering from a lack of nutrients:
Nitrogen deficiency
Symptoms: Nitrogen (N) (mobile nutrient) is essential for the growth of cannabis, as it facilitates the production of chlorophyll and intervenes in the process of photosynthesis. When there’s a nitrogen deficiency, growth slows down and the leaves of the lower part of the plant begin to turn yellow.
The older leaves at the bottom of the plant begin to turn yellow.
Phosphorus deficiency
Symptoms: Phosphorus (P) (mobile nutrient) is essential for vegetative growth, root mass development, proliferation of new shoots, and flowering.
When there’s a deficiency of phosphorus, the leaves remain small and wilt, showing curved edges and a bluish or reddish colour. The growth of the plant slows down and, in more advanced stages, brown spots appear on the leaves near the central stem.
Potassium deficiency
Potassium (K) (mobile nutrient) is a particularly important macronutrient for root development, chlorophyll production, and bud formation.
Symptoms: The plant slows its growth; some leaves curl downwards; and burnt edges appear and later dry out, acquiring a brown colour. At more advanced stages, brown spots also appear on the leaves.
Secondary nutrient deficiency in cannabis
These nutrients are called “secondary” because the plant only needs them in moderate amounts (i.e., lower than those of macronutrients, but higher than those of micronutrients).
Lack of calcium
Calcium (Ca) (immobile nutrient) strengthens the cellular structure of the plant, contributing to the development of its tissues and roots.
Symptoms: When there’s a calcium deficiency, the plant shows a decrease in vigour in its overall development, and the leaves become deformed, exhibiting irregular edges, burnt and twisted tips, etc. The new leaves grow with deformities and with little colour, which causes their early wilting.
Magnesium deficiency
Magnesium (Mg) (mobile nutrient) intervenes in different enzymatic processes of the plant, as well as in the production of chlorophyll and the photosynthesis process.
Symptoms: When a cannabis plant is suffering from a magnesium deficiency, its symptoms can often be confused with those of a nitrogen deficiency, as the older leaves at the bottom of the plant begin to turn yellow during the first stage. However, there are other symptoms that will help you to tell the difference between the two:
- Chlorosis in the leaves, with the nerves becoming more visible while the green tissue lightens and turns yellow.
- Brown or reddish spots appear on older leaves.
- The stems of the plant may display a reddish colour.
- If the deficiency isn’t resolved, chlorosis in the leaves intensifies over time, with the leaves turning completely yellow, curling up on themselves, and eventually dying.
- Although magnesium is not essential for the development of the flowering, a lack of it can cause the buds to be less dense and even become discoloured.
Sulphur deficiency
Although the cannabis diet doesn’t require great amounts of Sulphur (S) (immobile nutrient), a deficit of this secondary nutrient can trigger problems in development, since it contributes to the formation of some proteins and enzymes and to the regulation of hormones and vitamins. It also directly affects plant growth and the production of chlorophyll and terpenes.
Symptoms: A sulphur deficiency may result in slower plant growth, with petioles turning purple and pink tones sometimes appearing on the underside of the leaves. Another obvious symptom is that chlorosis appears on the new leaves at the top of the plant, starting at the base of the leaves and then spreading to the tips.
How to remedy nutrient deficiencies in marijuana plants?
As explained above, there are several causes that can lead to your plants experiencing deficiencies in one or more nutrients. Therefore, the first step involves analysing the source of the problem before correcting it:
- Measure and adjust the pH and EC of the soil and the nutrient solution.
- Test the water you’re using ― if it’s too hard, flush the roots to remove the salts accumulated in the substrate.
- Assess whether your substrate is of good quality.
- Provide the nutrient that your plants lack so they can recover, either through the use of fertilisers in the irrigation water or as a foliar solution that you can spray directly onto the leaves.
Timely detection and correction of nutritional deficiencies is key to abundant and high-quality harvests. By monitoring pH, EC, and nutrients, as well as by using a good substrate and water treatment, you will avoid blockages and boost the yielding capacity of your grow. Here’s wishing you a great harvest!