Less in more: the dangers of over-fertilising your cannabis plants

cannabis fertilisation poses a major challenge for growers: to find the perfect balance between nutrient supply and optimal yielding capacity. Although it may seem logical to provide an excess of nutrients to ensure vigorous growth, recent research refutes this practice, highlighting the dangers of the so-called “luxury consumption”, a phenomenon that not only leads to an inefficient use of resources but can also negatively affect the health of your marijuana plants.

In the search for cannabis plants that are as productive and lush as possible, it is easy for growers, especially for those who are just starting out in this noble art, to fall into the temptation of thinking that the greater the amount of nutrients, the better. This belief often leads to over-fertilisation, a practice that can do more harm than good. While nutrients are essential for cannabis growth, an excess of nutrients can disrupt the delicate balance needed for healthy development, generating a number of detrimental effects.

What happens when cannabis plants are over-fertilised?

One of the most immediate and visible consequences of over-fertilisation in cannabis plants is nutrient burn, a problem that manifests itself in the form of claw-shaped leaves (they curve downwards, adopting a claw-like appearance). The colour of the leaves may also be affected, with a dark green that’s more intense than usual, or brown or yellow tips which gradually spread as the plant struggles to cope with an excess of nutrients. Nutrient burn not only stunts growth but also reduces the overall vitality of the plant, resulting in lower yields and a lower quality of the final product.

Another major problem arising from over-fertilisation is the accumulation of salts in the substrate. Most fertilisers contain salts which can accumulate in the growing medium when they’re applied in excess. Over time, the salt concentration may become so high that it inhibits the plant’s ability to absorb water and essential nutrients. This results in what is known as “nutrient blocking” – despite the abundance of nutrients in the soil, the plant cannot access them, which leads to deficiencies and poor health.

Excessive fertilisation can also have detrimental effects on the root system of cannabis, which is the plant’s foundation and is responsible for the absorption of water and nutrients. When subjected to nutrient overload, this system can be damaged; and, in extreme cases, it can lead to root rot, a condition that can quickly kill a plant if not treated promptly.

Therefore, if you’ve overfed your cannabis plants, the first thing you need to do is to stop doing it. If you continue adding nutrients, you’ll only be prolonging the problem, so make sure to remove excess fertiliser and flush the roots with water. You may even need to transplant them into new soil if the soil is oversaturated with fertilisers.

But beyond the direct impact on plants, over-fertilisation also has wider environmental consequences, as the excess fertiliser can leak into the surrounding environment, contaminating water sources and damaging local ecosystems. This environmental damage is particularly worrying in areas where cannabis cultivation is widespread, as it may contribute to wider ecological degradation.

When, how and why to wash roots

Over-fertilisation in cannabis growing is commonly associated with the concept of “luxury consumption”, which can generate more harm than good for both the plants and their growers. This approach, which involves providing more nutrients than necessary in the hope of maximising the yield, can be counterproductive and costly.

The term “luxury consumption” refers to the belief that plants will absorb excess nutrients if they’re available, using them to grow faster or to produce higher yields. However, research shows that, beyond a certain point, plants not only fail to take advantage of these additional nutrients but may actually suffer negative consequences.

This has been demonstrated by extensive studies at both North Carolina State University (NCSU) and Israel’s Institute of Soil, Water and Environmental Sciences, which examined the effects of different fertilisation rates on plant growth, nutrient absorption, and cannabinoid concentrations.

One of the most relevant conclusions from these investigations is that cannabis may exhibit a luxury intake of several nutrients, including magnesium (Mg) and phosphorus (P). In tests with phosphorus fertilisation at levels between 15 and 180 ppm, it was found that increasing the dose above 15 ppm did not improve either plant growth or the total weight of the buds, although it did increase the concentrations of phosphorus in the leaf tissue.

Furthermore, studies on magnesium (with levels between 0 and 100 ppm) revealed that exceeding 50 – 75 ppm did not favour growth or the concentration of cannabinoids, despite the accumulation of this nutrient in the leaves. Therefore, these studies prove that taking in more nutrients doesn’t necessarily lead to higher yields but may instead create harmful internal imbalances.

The danger of nutritional antagonisms

Luxury consumption can generate conflicts (called “antagonisms”) between essential nutrients. Fertilisers provide nutrients in pairs to balance their loads, such as potassium (K+) or calcium (Ca2+) and nitrate (NO3-). When a nutrient is applied in excess, antagonism can occur, preventing another nutrient from being absorbed, even if it’s present in the fertiliser.

This phenomenon is common among most macronutrients. For instance, even if phosphorus toxicity isn’t detected, excessive accumulation can cause iron (Fe) deficiency (with iron being a critical micronutrient for photosynthesis).

Another example is the relationship between potassium (K), calcium (Ca), and magnesium (Mg), where maintaining a 4:2:1 ratio is highly recommended to avoid antagonism. Excess calcium (Ca) can cause magnesium deficiencies, even if the latter is applied in adequate amounts.

Less is always more

As these studies show, the management of your nutrient plan must be based on satisfying the real demands of the plants and avoiding excesses that don’t provide any additional benefits. By adjusting fertigation rates and designing adapted nutrient recipes, growers can prevent problems derived from luxury consumption, such as antagonisms and nutritional stress. This approach not only ensures plant health and bud quality, but also reduces fertiliser expenditure, making the process more efficient and sustainable.

Therefore, the key to successful cannabis cultivation is to abandon the idea that “more is better” when it comes to fertilising. Providing exactly what the plants need optimises their growth and yielding capacity, minimising the problems associated with luxury consumption. And with this balanced, scientifically based approach, growers can maximise both the yielding potential and sustainability of their grows.