You are growing a beautiful Cannabis plant, when suddenly you see that the leaves don't look as healthy as they did yesterday. You're sure you watered it yesterday so that can't be it. Could your plant be missing nutrients?
When your Cannabis plant is experiencing problems it can be tempting to look for a simple solution and blame a deficiency of one or more nutrients. The reality of growing Marijuana is that there are many factors that can influence the quality of your grow including light, ventilation, temperature etc.. Before blaming your nutrition, taking a look at the quality of life of your Cannabis plant in its grow space can often explain certain symptoms. For example, a plant grown too close to a light can certainly look like it is lacking nutrients, but it is most likely simply getting damaged by the light. If you have ruled out your growing conditions as the potential culprit, still keep in mind that many Cannabis nutrition problems are not caused by depleted nutrients. Especially in hydroponic Cannabis cultivation, nutrition problems are often caused by fluctuations in pH and/or E.C.. As we have outlined in our article about the role of pH and E.C. in Cannabis nutrition: if your pH is out of the 5,5 - 7 range in soil, or the 5 - 6,5 range in hydro you are most likely locking out several nutrients. E.C. values vary between types of medium and different strains of Cannabis, but should usually not exceed 1500 p.p.m.. For E.C. values it is important to remember that overshooting your E.C. can cause a lot of damage, whereas a too low E.C. will only stunt growth slightly.
Macronutrients are the nutrients that are used in the largest quantities by an organism, and are therefore very important to keep track of. Cannabis nutrition relies heavily on different macronutrients, a list of all of these macronutrients is provided below. There are different classes or macronutrients, this is also based on the amounts that are required. Carbon, hydrogen and oxygen are primary macronutrients, but these nutrients are mostly derived from the air. In the following section we discuss the primary macronutrients that a Cannabis plant draws from the growth medium: nitrogen, phosphorus and potassium.
Nitrogen is used for virtually everything a Cannabis plant does. While there is a lot of nitrogen in the air, a Cannabis plant is unable to use this form and must absorb nitrogen from the soil. When there is a lack of nitrogen available for the plant during any stage of its growth, the first signs are the lush green leaves will quickly turn to a pale yellow. Each leaf will bleach and become yellow until it withers and falls off. This happens because the plant will take the available nitrogen stored within its leaves and effectively cannibalize itself. It is natural to see some fan leaves begin to yellow towards the end of the flowering period as a Cannabis plant will direct as much energy towards bud development as possible. This is not caused by a nitrogen deficiency in most cases, but rather a sign that your crop is almost ready!
Phosphorus is used to promote the development of strong roots and branches and plays a central role in plant metabolism. A shortage of phosphorus during flowering causes weak branches and small budding sites. Phosphorus deficiencies often cause stems to become red or brown, but as this is also a natural phenotype of certain Cannabis strains like Gold Rush or Gold Bar Kush this is not a 100% reliable indicator. Stunted growth and drooping leaves can be signs of over fertilisation with phosphorus. If you overshoot phosphorus even more the leaves will form an orange tip burn, indicating you should lower the E.C. of your Cannabis nutrition.
Nutrition for Marijuana plants during flowering relies heavily on high levels of phosphorus and potassium (Potash) to produce large budding sites full of flowers. Potassium is very important for plants because it is used to regulate their internal “E.C. value” and is required for growth and breathing. A potassium deficiency can have some difficult to distinguish symptoms including bleaching of leaves, leaves turning slightly blue and scorched looking leaf tips. The clearest sign of a potassium deficiency however is dead spots on leaves. Usually this starts in the older fan leaves. Too much potassium can also be a problem for your Cannabis plant, so this should be dealt with quickly. Potassium overfeeding causes the fan leaves and the majority of the newest leaves to lose all green and become dark yellow and almost crispy. The effect of too much potassium can look like heat stress and can cease growth to a stand still. Once a Cannabis plant has problems at this level, the grower should flush the medium and carefully nurse the plant back to health.
Aside from nitrogen, phosphorus and potassium, there are three other macronutrients that a Cannabis plant uses in large amounts. The secondary macronutrients consist of calcium, magnesium and sulfur. A deficiency of any of these three nutrients can have enormous consequences on your grow.
In the same way humans rely on calcium for developing bones and magnesium for protein synthesis, calcium and magnesium also play key roles in Cannabis nutrition. These two nutrients are often equally depleted and can be taken up in a similar pH range. Cannabis growers frequently refer to calcium and magnesium together as cal mag. Many fertilizers contain N-P-K but lack calcium and magnesium, even though they are also used in large amounts. A lack of calcium can also cause a magnesium deficiency because calcium is needed for the uptake of magnesium. Cal mag deficiency causes weak cell walls and browning of the fan leaves. The leaves will have dark brown golden spots, which can look similar to bleaching from too much light. Other signs of a calcium magnesium deficiency are leaf tips that point upwards and a very dark shade of green in the centers of leaves. Once cal mag is restored to the correct levels, the plant will quickly regain its usual green characteristics. You can speed this process up by foliar feeding a nutrient mix that contains cal mag.
Another element that the Cannabis plant depends on is sulfur, which is required for photosynthesis. It can also be used as a replacement for potassium if there is a shortage. A sulfur deficiency will cause new growth to become yellow from the inside towards the leaf tip. This is in contrast with many other deficiencies which generally start with older growth in the bottom of the plant and then from the leaf tip inward. Too much sulfur can cause scorching of the leaves and will turn the once green leaf to a crispy light brown. The central area of the leaf will look as green as before yet the outside will look lifeless.
Micronutrients also play very important roles in Cannabis nutrition, but deficiencies are less common because they are used in such small amounts. This is especially true when growing in complex, soil-based media that will always contain small amounts of micronutrients. The list below is an overview of different micronutrients that are essential for a Cannabis plant.
Iron is used in many enzymatic processes and is an important part of Cannabis nutrition. Iron deficiency will cause new growth to start out bleached, and will eventually cause the rest of the Cannabis plant to bleach as well. Iron deficiency can look very similar to sulfur deficiency, but an iron deficiency will cause leaves to bleach completely while maintaining green veins. Check the pH of your nutrients and medium to distinguish between a sulfur and an iron deficiency as iron is much more pH sensitive than sulfur. A good flush can fix issues with availability of iron, but not sulfur. The availability of iron in soil is largely dependant on pH. If the soil pH is too high (8+), iron will effectively be locked out for plants. For an ideal absorption of iron from soil, aim for a pH between 6 and 6.5, this should be slightly lower in a hydroponic grow.
Cannabis nutrition problems with copper generally occur during flowering. Copper deficiency can usually be seen in the leaves as these will appear to have a blue and/or yellow glow. The leaves will also have yellowing edges which will feel hard to touch. The leaves closest to the buds can also begin to turn purple, yellow and a golden brown. This may look good from a distance, however when looked at closely you can see that the Cannabis plant is in trouble.
A boron deficiency will cause growing shoots to look burnt and eventually turn gray and die. It also causes brown patches on the leaves. A boron deficiency may be easier to identify than other deficiencies as the spots on leaves look like golden brown stains. Boron helps plants use calcium and is used for photosynthesis, maturation, pollen production, seed production, cell division, protein production and plant structure.
Despite the fact that many growers purposely leave tap water out overnight to remove chlorine, a Cannabis plant needs chlorine to aid moving water in and out of cells. Chlorine also plays an important role in sulfur metabolism. Chlorine deficiency leads to spots on the lower leaves and around the veins of the plant. Deficiency is rare however as chlorine is usually already mixed into our tap water at safe levels for humans and plants.
Silica is the second most abundant raw material on the planet, yet is often overlooked as a part of Cannabis nutrition. Silica is used in cell walls, which also form the outer layer of the plant and facilitate the transportation of liquids throughout a Cannabis plant. A healthy amount of silica ensures that a plant is more protected against insects which could otherwise easily penetrate the plant's outer cell walls. No picture of a silica deficient plant was included in this article because there are no clear symptoms of a silica deficiency. Some growers even argue that silica is not an essential nutrient. Silica can also be used to increase pH and can be absorbed through foliar feeding or from the medium. Supplementing silica can be a great tool if used correctly and can really strengthen plants and prevent Cannabis nutrition problems.
A zinc deficiency is uncommon in soil grows, but is known to occur among hydro growers. A common symptom is that fan leaves stay smaller, this has also been nicknamed “small leaf syndrome” by many growers. Further symptoms include white leaf tips and white areas between the veins. A zinc deficiency can be a consequence of an alkaline medium. Check the pH of your medium, feed and runoff regularly to prevent this. Make sure that for soil grows your pH is between 4,7 and 7,5, whereas for hydro grows these numbers are lower and should be between 4,2 and 5,7. In case your pH is out of this range, flush your soil and foliar feed your plants a weak nutrient solution that contains zinc. After this process, or if your soil pH was fine to begin with, make sure your nutrient solution contains zinc from this point on.
A molybdenum deficiency is another rare micronutrient deficiency that usually only occurs when the pH of the soil is out of a healthy range. Molybdenum is best absorbed from soil with a pH of around 6.5 or hydro media with a pH of around 5,5. A molybdenum deficiency looks similar to a nitrogen deficiency at first in that the older fan leaves will start to yellow. However this is soon followed by bleaching of the edges of younger fan leaves. This bleaching can cause fan leaf edges to turn different colours and pink, orange or red leaf edges are not unheard of. Similar to when treating a zinc deficiency, a molybdenum deficiency can usually be solved by flushing and pH adjusting your medium and foliar feeding a mild nutrient solution containing molybdenum.
Manganese is another micronutrient that a Cannabis plant requires in only very small amounts. Problems with manganese are again likely due to pH problems in the medium. Manganese deficiency starts with interveinal yellowing of younger leaves, which then spreads to older leaves and eventually causes dead spots. An excess of manganese can cause similar symptoms, but the dead spots on leaves will appear much faster. Excess manganese combined with high temperatures can deplete iron and zinc, which can make this situation look like a deficiency of these two nutrients. Again, with a soil pH of around 6.5 or a hydro medium pH of around 5,5 problems with manganese are very rare. Solving problems with manganese requires the same steps as were mentioned in the previous sections on zinc and molybdenum: flush your medium and adjust the pH, foliar feed a mild nutrient mix containing manganese and make sure your feed contains manganese afterwards.
Cannabis nutrition can be divided in macro- and micronutrients depending on the quantity that is used. Nutrient deficiencies can occur because certain nutrients are depleted and need to be added, but they can also be caused by a pH-dependant nutrient lockout. Macronutrient deficiencies are usually caused by missing nutrients as the pH range in which they can be consumed is wider than that of most micronutrients. Micronutrients are more commonly locked out due to problems with pH. In these cases the micronutrients are present, but they are locked in the medium and unavailable for your Cannabis plant. Micronutrient deficiencies are also more common in hydro than in soil grows, as soil will usually contain reasonable amounts of micronutrients to begin with. In a hydro grow your medium will be free of nutrients, and in combination with demineralized or R/O water the only source of these micronutrients will be your feed. It is also much more difficult to maintain a stable pH in hydro as the medium will contain less of a buffer, this can easily cause nutrient lock-outs. So in short: keep an eye on your plant, and check the pH and E.C. values of your feed, medium and runoff regularly. Also keep in mind that Cannabis nutrition gets more complicated with different types of growth medium, so start by growing Marijuana in soil if you have limited experience.