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Molybdenum Deficiency

Molybdenum Deficiency in Cannabis Plants

Molybdenum deficiency is one of the rarest nutritional problems among cannabis plants, with plenty of experienced growers never encountering it at all. If the issue is allowed to continue for too long though, it can do serious damage.

This is because the metallic element molybdenum is used by cannabis plants to break down nitrogen and synthesize important amino acids. If molybdenum deficiency persists for too long, a marijuana plant will grow slowly, and individual leaves will eventually succumb to necrosis.

Fortunately, molybdenum deficiency is normally fairly easy to diagnose and resolve. It most often becomes recognizable early on when leaves toward the middle of a cannabis plant begin to turn yellow.

The location is important, as this most notable sign of early-stage molybdenum deficiency can easily be confused with one often seen in plants that lack nitrogen. The symptoms of nitrogen deficiency appear first in the lower leaves of a plant and develop in somewhat different ways thereafter.

Once molybdenum deficiency has been diagnosed, it should normally be fairly easy to address via either adjustment of growing medium pH or supplementation. As long as the problem gets caught early on, the damage initially done by even significant molybdenum deficiency can be kept to a minimum.

While the average grower will not need to worry much about molybdenum deficiency, understanding why it is problematic and how to identify and resolve it can only be helpful. Being able to recognize when cannabis plants are suffering from a similar-looking but very different problem can make a real difference.

Molybdenum is an Especially Distinctive Micronutrient

Cannabis plants need to intake small quantities of eight micronutrients if they are to grow and flower without problems. Molybdenum is one of the most distinctive of these, for a number of reasons, such as the facts that:

  • Molybdenum is the only mobile micronutrient. From copper and iron to boron and manganese, none of the other micronutrients move around within cannabis plants after they have been deposited the first time. Cannabis plants are able to shuttle molybdenum to new locations later on, though, if needed. Many other types of micronutrient deficiency most often produce their earliest symptoms in new leaves which cannot be supplied with the associated element. When a cannabis plant lacks molybdenum, it will move the resource from older, established leaves to new ones, with the former showing the signs of the problem, as a result.
  • Molybdenum is the only micronutrient whose uptake decreases at lower pH. A cannabis plant’s ability to absorb the other seven micronutrients decreases when the pH in the root zone rises above a certain level. Molybdenum is the exception, with its uptake rate decreasing when pH drops too far, instead. In fact, it is possible for molybdenum deficiency to develop because of growing medium acidity that simultaneously fosters a toxic buildup of one or more other micronutrients. While this is even less common than molybdenum deficiency alone, it illustrates how counter-intuitive molybdenum management can be.
  • Cannabis plants need only tiny quantities of molybdenum. The eight “micronutrients” are so named because plants need only trace amounts of them. Molybdenum is a standout in this respect as well, with less of it being required than any other member of the group other than copper. This is one of the reasons why molybdenum deficiency is so rare, particularly given that most cannabis growers use nutrient mixes or growing media that include plenty of it.
  • Molybdenum is critical to the breakdown of nitrogen into ammonia and the synthesis of amino acids. Despite so little of it being required, molybdenum is fundamental to some especially important reactions within cannabis Tiny amounts of the metal are used to produce enzymes called “nitrogen reductases” that break nitrate down into nitrite. Since nitrate is the most widely available form of the nutrient nitrogen, a plant that lacks molybdenum will be deprived of one of its most important building blocks. That will inhibit its ability to produce critical proteins and is one of the reasons why certain symptoms of molybdenum deficiency resemble those common to plants that lack access to nitrogen.
  • Molybdenum toxicity is even less common than deficiency. As rare as molybdenum deficiency is in cannabis plants, toxicity is even less so. Unlike with several other micronutrients, cannabis plants typically tolerate excesses of molybdenum in their growing media or tissues fairly well. That makes supplementation a frequently reasonable way to address molybdenum deficiency, while it can be somewhat risky with deficiencies of other micronutrients.

Symptoms of Molybdenum Deficiency in Cannabis Plants

As with other types of nutritional problems, the best policy regarding molybdenum deficiency in cannabis plants will always be to recognize it as early as possible. Molybdenum deficiency has a characteristic progression that includes symptoms like:

  • Premature yellowing of leaves at the middle part of the plant. Much like with the more common nitrogen deficiency, molybdenum deficiency will first cause certain leaves of a cannabis plant to turn progressively more yellow. While this symptom of nitrogen deficiency always shows up first in the oldest, lowest leaves of a plant, it initially appears in leaves higher up when a lack of molybdenum is the culprit. This distinction is likely the single most important thing to understand about molybdenum deficiency as a grower, since being unaware of it can easily lead to serious, otherwise avoidable mistakes. It is also important to realize that a general yellowing of leaves after a plant has begun flowering is normal and probably not a sign of molybdenum deficiency at all.
  • Yellowing spreads to younger leaves and the twisting and curling of leaf edges begins. If molybdenum deficiency is allowed to continue, younger leaves higher up on a plant will start displaying the same progressive yellowing. The edges of leaves will also begin to twist, curl, and wilt.
  • Growth declines and leaves begin dying. Although it will normally take some time, persistent molybdenum deficiency will inhibit the growth rate of a cannabis plant in general. At this point, many leaves will have become pale or have acquired a scorched, burnt look. Even the old leaves lowest down on an affected plant will display symptoms of advanced molybdenum deficiency like curled tips and rolled edges.

Several Possible Solutions to Molybdenum Deficiency

It should always be possible to catch molybdenum deficiency at its earliest stages, when the only sign of the problem is the progressive yellowing of leaves near the middle of a plant. That will ensure that this rare problem can be resolved without reducing the yield much at all. Molybdenum deficiency in cannabis plants can normally be addressed by:

  • Raising root-zone pH. Even when there is plenty of molybdenum in the growing medium, the roots of cannabis plants struggle to absorb the metal when the environmental pH is too low. Generally speaking, pH should be kept above 6 if growing in soil and over 5.5 for hydroponic systems. It is important not to raise growing medium pH too high, since that can cause deficiencies of different micronutrients or other issues. As with other pH-related nutritional problems, flushing out the growing medium with an appropriately balanced solution will often be the best first step after molybdenum deficiency has been diagnosed. Within a week of a successful intervention, the plant should start looking better, if overly acidic pH was the problem.
  • Adding additional molybdenum. Nutrient solutions designed for cannabis plants should always contain plenty of molybdenum. If you are growing gorilla glue marijuana seeds, or any other strain, consider an engineered growing medium that contains enough of the substance already. Should pH levels seem high enough, though, it can make sense to provide additional molybdenum to plants that are showing signs of deficiency. Sprays applied to the leaves of plants are most often used for this purpose. While toxicity is very rare, adding too much molybdenum to a system should still be avoided.
  • Switching to a different nutrient. Cannabis plants need molybdenum to turn nitrate into nitrite and then ammonia. Nutrient mixes and fertilizers differ with regard to how much of each form of nitrogen they include. Although it is probably not common, molybdenum deficiency can arise, under certain circumstances, when using a fertilizer that includes too much nitrate-based nitrogen or too little of the ammoniacal kind. If nothing else works, it could be possible to resolve molybdenum deficiency by simply switching to a different nutrient or fertilizer product.

Especially robust, thoughtfully bred strains of cannabis can also be more resistant to molybdenum deficiency than others. In some cases, raising the sulfur and phosphorous levels in a growing medium will allow a plant to do a better job of taking in molybdenum, as well.

The Value of Being Aware of a Rare Problem

Molybdenum deficiency is one of the least commonly encountered nutritional problems that can affect cannabis plants, whether they are grown from high THC seeds or high CBD seeds. It should normally be fairly easy to diagnose and address, at least for those who are aware of issues like a potential similarity to nitrogen deficiency.

Many growers will never need to think much about molybdenum deficiency at all. A little bit of research on our i49seedbank grow guide should prepare anyone who does encounter it, and offer relatively easy solutions to resolve it.