Cobalt is not an essential element for higher plants. It is required for vitamin B12 and by the bacteria that fix nitrogen in legume root nodules, which is why it is sometimes listed as "beneficial". Cannabis does not form those nodules and has no known requirement for cobalt.
In practice
Some fertiliser blends and micronutrient premixes contain trace cobalt, mostly as a carry-over from formulas designed for legumes and other crops. Growers should neither add it nor worry about its absence. At higher concentrations cobalt is toxic and can interfere with iron uptake, causing chlorosis on new growth. If a label lists cobalt, the amount should be tiny, well below 0.1 mg/L (ppm) in the final solutionbeing reviewed.
Don't confuse with
- Copper (Cu): an essential micronutrient; easy to confuse by symbol.
- Molybdenum: essential for nitrate reduction; also needed by nitrogen-fixing bacteria.
- Nickel: essential in trace amounts for urease.
A micronutrient is an essential element plants need only in trace amounts, below a few mg/L: iron, manganese, zinc, copper, boron, molybdenum, chlorine and nickel.Yellowing of normally green plant tissue caused by too little chlorophyll, from nutrient shortage, poor root uptake, pH lockout, light damage, pests or normal ageing.Copper (Cu) is a micronutrient used in photosynthetic electron transport and lignin formation; plants need only traces, and excess is toxic to roots.Molybdenum (Mo) is the micronutrient needed in the smallest amount; plants need it to convert nitrate into usable nitrogen.Nitrate (NO3-) is the main form of nitrogen in hydroponic and soilless feeds; its uptake tends to push root-zone pH upward.Nickel (Ni) is a micronutrient required for urease, the enzyme that breaks down urea; plants need only traces, which most feeds and waters already contain.A beneficial element is one that improves growth or stress tolerance in some plants without being strictly essential, such as silicon, sodium, cobalt and selenium.
