Molybdenum is a micronutrient absorbed as the molybdate anion (MoO4 2-). It is the metal centre of nitrate reductase, the enzyme that turns nitrate into nitrite on the way to amino acids. Without it, a nitrate-fed plant can starve for nitrogen while nitrate builds up in its tissue. Unlike most micronutrients, molybdenum becomes more available as pH rises.
In practice
Deficiency is uncommon. It shows as pale, yellowing middle and older leaves with cupped or scorched margins, resembling nitrogen deficiency. Low pH makes it more likely. Hydro feeds carry tiny amounts, often around 0.03-0.05 mg/L (ppm) Mobeing reviewed, usually from sodium molybdate or ammonium molybdate.
Don't confuse with
- Nitrogen deficiency: very similar look; Mo deficiency can occur with plenty of nitrate in the feed.
- Manganese: different element; the symbols Mn and Mo are easy to mix up.
- Magnesium (Mg): also easy to confuse by symbol.
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.Nitrate (NO3-) is the main form of nitrogen in hydroponic and soilless feeds; its uptake tends to push root-zone pH upward.pH measures how acidic or alkaline a solution is on a 0–14 scale; in growing it controls nutrient uptake, with soil best near 6.0–7.0 and hydro 5.5–6.5.A nutrient deficiency is a shortage of an essential element in the plant tissue, causing characteristic symptoms; the cause is often pH or uptake rather than an empty feed.Nitrogen (N) is the macronutrient plants use most, built into proteins, chlorophyll and nucleic acids; it drives leaf and stem growth and is fully mobile.Sodium molybdate dihydrate (Na2MoO4·2H2O), about 39.7% molybdenum, is the common Mo source for micronutrient mixes, used in microgram-per-litre amounts.Ammonium (NH4+) is the positively charged form of nitrogen; a small share stabilises root-zone pH, but too much reduces cannabis yield and quality.Manganese (Mn) is a micronutrient that splits water in photosynthesis and activates many enzymes; low pH can raise it to toxic levels.A mobile nutrient is one the plant can move from older leaves to new growth, so its deficiency shows first on lower, older leaves: N, P, K, Mg, Cl, Mo.
