Boron (B)
A trace micronutrient plants need at very low concentrations. Included in most complete trace mixes; rarely tested alone.
What it is
Boron is a plant micronutrient involved in cell-wall structure and the transport of sugars within the plant. Required in extremely small amounts — typical aquarium target is 0.05–0.5 mg/L, against an upper toxicity threshold around 2 mg/L.
Why it matters
Boron deficiency is one of the rarer planted-tank problems, mostly seen in tanks running heavy RO water with an incomplete remineraliser. Symptoms are subtle: stunted apical growth on stem plants, with curled or distorted new leaves. Most diagnoses come from elimination — when Fe, Mg, N, P, K, and the more common deficiencies are ruled out, B is the usual suspect.
How to test
Boron-specific aquarium kits are uncommon. Most planted-tank keepers trust a complete trace mix to maintain B alongside the rest.
What high and low look like
Below 0.05 mg/L sustained, in a tank with otherwise complete fertilisation: distorted new leaves, "claw-shaped" shoots on stem plants. Above 2 mg/L: leaf-margin necrosis (browning edges); this is the kind of damage that follows accidental over-dosing.
How to fix
A complete micro fertiliser includes boron in the correct trace proportion. If you suspect deficiency, increasing the dose of your current trace mix by 25% for two weeks is usually enough to see new growth normalise.
