What it is
Cobalt is a trace element, part of vitamin B12 and needed by legumes in very small amounts. It is measured in the hydroponics package because the margin between enough and too much for plants is narrow.
Where it comes from
Cobalt dissolves from natural minerals and is raised locally by mining, metal working and some fertilisers. Levels in UK water are normally very low.
Why it matters
There is no UK drinking-water limit for cobalt, and at natural levels it is not a health concern. For plants, excess cobalt is toxic, so a raised result matters to a grower even though it would not to a householder.
What can be done about it
If a grower needs a lower baseline, reverse osmosis removes cobalt along with other dissolved metals, and the feed is then built up from a known starting point.
Information last reviewed 5 October 2026 by Pozzani Pure Water.