If I find Sphagnum, is it definitely a bog?

No. Sphagnum also grows in poor fens and on fen margins, so the moss alone proves nothing. Confirm with the water source and a pool reading of pH 3.0-4.5 before calling it a bog.

Can one pH strip reading settle bog versus fen?

Not by itself. A puddle filled by fresh rain reads acid even inside a fen, so test an established pool or a hole dug to the water table. Treat the strip as one of six checks, not a verdict.

Can a fen turn into a bog?

Yes, over centuries. As peat builds at 0.5-1 mm a year, the surface can rise beyond the reach of groundwater; from then on rain alone feeds it, and the fen has become a bog.

How old is the peat under a bog?

Divide the depth by the growth rate. At 0.5-1 mm a year, one metre means roughly 1,000-2,000 years and three metres means 3,000-6,000 — probe depth is a clock.

3%of the world's land surface is peatland — roughly 4 million km²

500-700 Gtof carbon stored in peat, about twice all living forests

0.5-1 mmof new peat added in a typical year

pH 3.0-4.5Sphagnum bog pools

Bog versus fen, answered in numbers

La Boganii — Bogs, fens and peatlands: how each | la-boganii.com

A bog is fed only by rain, so its water stays acid — Sphagnum pools read pH 3.0-4.5 — and its nutrients stay scarce. A fen is fed by groundwater as well, so its water sits near pH 6.0-7.5 and sedges replace the moss carpet. Everything else in this guide follows from that one difference.

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An independent field notebook on bogs, fens and the peat beneath them.

Numeric index

The peatland numbers

pH 3.0-4.5Sphagnum bog poolsRain-only feeding, with no mineral groundwater to buffer it, keeps bog water this acid.

10-40 cmWater table in a living raised bogOnce it falls below roughly 50 cm, the peat dries, shrinks and starts oxidising.

15-20×Water held by Sphagnum, by dry weightThat sponge effect is what carries a bog through a dry spell.

Bog, fen, swamp and marsh, side by side

Four wetland types sorted by the variables that decide the bog-versus-fen question in the field.
Wetland type	Main water source	Typical pH	Nutrient level	Typical plants
Bog	Rainfall only	3.0-4.5	Very low	Sphagnum moss, heather, cotton-grass
Blanket bog	Rainfall only; needs about 1,000 mm a year	3.0-4.5	Very low	Sphagnum and cotton-grass over whole hillsides
Poor fen	Rain plus some groundwater seepage	Between 4.5 and 6.0	Low	Sphagnum mixed with sedges
Rich fen	Groundwater plus rain	6.0-7.5	Moderate to high	Sedges, brown mosses, reeds
Swamp	Surface water and groundwater	Near neutral	High	Trees and shrubs on wet soil or shallow peat
Marsh	Surface flooding	Near neutral	High	Grasses, sedges and rushes; little or no peat

Four wetland types sorted by the variables that decide the bog-versus-fen question in the field.

The short answer: follow the water

A bog is fed only by rain; a fen is fed by groundwater as well — and that single difference sets the acidity, the plants and the peat.

Hydrologists call a rain-only wetland ombrotrophic: every drop it holds fell from the sky, carrying almost no dissolved minerals. A fen is minerotrophic: its water arrives after moving through soil and rock, picking up calcium and other bases on the way.

Because nothing buffers a bog's water, Sphagnum bog pools usually measure pH 3.0-4.5. Rich fen water, fed by mineral groundwater, sits near pH 6.0-7.5 — a gap of about three pH units, roughly a hundred- to thousand-fold difference in acidity.

Nutrients follow the same divide. Groundwater delivers bases and minerals, so fens support a richer flora; bogs run on rain and airborne dust alone, and only highly specialised plants cope.

Terms worth keeping

Peat: Partly decayed plant matter that builds up where waterlogging stops full decomposition. It accumulates at roughly 0.5-1 mm a year.
Bog: A peat wetland fed only by rainfall — ombrotrophic — acid at pH 3.0-4.5 and carpeted in Sphagnum.
Fen: A peat wetland fed by groundwater as well as rain — minerotrophic. Rich fens measure pH 6.0-7.5 and grow sedges and brown mosses.
Ombrotrophic: Literally 'rain-fed': a wetland whose only supply of water and nutrients is precipitation.
Minerotrophic: Literally 'mineral-fed': a wetland that also receives groundwater carrying dissolved minerals from soil and rock.
Sphagnum: The bog-building moss. It holds 15-20 times its dry weight in water and keeps the surface wet and acid.

What visitors push back on

If I find Sphagnum, is it definitely a bog?: No. Sphagnum also grows in poor fens and on fen margins, so the moss alone proves nothing. Confirm with the water source and a pool reading of pH 3.0-4.5 before calling it a bog.
Can one pH strip reading settle bog versus fen?: Not by itself. A puddle filled by fresh rain reads acid even inside a fen, so test an established pool or a hole dug to the water table. Treat the strip as one of six checks, not a verdict.
Can a fen turn into a bog?: Yes, over centuries. As peat builds at 0.5-1 mm a year, the surface can rise beyond the reach of groundwater; from then on rain alone feeds it, and the fen has become a bog.
How old is the peat under a bog?: Divide the depth by the growth rate. At 0.5-1 mm a year, one metre means roughly 1,000-2,000 years and three metres means 3,000-6,000 — probe depth is a clock.

How this guide is assembled

Every figure here is a widely published peatland range — accumulation at 0.5-1 mm a year, bog pools at pH 3.0-4.5, rich fen water at pH 6.0-7.5 — cross-checked against Ramsar Convention reporting. The field procedure is ordered so that each step rules out one common misidentification before the next begins.

Where the numbers come from

Numbers follow ranges commonly published in peatland science and in Ramsar Convention materials; no single study is privileged over the consensus range.