pH & lime: why blueberry and thyme can't share a bed

pH counts the hydrogen ions floating in your soil water. More hydrogen, more acid, lower number. Seven is neutral, below is acid, above is alkaline. The scale runs from 0 to 14, but your garden lives between 4 and 8.

Here's the catch: the scale is logarithmic. Every step down is ten times more acid than the one above. pH 5 isn't a bit more acid than pH 6. It's ten times more. pH 4 is a hundred times.

Where you live decides where you start. The Campine sands north of the Demer sit naturally around pH 4.5 to 5.5: bilberries, heather, pines. The Haspengouw loam to the south sits near neutral and sometimes carries lime. Diepenbeek is on the seam, which is why two gardens on one street can disagree about everything.

Here's the tea: pH 5 to pH 6 isn't a step, it's a ten-fold jump. Treat every half point like a whole song on the setlist.

In 1946 a soil scientist called Emil Truog drew a chart that has hung in every agronomy classroom since: nutrients down the side, pH along the bottom, and for each nutrient a band that gets fat where roots can reach it and thin where they can't. Here it is, redrawn as what it really is: a pantry with a lock.

Below pH 5.5, phosphorus, calcium, magnesium and molybdenum lock into forms roots can't take up. Worse, aluminium, harmless in neutral soil, dissolves and burns root tips. That's why acid sand grows short, stubby roots.

Above pH 7 the trace elements go: iron, manganese, zinc, copper and boron bind to the lime and stay there. The soil still contains them. A lab would find plenty. The plant can't touch them. It's the roadie with the wrong laminate, babe: the gear is in the building, it's just not getting on stage.

The fat part of nearly every band overlaps between 6 and 7. That's why most fruit trees, vegetables and herbs list a range around there. It's not a preference. It's where the pantry door is open.

Here's the tea: the nutrients are usually there. pH decides whether your plant is allowed to touch them.

Lime chlorosis is the classic pH illness, and easy to spot once you've seen it. The young leaves at the tips go yellow while the veins stay green: a yellow leaf with a green skeleton. The old leaves lower down look fine.

That pattern is the diagnosis. Iron can't travel inside a plant, so when uptake stops, the newest growth runs short first. Magnesium is the opposite: it's mobile, so a plant short on magnesium strips its old leaves to feed the new ones, and the yellowing starts at the bottom. Same colour, opposite end of the branch, opposite cause. Lesson 105 has the full line-up.

Spraying iron on the leaves is a cover version: sounds right for a few weeks, doesn't last. Adding iron to a limey soil does nothing at all, it locks up on arrival. The fix is the pH, or the plant.

Here's the tea: young leaves yellow, veins green, on limey soil. That's not a hungry plant. That's a locked pantry.

Blueberry, cranberry, lingonberry, heather: the whole heath family grew up on acid peat at pH 4.5 to 5.5. They hire a special fungus, ericoid mycorrhiza, that only works in acid soil, and they take their nitrogen as ammonium, which acid soil keeps and neutral soil converts away. Plant a blueberry at pH 7 and it doesn't die at once. It sulks, yellows and shrinks for three summers, then quits the band.

Thyme, rosemary, lavender, sage and most of your Mediterranean herbs grew up on limestone hills. They're happy from 6.5 up to 8, and they hate the two things blueberries love: acid and wet. Same bed, one loser, guaranteed.

So read the pH range on the datasheet before the price on the label. It's the rider, honey: what the artist needs before she'll perform. A plant listing 4 to 5.5 on pH 7 loam belongs in a container. A plant listing 6 to 8 doesn't care about this lesson at all.

Here's the tea: plant for the pH you have, not the pH you'd like to have. The soil wins that argument every time.

Clay particles and humus carry a negative charge, so they hold on to positively charged things: calcium, magnesium, potassium, and hydrogen. Soil scientists call that holding capacity CEC, cation exchange capacity. Translation: the size of the soil's pantry shelves.

Sand has almost no shelves. Its pH changes fast in both directions: lime moves it in weeks, rain pulls it back down within a few years. Loam and clay have deep shelves. They resist change, so you need two or three times as much lime for the same shift, but once moved, they stay moved.

That's why a lab wants your texture before it gives you a dose, and why "add a handful of lime" is not advice, sweetheart.

Here's the tea: sand is a mood, loam is a personality. One changes in a week and changes back. The other takes months and means it.

Up is the easy direction: ground limestone, or dolomite if you want magnesium too, spread in autumn or winter and given months to work. A few hundred grams per square metre shifts sand about half a point; loam and clay need two to three times that. Test first: the Belgian Soil Service reports that most Flemish gardens are already over-limed, not under. And don't test within three months of liming, because the reading lies.

Down is the hard direction. Elemental sulphur is the only thing that lasts. Soil bacteria turn it into sulphuric acid, slowly, and only while the soil is warm (above roughly 13 °C) and moist. Count a full season, often longer. Illinois Extension's numbers for dropping from 6.0 to 5.5: about 25 grams per square metre on sand, 35 on loam, 50 on clay. Split bigger jobs over several rounds months apart, so you don't scorch the roots or the biology.

And the trap: soil with free lime in it (chalky loam, old builder's rubble, limestone gravel) cannot be acidified. Every gram of acid you make meets the next gram of lime. A Belgian Soil Service report on a limey garden puts it plainly: growing lime-haters there is high risk. Raised bed or container with an ericaceous mix, watered with rainwater, because Flemish tap water is hard and brings the lime straight back.

Here's the tea: lime is a season. Sulphur is a season plus patience. Free lime is forever. Go up, or go in a pot.

Every plant in the database carries a pH range: ph_min and ph_max. Now you can read it. A narrow low range (4 to 5.5) means acid or nothing. A wide range (5.5 to 7.5) means the plant forgives you. A range that starts at 6.5 wants lime and drainage: your herbs, your brooms, your figs.

One trap for Belgians, flowers: datasheet numbers are pH in water, the international standard. Belgian lab reports give pH-KCl, which reads roughly half a point to a full point lower. If your report says 5.5, you're around 6 to 6.5 on the datasheet scale.

Here's the tea: the datasheet range is the plant's pantry hours. Your lab number tells you when you're arriving.

Real talk: Ingham's masterclass says bacteria push soil alkaline and fungi push it acid. At the scale of a root hair, sure. Biology shifts the pH a fraction, locally, for a while. At the scale of your garden, pH is set by the rock the soil came from, the lime in it, and a few thousand years of rain. No compost tea turns pH 7.8 loam into blueberry ground.

Real talk: Truog's chart is from 1946, drawn for mineral soils, and it's a simplification. Peaty soils behave differently, and real plants are more forgiving than the bands suggest. Use it as a map, not a law.

Real talk: cheap probe meters, the metal spike you push into the ground, are mostly guessing. A dye test kit gets you within half a point. A lab gets you the truth plus your texture, your organic matter and whether there's free lime, which no kit detects. And in Flanders the common mistake is too much lime, not too little.

This lesson can't promise you a blueberry harvest on loam. It can promise you'll stop blaming the plant.