Some springs you tuck twenty tomato seeds into a starter tray, wait ten days, and fewer than half come up. It's tempting to blame the packet — except seeds from that very same packet may poke through in the pot beside it in three days flat. If you kept the temperature and the watering identical and the results still split, the one condition left is the pH of your soil.
When pH Swings, Seeds Stall
A team of plant physiologists set tomato seeds across a range of conditions from pH 3.5 to 10.5 and watched how they germinated. The further the pH drifted — acidic side or alkaline side — the further germination rates fell. Even the seeds that did sprout put out shorter roots and a shorter hypocotyl, the stem below the seed leaves, and the seedlings weighed less overall. Out at pH 3.5 and 9.5, the share of seeds that stayed viable at all dropped.
Tomatoes are happiest between pH 6.0 and 6.8. A garden bed worked for years rarely strays far outside that band, but seed-starting mix and containers are another story. A mix heavy on peat moss tilts acidic, while generously limed soil — or a pot you keep watering with strongly alkaline well water — drifts the other way. If germination has flopped several seasons running, suspect the bag of potting mix before you suspect the seeds.
What's Happening Inside While the Roots Fall Short
When pH tips off balance, reactive oxygen species surge inside the seed. These molecules do double duty: in small amounts they act as a signal that germination is underway, but in excess they oxidize the fats in cell membranes and kill cells outright. In the study, the balance among the seed's antioxidant enzymes broke down, and the cells lost some of their grip on water at the same time.
Down in the roots, auxin levels dropped. Auxin is the plant hormone that drives roots and stems to elongate. Seedlings that had been through acidic or alkaline conditions showed reduced expression of both the genes that produce auxin and the genes that loosen and repair cell walls. If the cell walls can't stretch, the roots can't lengthen. Fewer root cells were alive, too.
The Soak That Brought Seeds Back — and What to Do Today
The researchers tested a way to recover as well. Before exposing seeds to alkaline conditions, they pre-soaked them in a solution of indole-3-acetic acid, a form of auxin. Germination and early growth came back in large part. That said, the result came out of laboratory conditions, and there's no settled answer yet on what concentration to use in a home garden or how long to soak. It's a promising avenue for rehabilitating salty, alkaline ground, and one worth testing further.
What you can do right now is bring the pH back where it belongs.
- Before you sow, test both your seed-starting mix and your garden soil once, with test strips or an inexpensive meter. Even fresh mix varies from bag to bag.
- If it leans acidic, work in dolomitic lime a little at a time, wait about two weeks, then test again. Dump it all in at once and you'll overshoot the other way.
- If it's alkaline, mix in peat moss or well-rotted leaf compost, and water with collected rainwater when you can.
- When you presoak seeds before sowing, use plain lukewarm water — skip anything with plant food or fertilizer mixed in.
If your tomatoes came up thin this spring, stir a handful of your seed-starting mix into water and check the pH before the next sowing. Just landing somewhere around 6.5 will change what you're looking at ten days later.
Source — a 2026 study in the journal Plants on pH stress in tomatoes and its effects on germination and seedling growth
