The honest answer is that plants disliking sulfate of potash are rarely a fixed species list. The real culprit is the soil they sit in. Any plant already growing in potassium-rich soil gains nothing from more potash, and some will suffer for it. Sulfate of potash is a useful feed, but only when your ground actually runs short on potassium.
So before you reach for the bag, look at the bed, not the plant tag. A tomato wants potash while it sets fruit. But a tomato in rich soil does not need a top-up at all. The plant did not change. The soil did the work for you. This is why two gardeners can feed the same crop and get opposite results from one bag of potash.
Here is the technical reason extra potash can backfire. Potassium, magnesium, and calcium all compete for the same uptake pathways in the root. Pile on too much potassium and you crowd out the other two. D.T. Brown flags this caveat, and extension labs report the same pattern. The result is nutrient lockout, where the magnesium and calcium sit in the soil but the plant cannot pull them in.
You see this lockout on the leaves first. Magnesium shortage yellows the older leaves between the veins, while the veins themselves stay green. Calcium shortage shows up as stunted new growth and weak, distorted tips. Both can trace back to one heavy-handed potash feed on ground that never needed it. The plant looks hungry, but it is actually swamped. That is the trap. You read the yellow leaves as a feeding cue, add more, and make the lockout worse.
Some crops also dislike the form, not just the dose. Sulfate of potash is the low-chloride choice for a reason. Chloride-sensitive plants like potatoes, berries, and citrus do better with it than with muriate of potash. So these are good candidates when a feed is warranted. But on a bed that is already rich, even the gentle sulfate form adds salt and pressure with no payoff.
- Fruiting crops on potassium-low soil confirmed by a test.
- Chloride-sensitive plants like potatoes, berries, and citrus.
- Heavy potassium users such as tomatoes during cropping.
- Anything in soil a test shows is already potassium-rich.
- Beds recently fed with a balanced or high-potassium feed.
- Plants showing magnesium or calcium shortage, where more potassium worsens lockout.
Watch out for beds you fed not long ago. If you worked in a balanced feed or a high-potassium tomato feed this season, the potassium is still there. A second dose of sulfate of potash on top of that is the classic way gardeners tip healthy soil into excess. More is not better here. The same goes for ground near a heavy compost border or an old manure pile. Potash can build up there on its own over the years. You never had to add a thing.
So what should you do instead of feeding on autopilot? Soil test first. A basic test costs little and tells you whether potassium is short or already fine. If the number sits in the adequate range, skip the potash and put your money into the nutrient that actually reads low. Routine feeding without a test is how good soil goes wrong. A test also shows your magnesium and calcium levels, so you can spot the start of lockout before your plants ever wave a yellow flag.
The takeaway is short. Healthy, well-fed ground almost never needs more potassium, and adding it risks more harm than good. Test, then decide. Feed the plants that genuinely run short, and leave the rest alone. Your beds will thank you for the patience.
Read the full article: Sulfate of Potash: A Complete Guide
