Yes, your tomatoes love it. Reaching for sulfate of potash for tomatoes gives them a clean, chloride-free dose of the potassium they crave once the fruit swells. You may have seen lists that leave tomatoes out. Those lists sort plants by chloride sensitivity, and the tomato is not strictly chloride-sensitive. So they assume the source you pick does not matter. It does. Your tomato is one of the heaviest potassium users in the whole garden. It feeds on the nutrient itself, no matter how it reaches the roots. That is why a good potash feed pays you back at harvest.
I picked Sungold tomatoes warm off the vine and ate half of them before they ever hit the bowl. That bed sat against the south-facing fence. The year I switched to a chloride-free potash feed, those plants cropped sweeter and firmer than I had ever managed there. The summer before, the same bed threw watery fruit that split in the heat. I had used a flat, harsher feed that year, and the taste came out thin. I changed only the potash between the two seasons. The trusses ripened evenly right down the vine. You could see the difference before you tasted it.
Here is the technical part. Potassium is the nutrient your tomato burns through fastest as it fruits. It uses far less while it is still growing leaf. As a tomato potassium feed, sulfate of potash drops potassium into the soil with sulfur on it, not chloride. Your plant grabs the potassium it needs. It pays no mind to the source debate at all. That is the simple reason some lists skip the tomato. They sort plants by chloride, not by how much potassium the plant truly wants. So the tomato slips off a chloride list while still being a hungry potassium feeder underneath.
Potassium does real work inside your fruit. It moves sugars from the leaves into the tomato. That is why flavor and sweetness climb with a steady supply. It also firms up the cell walls, so your fruit holds together and resists splitting on the vine. Ripening runs more even across the truss too. You get fewer green shoulders and fewer hard cores in the middle. The low salt index helps you in summer as well. In a hot, dry spell, a harsh feed piles salt stress onto the roots. Sulfate of potash carries less salt, so your plant keeps drinking and feeding instead of stalling in the heat.
Timing is simple. Start to feed as the first fruit sets. Keep feeding right through cropping, since that is when demand spikes hardest. Use it as your main fruiting fertilizer once the flowers turn into pea-sized fruit. Keep your rates modest. Feed little and often instead of one big hit. A heavy slug of potassium can crowd out magnesium in the root zone. Then your plant struggles to pull magnesium from the soil. You will see it first as yellowing between the veins on the older, lower leaves.
So pair your potash with a magnesium source to hold that balance steady. A light watering of Epsom salts every two weeks covers most plants. You can also pick a feed that already carries magnesium in the mix. Watch your older leaves as your gauge through the season. If they stay green and your fruit ripens firm and sweet, your ratio is right. Feed at the roots and water it in well each time. Then let your plant sort out the rest through the rest of the summer.
Read the full article: Sulfate of Potash: A Complete Guide
