Potash top dressing for tomatoes. Why do tomatoes need potassium? A large amount of potassium is needed, first of all, for young leaves, buds and flowers. The emerging ovaries also need potassium, like the fruits. Potassium fertilizers for tomatoes can be prepared with your own hands, from folk it is ash. But if you do not have ash, then you can use other drugs wisely. Potassium nitrate is a very good fertilizer when used correctly. Potassium supplementation is very helpful in preventing top rot on tomatoes. Potash fertilizers are the key to a good harvest. Happy viewing.