Tomato fertilizers are ideal for use on potatoes as those sold to encourage flowering and fruiting when growing tomatoes are higher in potassium and phosphorus than nitrogen. A typical ratio of NPK for tomato fertilizers has the second two numbers higher than the first. Anne Swithinbank, expert for Amateur Gardening, claims potatoes ‘respond well’ to such high potassium fertilizers and that will ‘encourage higher yields’ from your plants. An example of such a tomato feed is the Jack's Classic Tomato Feed available on Amazon.