Fertilizer Numbers: What They Mean and How to Use Them to Grow Better Adding a fertilizer high in nitrogen (such as 6-2-1 or 10-5-5) to a fruiting or flowering plant, like a tomato or a petunia, will result in excessive green growth often at the expense of flower and fruit production. But adding it to a green, leafy vegetable plant, such as spinach or lettuce, makes much more sense.