Material you add to a garden to increase the nutrients in the soil and help plants grow is called fertilizer. If your rosebushes are looking a little sad, you might want to try using fertilizer.
To fertilize is to make something fertile, or to encourage it to grow or reproduce. These words, along with fertilizer, come from the Latin fertilis, "bearing in abundance, fruitful, or productive." The most common kind of organic fertilizer is animal manure or peat, and other fertilizers are made of specific nutrients like nitrogen or potassium. Fertilizers are usually solid, though they come in liquid form as well.