The designation "13-13-13" on a bag of fertilizer refers to the number of pounds of each of the three primary nutrients contained in the bag. Nitrogen, phosphorous and potassium are each included in fertilizer in varying degrees, depending on the fertilizer's intended use.
A bag labeled "13-13-13" is a quick-release, general-purpose fertilizer that contains equal amounts of the three nutrients that all plants require in order to grow properly. Nitrogen aids in chlorophyll formation, while phosphorous stimulates the growth and development of the plant's root system. Potassium gives the plants the nutrients that they need to survive drought and other issues that affect growth.