The middle number refers to the concentration of phosphorous. Phosphorous contributes to many fundamental plant processes such as rooting and setting flower buds. A 5-10-5 fertilizer would contain 10% phosphorous by weight or .1 pounds of phosphorous.
Originally posted by: FoBoT
http://gardening.about.com/od/gardenprimer/ht/fertilizerlabel.htm
you want more Phosphorous, the second number