What Do the Numbers on Fertilizer (NPK) Mean?

Every fertilizer label carries three numbers in order: nitrogen (N), phosphorus (P), and potassium (K). Nitrogen drives the green, leafy top growth. Phosphorus supports roots and is important for establishing new lawns. Potassium helps the lawn handle stress, drought, and cold.

The numbers are percentages by weight. A 17-0-4 blend is 17% nitrogen, 0% phosphorus, and 4% potassium; the rest of the bag is carrier and other nutrients. So a higher first number means more nitrogen for color and growth, which is why a summer feeding blend leads with nitrogen.

If you have noticed that Wisconsin lawn fertilizers usually show a 0 in the middle, that is by design. State law restricts applying phosphorus to established turf, with exceptions for new lawns or where a soil test shows a deficiency. A 0 in the phosphorus slot means the blend is made to be used legally on an established lawn here.

The right ratio depends on the season and the goal: more nitrogen for growth and color in the active season, more potassium heading into winter for hardiness. You do not need to memorize this — but understanding it helps you see why a program uses different blends at different times.

Our lineup shows this in practice: 17-0-4 for summer color and feeding, 10-0-10 with Iron for fall hardiness and color, and 11-11-11 as a balanced all-purpose option for situations where phosphorus is permitted, such as gardens.