Know The Fertlizer Numbers – Center for Urban Agriculture

Source(s): Clint Waltz, Extension Turfgrass Specialist, The University of Georgia

Most folks have heard of 10-10-10, but they may not know what the numbers stand for. So what are the numbers?

All plants require nutrients to grow and develop properly. Various nutrients affect plants differently and are needed in varying amounts.

Essential nutrients are just that: essential. Plants have to have them to sustain the vigor they need to resist environmental stresses, weeds, diseases, insects and other pests.

The 16 essential nutrients are broken into two categories, primary (nitrogen, phosphorus and potassium) and secondary (calcium, magnesium, sulfur, iron and others).

The primary nutrients get the most attention because they’re required in the greatest amounts. Fertilizers are sold based on their amounts.

The numbers

On a fertilizer bag, the numbers refer to the percentage of actual nitrogen (N), phosphate (P2O5) and potash (K2O). So 10 percent of the weight of a bag of 10-10-10 is nitrogen, 10 percent is phosphate and 10 percent is potash.

It’s easy to figure the actual weight of nitrogen. The percentage is listed on the bag. In a 50-pound bag of 10-10-10, the nitrogen would weigh 5 pounds (0.10 times 50).

It’s not as simple, though, for phosphorus (P) and potassium (K). To find the amount of phosphorus, multiply the phosphate number by 0.44. Get the potassium amount by multiplying the potash weight by 0.83.

How it adds up

So that 50-pound bag of 10-10-10 has 5 pounds of phosphate times 0.44, or 2.2 pounds of phosphorus. It has 5 pounds of potash times 0.83, or 4.15 pounds of potassium. With the 5 pounds of nitrogen, then, it has 11.35 pounds of primary nutrients.

Using the same formula, a 50-pound bag of 16-4-8 would have 8 pounds of nitrogen, 0.88 pounds of phosphorus and 3.32 pounds of potassium. That’s 12.2 pounds of primary nutrients.

So what’s the rest of the weight in the bag? Some of it may be secondary nutrients. The rest is filler material to make it easier to apply.

A fertilizer bag containing all three nutrients is considered a “complete” fertilizer. A product with any nutrient missing is called “incomplete.”

Center Publication Number: 146