N-P-K numbers?

[TheBeast]

Every label carries three conspicuous numbers, usually right above or below the product name. These three numbers form what is called the fertilizer's N-P-K ratio — the proportion of three plant nutrients in order: nitrogen (N), phosphorus (P) and potassium (K).

By Dr. Lynette Morgan
[font='Memphis W01', Times, serif]What do the numbers and letters mean?[/font]
If you (vaguely!) remember the periodic table from high school chemistry, you know that N stands for Nitrogen, P for Phosphorus, and K for potassium. These nutrients are the three numbers on a fertilizer bag listed in order (N-P-K). So, a fertilizer that contains 5-10-10 means it has 5 percent nitrogen, 10 percent phosphorus (phosphate), and 10 percent potassium (potash). A “complete” fertilizer contains all three.



What do the different nutrients do?

Each nutrient plays a different role. Nitrogen boosts green leafy growth. “It’s like the gas pedal,” says John Esslinger, horticulture educator at Penn State Extension. “That’s why many lawn fertilizers are high in nitrogen to promote leafy growth.” Phosphorus helps strong roots form, so plants lacking in phosphorus may be purple-ish and slow-growing. Potassium helps promote vigorous growth and hardiness, so a deficit may result in wimpy fruit or spindly plants that fall prey to pests and diseases.



How do I pick a formula?

Now you’ve got some homework to do. “Before you add anything to your lawn or garden, get a soil test,” says Esslinger. “Otherwise, you have no idea where you’re starting from and what your soil needs or doesn’t need.” In fact, some nutrients, such as phosphorus, are good at staying in the soil; you may not need to add them every time you fertilize. And more is not better. It’s not only a waste of money to add stuff you don’t need—it’s also bad for the plants. For example, too much nitrogen will grow monster tomato plants, but you may not get any fruit. And really, then, what’s the point?

Get your soil test kit from a local garden center or your area’s coop extension service; they’re typically about $10 to $20, and you only need to do one every few years. The master gardener at the county extension service can help you decipher the results and explain how much of each nutrient you need to apply. A balanced fertilizer (with all the same numbers, such as 5-5-5) may be okay for many situations such as flower and vegetable gardens, but only a soil test will tell for sure.
[font=Charter, Georgia, Times, serif]Finally, your test also will include your soil’s pH, a measure of how acidic or alkaline your soil is. It’s another important aspect of soil fertility because if your pH is too high or too low, your plants may not be able to use the nutrients you apply, says Esslinger. Based on your test results, an application of lime to raise pH or sulfur to lower pH may be recommended.[/font]