Everyday Math

Math and the Outdoors

What numbers are important when people fertilize their plants?

Not only do you have to pay attention to the zone where you plant, but how and when to use the best and complete fertilizers. In many cases, your average soil will not have enough nutrients to support certain plants, especially if you decide to cultivate a vegetable garden.

All plants need oxygen, carbon, and hydrogen, which are collected by the plant from the air, sunlight, and water, respectively. But they also need a range of other chemical elements, such as nitrogen (N), phosphorus (P), and potassium (K). These are usually present in what are called the complete fertilizers (which means it contains all three elements). Most general fertilizers—liquid or dry—that you find in your local nursery, farm-based, or even hardware store have these three elements in varying amounts.

For example, if a bag of fertilizer reads “10-5-6” that means the ratio of these three elements is 10:5:6, or that the fertilizer contains mostly nitrogen and lesser (and almost equal) amounts of phosphorus and potassium. This does not mean the bag contains all three of these elements only; it means that the entire bag of fertilizer contains 10 percent total nitrogen, 5 percent phosphate, and 6 percent potassium (often called potash). If a bag reads 5-5-5, then all three elements are found in equal parts within the fertilizer.

When buying such fertilizer for your vegetable (or even flower or herb) gardens, you should pick the best fertilizer based on what your plant needs, such as nitrogen for better leaf growth, or phosphate for activating root growth. In most cases, the fertilizer is either added to the soil before you plant in the spring, or added as a side-dressing as the plant grows. Each bag usually tells you how much you should add to the garden per square foot. Liquid fertilizers can also be used as something called foliar feed, which is sprayed on the plants’ foliage and absorbed quickly by the leaves. In the majority of cases, nitrogen is needed as a foliar feed, thus the container will have the first number (representing nitrogen, or N) as the largest number.


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