What’s in the Bag?

The hyphenated numbers on the front label of any package of fertilizer stand for the major nutrients plants need — nitrogen (N), phosphorous (P), and potassium (K). The numbers, always given in the same order, represent the percentages each of N, P, and K in the bag (by weight). Other nutrients are often found in fertilizer, but not in as large amounts as N, P, and K. The percentages of other nutrients are listed under the Guaranteed Analysis.

The Three Main Nutrients (N, P, and K)

Nitrogen (N) is the main nutrient for new, green growth. Plants that are almost all leaf (such as lawn grasses) need plenty of nitrogen, so the first number is especially high in fertilizers for lawns because grass must continuously renew itself after mowing. The higher the number, the more nitrogen the fertilizer provides.

Phosphorous (P) promotes root development which helps strengthen plants.It also increases blooms on flowers. Lots of phosphorous is great for bulbs, perennials, and newly planted trees and shrubs. They depend on strong roots, so fertilizers meant for these plants often have high middle numbers.

Potassium (K) improves the overall health of plants. It helps them withstand very hot or cold weather and defends against diseases. Most soils already have some potassium, so the third number in the fertilizer analysis is usually smaller than the other two. Fertilizers meant for fall, such as Winterizer, also contain extra potassium to help prepare plants for cold weather.

Other Important Nutrients

Calcium (Ca) improves general plant vigor and promotes growth of young roots and shoots.

Magnesium (Mg) helps regulate uptake of other plant foods and aids in seed formation. It is important in development of the dark green color of plants and to assist the plant in the manufacture of food from sunlight.

Sulfur (S) helps maintain a dark green color while encouraging more vigorous plant growth.

Minor, or trace elements, are elements used in small amounts but which are still essential to plant health. They help insure dark green color, vigorous development, and healthy growth. Iron (Fe) is the minor element most often lacking from poor soils; it helps plants maintain a dark green color.

Look carefully at a label to shop wisely for fertilizers. The numbers on the bag tell only one part of the story. The most important ingredient, nitrogen, comes in many different forms. That is what distinguishes commodity fertilizers and some slow-release fertilizers from the latest technology such as timed-release nitrogen.