Meaning of the Numbers on a Fertilizer Bag

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The analysis is actually the three numbers you see on every fertilizer label – put there by law. These numbers represent the percentage (by weight) of the three major nutrients required for healthy plant growth, always in the same order: nitrogen-phosphorus-potassium (N-P-K).

Each of these nutrients affects plant growth differently, and the formulation you select should depend on your specific gardening needs. 

The first number is the percentage of nitrogen in the bag. So a bag of 24-8-4 has 24 percent total nitrogen. Nitrogen provides plants with the ability to produce more chlorophyll, which in turn allows plants to grow quickly. With each additional nitrogen application, plants will grow taller and develop a darker green color. So if you want a dark green lawn, use a lawn fertilizer that’s high in nitrogen – but then expect to mow more often. 

The second number in the analysis is the percentage of phosphorus in the mix. Phosphorous aids in root development and increases flowering ability and bloom size. High-phosphorous fertilizer should be used when plants are being established in your garden – when sowing a new lawn or planting a new tree, for instance. 

The third number represents the percentage of potassium found in the product. Potassium has many functions: It guards the plant against diseases and aids in drought protection and cold tolerance. It also serves a role in improving root development and helps in the process of photosynthesis. You might consider using a high-potassium fertilizer at the start of winter and summer to protect crops from temperature extremes or when insects and disease have caused damage to your plants. 

The sum of the percentages doesn’t equal 100 percent. That’s because there are other nutrients and filler product in fertilizer mixtures. This filler helps to apply the nutrients evenly over an area. 

Learn to recognize a plant’s need for fertilizer. For example, plants that are deficient in nitrogen may start turning light green or yellow. Similarly, purple foliage (on an otherwise green plant) is a telltale sign of phosphorus deficiency. The only true way to determine how much fertilizer a crop needs is to conduct a soil test. Most states offer soil samples through their Cooperative Extension Service (for a small fee). A soil test ensures that the correct amount and type of fertilizer will be used on your plants.