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March 29, 2024

What can you expect to find on all bags of fertilizer?

  • Brand Name
  • Manufacturer Name & Address
  • Guaranteed analysis of package contents
  • Net Weight (Dry Fertilizer)
  • Potential acidity of the product

While you will likely recognize a Brand Name and the manufacturer may even be familiar to you, it is the guaranteed analysis of the contents where most of us get confused. It is common for a manufacturer to include more of a nutrient than shown on the label, but they cannot deliver less than what the label shows. Those numbers on the bag represent the percentage by weight of each nutrient listed inside the bag.

The nutrients listed may describe what is referred to as a complete or incomplete fertilizer. A complete fertilizer will contain N-nitrogen (greens up the grass) P-phosphorus for root development, and K-potassium for overall plant health and vigor. If any one of the nutrients listed above are not contained in the bag, it cannot be called a complete fertilizer. The image below shows a bag with no Potassium and thus would make this bag a “non-complete” fertilizer.

A “complete” fertilizer is not always required for effective fertilization. All three nutrients should be present when seeding however.

The pictures above show “Complete” fertilizers.

Secondary and Micronutrients

Secondary nutrients also foster healthy growth in plants, though plants need smaller amounts of these than nitrogen, phosphorus and potassium. Calcium, magnesium and sulfur are all secondary nutrients. Micronutrients, which plants need in very small amounts, include chlorine, zinc, manganese, boron, iron, copper and molybdenum. Fertilizer labels often list which secondary nutrients and micronutrients they contain, and soil tests often reveal whether a soil needs more secondary or micronutrients.

Choosing a Fertilizer

N-P-K ratios are useful because soil test results often give fertilizer recommendations in ratios. A fertilizer should have the same proportions as a soil test recommendation for nutrients, but it does not need to have exactly the same nutrient concentrations. For example, a gardener could use a 2-1-1 ratio fertilizer for a fertilizer recommendation of a 2-1-1 N-P-K ratio, and a 10-5-5 or 20-10-10 ratio fertilizer would also work. As long as the relative proportions are the same, any fertilizer with the correct ratio will work fine. Higher numbers mean higher nutrient concentrations. For example, a gardener will have to use less of a 20-20-20 ratio fertilizer than a 10-10-10 one to get the same results. fertilizer test results also give recommendations for secondary nutrients and micronutrients, which should be listed on fertilizer labels.

Label Terminology

Many fertilizer labels use certain standardized descriptive terms. You'll need to understand these terms when reading fertilizer labels or information about plant fertilizer preferences. A reference to a "complete fertilizer " means that it contains at least some of each primary nutrient. A "balanced fertilizer " has equal amounts of nitrogen, phosphorus and potassium. For example, 1-1-1 and 15-15-15 fertilizers are balanced ratios. Some fertilizer labels say "phosphate" rather than phosphorus. Other fertilizer labels also say "potash" rather than potassium. "Slow-release fertilizers" release nutrients into the soil slowly over time. They have some advantages over quick-release fertilizers. For example, they are less likely to cause polluted runoff, and they also require less frequent applications.

Label Application Rates

fertilizer labels often suggest application rates for specific types of plants. Labels may recommend a specific volume of fertilizer for a given amount of garden space. For example, a label might suggest a cup of fertilizer for every 15 square feet of rose garden space.

Application Rates Based on Testing

If soil tests recommend a specific amount of nutrients by weight for a given area, it is possible to calculate how much fertilizer to apply by looking at the N-P-K ratio. For example, a soil test might reveal that the gardener should add 10 pounds of nitrogen, 5 pounds of phosphorus and 5 pounds of potassium to 1/2 of an acre of garden space. First, the gardener should choose a 10-5-5, 2-1-1, 20-10-10 or similar ratio fertilizer. Second, the gardener can determine how much of the fertilizer to use by looking at the percentage by weight of the nutrients in the fertilizer. Because a 10-5-5 ratio fertilizer is 10 percent nitrogen, 5 percent phosphorus and 5 percent potassium, the gardener would need to use 100 pounds of the fertilizer to add 10 pounds of nitrogen and 5 pounds each of phosphorus and potassium. Since a 20-10-10 ratio has twice the concentration of nutrients, the gardener would need to use just 5 pounds.

Although nitrogen supports vegetative growth in a garden, phosphorus and potassium are important for strong rooting, flowering and fruit development. They're particularly vital in a fruit, vegetable or flower garden. But before you rush to douse your garden in these nutrients, you'll need to make sure that your fertilizer has a balanced amount of the nutrients your plants require.

NPK Fertilizers

In NPK fertilizers, the P indicates phosphorus, while the K represents potassium. For instance, a 10-10-10 fertilizer has 10 percent nitrogen, 10 percent phosphorus and 10 percent potassium. If you're looking for a fertilizer with high phosphorus and potassium content, the second and third numbers should be larger than the first, such as they are in a 10-20-20 fertilizer.

Benefits of Phosphorus

Phosphorus is the nutrient that aids root development, flowering and fruit production. This makes it especially important for flowers, fruit trees and vegetables. It's also the most important ingredient in bloom booster formulas, which encourage flowering plants to produce blossoms. An example is a 10-20-10 fertilizer.

Potassium's Role

Most balanced fertilizers have a fair amount of potassium, which is responsible for a plant's overall metabolism. It also has a number of supporting roles, such as fueling root growth and improving the quality and size of some plants' nuts and fruits. Because it is necessary for other nutrients to succeed, it's a key ingredient in most fertilizers, particularly balanced blends.

Potential Problems

Overdosing a garden with phosphorus and potassium can be just as detrimental as not fertilizing. For instance, in a vegetable garden, applying phosphorus when it's not necessary can make the vegetables less capable of absorbing micronutrients such as iron. Meanwhile, too much potassium can cause salt buildup in the soil, which burns roots.

Fertilizers are legally defined as soil amendments with guaranteed minimum nutrient percentages. Fertilizers come with ratios of nitrogen, potassium and phosphorus. For example, a 10-5-5 ratio fertilizer is 10 percent nitrogen, 5 percent potassium and 5 percent phosphorus by weight. Different plants require different nutrient amounts, and gardeners can calculate fertilizer application rates based on the plant requirements, garden square footage and fertilizer nutrient concentration. Nitrogen is the most important nutrient for lawns and many other leafy plants, so gardeners often calculate fertilizer rates based on nitrogen contents. Rates can also be calculated for bloom- and fruit-producing nutrients like potassium and phosphorus.

1. Calculate the size, in square feet, of the area to be fertilized. For a square or rectangular planting space, do this by multiplying the length of the rectangle, in feet, by its width.

2. Divide the recommended amount of a given nutrient by the percentage of the nutrient in your fertilizer. For example, many lawns need a minimum of two pounds of nitrogen per 1,000 square feet. You would need to divide two pounds by the percentage of nitrogen in your fertilizer to get the amount of fertilizer required for a 1,000-square-foot space. If the fertilizer contains 10 percent nitrogen, you would divide two pounds by .10 and discover that you need 20 pounds for a 1,000-square-foot space: 2 lbs/.10 = 20 lbs per 1,000 square feet.

3. Calculate the amount of fertilizer needed for the precise amount of space you have by dividing the size of your garden by the size given in the fertilizer recommendation. Then, multiply this number by the application rate of the fertilizer. For example, you would determine the amount of fertilizer to use for a 500-square-foot space and an application rate of 10 pounds per 1,000 square feet by dividing 500 by 1,000 and multiplying by 10: 10(500/1,000) = 5 lbs.

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