There are always three numbers listed on every package of commercially sold fertilizer. These numbers indicate the amount of nitrogen, phosphorous and potassium in the fertilizer. Understanding these numbers is important because using the wrong fertilizer, or too much or too little of a nutrient can affect crop yields and harm plants.
Complete fertilizers contain nitrogen, phosphorous and potassium, but not necessarily in equal amounts. This type of fertilizer is often formulated for specific types of plants, or as an all-purpose fertilizer. A complete fertilizer may list the ingredients as 10-10-10, 5-3-3 or any other combination of numbers. These numbers represent the percentage of nitrogen, phosphorous and potassium in the fertilizer and are always listed in that order. There will always be a value for each ingredient. Some fertilizers may have a considerably higher concentration of one nutrient than the others. Roses flourish with a steady diet of 7-8-5 fertilizer. Tomatoes, peppers and eggplants on the other hand, can be harmed by too much nitrogen and will do better with a lower concentration.
A balanced fertilizer is actually a complete fertilizer, and sometimes is referred to as a balanced, complete fertilizer. This type of fertilizer always contains equal amounts of nitrogen, phosphorous and potassium. They may be 10-10-10, 5-5-5 or some other value. Some products are very low in nutrients while others have higher, yet equal concentrations of the main nutrients. Balanced, complete fertilizers are often incorporated into soil before planting a vegetable or flower garden. They are also used on a variety of fruit trees for annual applications.
Choosing the Right Fertilizer
Different plants have different nutrient requirements. Some need large amounts of nitrogen, others less. Knowing a plant’s needs will help determine the right fertilizer for the job. Leafy greens, broccoli and corn are heavy nitrogen users. Some plants, such as peppers, tomatoes and eggplant require less nitrogen once they begin fruiting. Other factors that affect what fertilizer to use are the natural fertility of the soil and the amount of organic matter in the soil. A soil test is the best way to determine the fertilizer requirements of the soil.
Chemical vs. Organic Fertilizers
There are both chemical and organic complete and balanced fertilizers. Chemical fertilizers are made up of synthetic compounds and are formulated to yield fast results. They are quickly absorbed into soil and usually require repeated applications throughout the growing season unless a slow-release formula is used. Organic fertilizers are derived from plant, animal or naturally occurring mineral sources. Most contain more nutrients than chemical fertilizers. Many of these nutrients are converted into inorganic forms by bacteria and fungus in the soil before they are absorbed by a plant, which means they are released slowly into the soil over time.
Slow and Quick Release Fertilizers
Complete and balanced fertilizers are available in both slow and quick-release formulas. By nature, organic fertilizers are slowly released into the soil. The term slow-release has become synonymous with natural or organic fertilizers. Chemical fertilizers, however, are also available in slow-release formulas. Slow-release fertilizers are not applied as often in the garden because they have low water solubility, are slowly released into the soil or require nitrification in the soil. Chemical, slow-release fertilizers may have a sulfur coated urea or plastic resin that slows the release of nutrients into the soil. Applications every two to three months are recommended for slow-release fertilizers. Quick-release formulas often require multiple applications, as often as every week to 10 days during the growing season.
- PBS: Soil 101
- University of Arizona: Soils and Fertilizers: Fertilizers
- Mississippi State University Extension Service: Discover Garden’s Fertilizer Needs
- Oregon State University: Here's the scoop on chemical and organic fertilizers
- University of Arizona Cooperative Extension: Easing the Roller Coaster Ride with Slow Release Fertilizer
- General Garden Fertilizer
- The Importance of Fertilizers to Farmers
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