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Your lawn and garden need a variety of nutrients to grow and stay healthy. Soil is an important source of key nutrients like nitrogen (N), phosphorus (P), and potassium (K), but it doesn’t always contain them in adequate amounts. That’s why many homeowners rely on commercial fertilizers as a supplement. Fertilizers come in an extensive variety of types and nutritional profiles, each of which impacts your plants—and the environment—in a unique way. Read on to acquaint yourself with the types of fertilizers available on the market today, so you can shop your garden center for the right solution with confidence.
Understanding NPK Values
Fertilizer supplies plants with three primary nutrients: nitrogen, phosphorus, and potassium. This nutrient trifecta is so important to the health of plants that all fertilizers display an NPK value on their packaging. The NPK value represents the percentage by volume of nitrogen (N), phosphorus (P), and potassium (K) that a fertilizer contains. For example, a fertilizer with an NPK value of 16-16-16 contains 16 percent nitrogen, 16 percent phosphorus, and 16 percent potassium (the remainder of the fertilizer comprises filler ingredients). It also means that the fertilizer has an NPK ratio of 1:1:1; that is, it contains equal amounts of the three main nutrients. Similarly, types of fertilizer with the NPK value of 24-8-16 contain 24 percent nitrogen, 8 percent phosphorus, and 16 percent potassium—that’s an NPK ratio of 3:1:2.
Before choosing a fertilizer, determine the optimal NPK ratio for your soil by determining the existing nitrogen, phosphorus, and potassium levels using a soil nutrition testing kit (available for $8 to $25 at home centers, nurseries, and online).
- If the soil test reveals that all three nutrients are present in a roughly equal amount, opt for an all-purpose fertilizer with an NPK ratio of 1:1:1. These fertilizers contain a balanced nutritional profile that is suitable for flowers, vegetables, shrubs, trees, and lawns. A minimum NPK value of 3-3-3 is recommended, but fertilizers in the 1:1:1 ratio come in various NPK values; some popular options include 5-5-5 and 10-10-10. The difference between these two fertilizers is that the nutrients are twice as concentrated in the 10-10-10 option, which means you can apply half as much of it to supply your soil with the same level of nutrients.
- If, on the other hand, the soil test indicates that your soil contains too little or too much of one or more of the three key nutrients, opt for a specialized fertilizer that contains a specialized ratio. For example, if your soil is nitrogen-poor but richer in phosphorus and potassium, you could choose a fertilizer with an NPK ratio of 3-1-2 (such as a fertilizer with an NPK value of 24-8-16). Alternatively, you could select a fertilizer with no phosphorus or potassium (with an NPK value such as 10-0-0 or 21-0-0). Specialized fertilizers are also recommended when a plant demands a higher or lower amount of one of the three nutrients. Sweet corn, for instance, thrives in soil with high nitrogen and phosphorus content, which is why fertilizers with an NPK ratio of 2:2:1 are commonly used in sweet corn soil beds.
Types of Fertilizers
With the proper NPK ratio for your soil in mind, you’ll seek out that set of numbers in one of the two main types of fertilizers on the market: organic and synthetic.
Organic Fertilizers
Plant, animal, or mineral remains that are packaged and sold either in their raw state or as pellets are called organic fertilizers. These environmentally-friendly fertilizers typically contain naturally occurring and therefore lower concentrations of individual nutrients than synthetic fertilizers. Home gardeners may pick them for this reason in order to help keep the fertilizer from building up in soil and either burning (i.e. killing) plants or contaminating local water sources via runoff.
Organic fertilizers—which include cottonseed meal, fish emulsion, bone meal, and blood meal—contribute beneficial microorganisms to soil that will assist in improving its structure over time, which makes it better able to hold water and nutrients. Organic fertilizers also water-insoluble, meaning they won’t dissolve with irrigation. One downside, however, is that organic fertilizer takes significantly longer to improve your plants than liquid or quick-release synthetic fertilizer does. Once spread over the soil surface, organic fertilizers must be organically decomposed by bacteria before releasing nutrients to the soil. Depending on the specific brand and mixture, most organic fertilizers supply nutrients for between three months and 10 years—so even though these fertilizers cost more than the average synthetic fertilizer (typically running anywhere from $.75 to $3 per pound), they generally require less frequent reapplication. Organic fertilizers work well for both garden or lawn applications.
Synthetic Fertilizers
Synthetic fertilizers are formulated with chemically processed compounds such as ammonium nitrate or urea. These fertilizers provide a robust source of primary nutrients but don’t contribute any microorganisms to the soil, and therefore the soil’s structure and water retention remains the same. Compared with organic fertilizers, synthetic fertilizers are more affordable (around $.50 to $2 per pound) and improve plants more quickly. The flip side to that fast action, however, is that they won’t supply nutrients to the soil for as long—only two to eight weeks, depending on the fertilizer—while organic fertilizers provide nutrients for as long as it takes the microorganisms to decompose them. While these types of fertilizers require more frequent application, over-application can lead to fertilizer burn, run-off into local water sources, and even the destruction of existing microorganisms in the soil. If you choose a synthetic option, always carefully follow manufacturer’s recommended amount and instructions.
- Liquid fertilizers are water-soluble fertilizers that should be diluted in water and then applied to the soil with a hose-end sprayer or watering pot. These fertilizers (including both liquid concentrates like Miracle-Gro Liquid All Purpose Plant Food Concentrate and dry powders like Jacks Water-Soluble Fertilizer) rapidly release their nutrients to the soil, and they often improve plants within a few days of application. However, since they supply nutrients to the soil for only two to three weeks, liquid fertilizers require frequent re-application. These types of fertilizers are most often used when growing outdoor plants and houseplants.
- Quick-release granular fertilizers are comprised of small, solid, water-soluble granules. Gardeners must sprinkle or spread the granules over the soil surface, mix them into the topsoil, and water them in order for the fertilizer to decompose and release nutrients into the soil. These fertilizers (such as Jobe’s Synthetic All-Purpose Fertilizer) supply nutrients for three to four weeks, so they require less frequent reapplication than liquid fertilizers. However, quick-release granular fertilizers can take slightly longer—usually a week—to improve plants, due to their slower decomposition rates. These fertilizers are commonly used for both lawn and garden applications.
- Slow- and controlled-release granular fertilizers are made of water-insoluble granules that are either uncoated or coated with a semi-permeable material. These fertilizers are spread on the soil surface, then covered with soil or compost. The fertilizers decompose and release their nutrients into the soil more gradually than liquid or quick-release granular varieties, supplying nutrients to the soil for anywhere from two to nine months and improving plants in as little as two weeks after application. The slow release diminishes the risk of burning the plant. Controlled-release fertilizers tend to release nutrients at a predictable rate that depends on soil temperature, while the rate of nutrient release in slow-release fertilizers depends on a number of factors including soil temperature, moisture, and pH. Although slow-release granular fertilizers (such as Osmocote Smart-Release Plant Food) are slightly more expensive than quick-release fertilizers, their staying power in soil saves you fertilizer (and money) over time. In fact, gardeners can get away with applying the fertilizer only once or twice per growing season. These fertilizers are commonly used in the lawn and garden.
- Fertilizer spikes allow gardeners to avoid the hassles of storing fertilizer bags and spreading fertilizer. The user merely has to drive the solid, water-insoluble sticks (like Jobe’s Fertilizer Spikes) into moist soil, where they decompose and release their nutrients. Although fertilizer spikes supply nutrients to the soil for one to three months, they only work on small soil sections. This makes them a better source of nutrients for small trees, plants, and shrubs than for large trees or lawns. Thanks to the tendency of plant spikes to create high concentrations of nutrients in a limited area, they can also contribute to unequal root growth or the dreaded fertilizer burn. To prevent this issue, leave at least three feet between spikes to prevent too much fertilizer from building up in any one area of soil. If the spikes are properly planted, you can expect plants to improve in as little as one week.