By Noelle Johnson, Houzz
Spring is a great time to fertilize plants, once freezing temperatures recede, but it’s easy to become overwhelmed when faced with the large variety of fertilizers at your local garden center. Several questions may go through your head as you ponder which one is right for your plants. What do those three numbers mean? What’s the difference between synthetic and organic fertilizers, and what are “complete” and “incomplete” fertilizers? Let’s take a step back and talk about fertilizer basics and what you need to know to find the right one for your garden.
Plants need nutrients to grow, and the soil is their primary source of nutrients. Often, plants don’t need any fertilizer at all, as they get what they need from the soil. However, if the level of a particular nutrient or nutrients is low in the soil, or you have a particular type of plant that requires high amounts of nutrients, fertilizer can be used to supplement the amount of nutrients present in the soil.
Fertilizers contain one or more of three vital nutrients: nitrogen, phosphorus and potassium, which plants use for growth. These nutrients are also known collectively as NPK, and the percentage of each contained in a bag of fertilizer is indicated by three numbers separated by dashes.
To better understand fertilizer and how it helps plants, let us learn how each type of nutrient works.
Nitrogen is the most common nutrient found in fertilizer and is the most important one for plants. It stimulates plant growth, such as the production of lush green leaves. This important nutrient helps plants make food as well as defend against pests. Nitrogen is the most common nutrient that plants can become deficient in, as it is quickly leached out of the soil. A sign of nitrogen deficiency is yellowing leaves, beginning with the older leaves found on the lower parts of a branch.
Phosphorus is the nutrient that promotes plant growth while also playing a vital role in flower, fruit and seed formation. It also helps with root growth. Fertilizers that stimulate flowering are high in phosphorus. Signs of phosphorus deficiency are leaf tips that appear burnt and leaves that have a reddish-purple tint.
Potassium is the second most needed plant nutrient after nitrogen. It maximizes the way plants use other nutrients, aids in defense against disease and also plays an important part in root, stem and flower formation. Plants that are deficient in potassium may have leaves with a scorched appearance on the outer edges. In addition, they may show signs of chlorosis (yellowing in between the veins) on older leaves — not to be confused with iron deficiency, which occurs on newer leaves.
Fertilizer labels. Now that we know what the three main nutrients are, we can talk about what the numbers on the fertilizer bag mean. Fertilizer labels have three numbers that are separated by dashes, as we see in this photo. These numbers tell you the percentage of nitrogen, phosphorus and potassium contained in the fertilizer. For example, a bag with the numbers 15-15-15 contains 15 percent nitrogen, 15 percent phosphorus and 15 percent potassium. A different fertilizer may have the numbers 12-0-0, which means that it has 12 percent nitrogen, 0 percent phosphorus and 0 percent potassium.
Common Fertilizer Definitions
There are several common terms describing fertilizers, and it’s helpful to know what they mean before choosing a fertilizer.
â—‹ General-purpose fertilizers have all three nutrients and are formulated to meet the needs of most plants.
â—‹ Balanced fertilizers contain equal amounts of nitrogen, phosphorus and potassium.
â—‹ Special-purpose fertilizers are formulated with higher levels of certain nutrients to bring about desired results for specific plants. Examples are lawn fertilizers, which have a high level of nitrogen, and fertilizers with a higher level of phosphorus, which promotes flowering. Other special-purpose fertilizers are created for specific kinds of plants, such as citrus, houseplants and roses.
â—‹ Incomplete fertilizers are missing at least one of the major nutrients and are used to correct a specific nutrient deficiency.
â—‹ Slow-release fertilizers are general fertilizers that release their nutrients slowly, usually over about three to six months.
â—‹ Liquid fertilizers are water-based and can contain all three major nutrients, or one, and are used around the roots or sometimes on the leaves of a plant. They work very quickly compared to other fertilizers.
Chicken manure is a good organic source of nitrogen.
Organic and Synthetic Fertilizers
When shopping for fertilizer, you’ll see that some types are marked “organic.” Those that aren’t organic are of synthetic origin, derived from chemicals. Both types of fertilizers are used to provide the same nutrients to plants, but the nutrients come from different sources and vary in the way they work.
Organic fertilizers. These come from plants and animals and contain all three major nutrients as well as some micronutrients, which also benefit plants. The amounts of nutrients present in organic fertilizer are lower than those in synthetic form, but the nutrients last much longer. They are released slowly, because they need to be converted into forms that the plants can use. This is done by beneficial soil microbes that are already present in the soil. This process helps to enrich the soil. Because organic fertilizers benefit and improve the soil, future use of fertilizer may not be needed. They are from natural sources, so they are unlikely to “burn” plants if over-applied, and they don’t pollute groundwater, as synthetic fertilizers can.
Popular organic fertilizers:
â—‹ Chicken, cow, horse and rabbit manure are great fertilizers. It’s important that manure is aged over several months before being applied. All manures are good sources of nitrogen.
â—‹ Blood meal, fish emulsion and seaweed are natural sources of nitrogen. Blood meal is also a good source of phosphorus.
â—‹ Ash from burnt wood is a source of potassium and is best used in small amounts mixed with compost.
â—‹ Alfalfa and cotton meal are plant-based fertilizers that provide a mixture of nitrogen, phosphorus and potassium.
Roses benefit from being fertilized in spring.
Synthetic fertilizers. These are created from chemicals. They are fast-acting and, unlike with organic fertilizers, their nutrients don’t need to be converted into a different form before being used by plants. Caution should be used when applying, as plants can be “burned” if too much is used. While synthetic fertilizers work quickly to correct nutrient deficiency, their effects don’t last long and they must be reapplied more frequently than fertilizers from organic sources.
These chemical-based fertilizers have a detrimental effect on beneficial microbes in the soil as well. Synthetic fertilizers are often sold in granular form, which helps them release their nutrients quickly. Slow-release synthetic fertilizers have coated granules that gradually disintegrate. Liquid formulations are also available.
Common synthetic fertilizers:
â—‹ Ammonium nitrate, ammonium sulphate and urea are sources of nitrogen only and work quickly. They do tend to make soils more acidic with repeated use.
â—‹ Rock phosphate and superphosphate are chemically based sources of phosphorus.
â—‹ Muriate and sulphate of potash are synthetic sources of potassium.
Which to choose: organic or synthetic? Organic fertilizers are the better choice, as they not only add needed nutrients but also improve the health of the soil by nurturing the beneficial microorganisms within it and increasing the soil’s ability to hold on to moisture and nutrients. It helps to remember that healthy soil equals healthy plants. Because they come from natural resources, organic fertilizers are renewable and biodegradable.
Synthetic fertilizers are useful if plants need nutrients immediately due to severe nutrient deficiency. However, it’s important to keep in mind that they do have negative environmental impacts, as they kill beneficial microorganisms in the soil, aren’t renewable and can leach into groundwater and other water sources, polluting them.
To fertilize or not? Before running out and buying fertilizer, take a look at your plants. Do they look healthy? If so, chances are that they don’t need fertilizer. Many plants are adapted to soils with low nutrients and do best without supplemental nutrients. Sometimes plants are fertilized when they don’t need it, which can result in sprawling growth and decreased flowering. Native plants as well as wildflowers are particularly well-adapted to growing without any fertilizer.
However, if your plants show signs of nutrient deficiency, it’s important to test your soil first to see what nutrients they may be deficient in. Healthy soil can be created by incorporating organic matter, such as compost, once or twice a year, which helps to create fertile soil over time and reduces the need for fertilizer. Specialty plants such as fruit trees and roses may require supplemental fertilizer to ensure the production of delicious fruit and large blooms.