Adding compost and other organic matter is important for improving the long-term health and fertility of your soil, but fertilizers play an important role in providing the readily available nutrients that keep plants growing strong.
Here’s a quick overview of how plants use nutrients, followed by an explanation of how to read a fertilizer label, the role of soil pH, the difference between organic and chemical fertilizers, and several options for ways to fertilize your plants.
Most garden fertilizers contain the three primary plant nutrients: nitrogen (N), phosphorus (P) and potassium (K), plus small amounts of the minor nutrients: magnesium, calcium, sulfur, iron and boron. The N-P-K ratio on the package tells you the percentage – by weight – of each major nutrient.
Nitrogen is essential for fueling new growth. Most soils are low in nitrogen because it’s a water-soluble nutrient that gets washed away by rain or gets used up by plants and microbes.
Phosphorus stimulates root growth and helps plants transfer energy between their roots, leaves and flowers. Sometime phosphorus is present in the soil, but is not available to plants due to soil temperature, microbial activity or the pH of the soil.
Potassium helps plants move water, nutrients and carbohydrates from one area to another. It is also responsible for stimulating early growth, improving hardiness and increasing resistance to insect pests and diseases.
In addition to these primary nutrients, plants also require small amounts of other nutrients including magnesium, calcium, sulfur, iron and boron. Each has an important role in maintaining plant health and productivity.
Most garden fertilizers contain the three major nutrients nitrogen (N), phosphorus (P) and potassium (K) as well as small amounts of magnesium, calcium, sulfur, iron and boron. The N-P-K ratio on the package tells you the percentage – by weight – of each major nutrient.
For example, a 50-pound bag of 10-5-5 fertilizer contains 10% (5-pounds) of nitrogen, 5% (2.5-pounds) of phosphorus and 5% (2.5-pounds) of potassium, with the rest being filler to help disperse the chemicals.
Note: If you purchase an organic fertilizer, you may notice that the N-P-K rating is relatively low compared to most synthetic fertilizers. This is because by law, the label can only reflect immediately available nutrients. Most organic fertilizers contain slow-release nutrients and trace elements that will become available over time.
Plants absorb nutrients through their roots, so the nutrients need to be dissolved in water. Some nutrients won't dissolve if the soil is very acidic (pH 4-5) or very alkaline (pH 8-9). This means even if they’re present in the soil, they won’t be available to plants. When the pH is between 6 and 7.5, most soil nutrients dissolve relatively easily, which is why most plants prefer this “neutral” range. That said, some plants actually prefer a high or a low pH. Blueberries and rhododendrons, for example, grow best in acidic soil with a pH of between 4 and 5.
Soil is largely composed of weathered rock fragments and it’s these rocks that determine the soil’s natural pH level. Soil pH is also affected by rainfall, temperature and vegetation. Some parts of the country have characteristically high or low pH levels. The only way to know for sure is to test the pH of the soil -- something that can be done by mailing a soil sample to a lab (look online to find a soil testing location near you).
Based on the test results, the lab will make recommendations for adjusting the pH (if needed) and balancing the major and minor nutrients. Changing the pH level may require you to add lime (to raise the pH) or elemental sulfur (to lower the pH). Always take a conservative approach to adjusting pH levels. Changes happen slowly and it’s easy to go too far in the other direction.
Adding organic matter to the soil (such as compost, shredded leaves or leaf mold) has a “buffering” effect on pH and can help keep levels relatively consistent.
Organic and synthetic fertilizers provide nutrients in different ways.
Organic fertilizers are made from naturally occurring mineral deposits and organic components such as bone meal or composted manure. Over time, they help to increase a soil’s baseline fertility. They also stimulate beneficial soil microorganisms, help improve the texture and structure of the soil, and add important secondary nutrients and trace elements that may be lacking. Most organic fertilizers are not water-soluble, so their nutrients become available to plants slowly, over a period of months or years. Liquid fish or seaweed emulsion and compost tea are exceptions.
Synthetic fertilizers often start as organic, but are chemically treated to make them water soluble and more immediately available to plants. This can be valuable when plants are growing rapidly and need nutrients, but used exclusively, synthetic fertilizers do not feed beneficial soil life and do not improve the fertility of the soil. Synthetic fertilizers are available in several forms: granular (in bags), slow release (like Osmocote), and water soluble (like Miracle-Gro).
With all fertilizers, it's important to not exceed the recommended application rate. This is especially true with synthetic fertilizers, as too much fertilizer can “burn” foliage, over-stimulate growth and pollute waterways with nutrient runoff.
Not all plants need the same amount of nutrients or need them at the same time during the season. Always follow package directions, because application rates vary depending on the nutrient content, whether it’s a synthetic or organic fertilizer, and whether it is granular, timed-release or water-soluble.
Trees and Shrubs. When planting new trees and shrubs, incorporate fertilizer into the root zone to ensure the plants have access to a slow and steady supply of nutrients. Established trees, shrubs may be fertilized once a year, in early spring. If the soil is fertile and the plantings are healthy and well established, there may be no need for fertilizer.
Perennials. When planting new perennials, mix an organic, all-purpose fertilizer into the planting hole to make sure the plants have access to the phosphorus, potassium and other nutrients they will need. Established perennial gardens, berries and perennial food crops such as asparagus and rhubarb, can be fertilized once a year in early spring.
Flower Bulbs. Spring-blooming bulbs that are treated as annuals, such as tulips and hyacinths, do not need to be fertilized. Bulbs grown as perennials, such as daffodils and alliums, may be fertilized before or after flowering. Summer-blooming bulbs such as dahlias, gladiolas and lilies can be fertilized at planting time. Dahlias, which are particularly heavy feeders, benefit from monthly applications of a low-nitrogen fertilizer.
Annuals. Most annual flowers and vegetables are hungry plants. Use a granular all purpose fertilizer at planting time and then apply a low nitrogen liquid fertilizer once or twice a month right through August. If you garden where summers are extremely hot, you'll want to back off on the fertilizer during the heat and then resume in early fall. Fertilizing regularly is especially important for container plants.
Though you don’t want to let plants go hungry, applying too much fertilizer can cause problems. Plants that get too much nitrogen will produce lots of foliage but few flowers and fruit. Too much phosphorus in the soil makes it difficult for plants to absorb iron and zinc; too much potassium prevents them for absorbing calcium. Excess fertilizer can also wind up in groundwater and waterways, damaging ecosystems and polluting drinking water.
Always follow package instructions for application rates and timing. If in doubt, be conservative. And don't forget to to feed the soil as well as your plants. You can learn more about improving your soil here: How to Prepare Soil for Planting.