Understanding the ins and outs of fertilizing your garden can sometimes feel overwhelming. The shelves of colorful products at the local shop, sometimes with confusing labels, promise magical results. By keeping in mind some basic facts about fertilizers and fertilizing, you may approach the task with much less apprehension.
Know what you’re trying to accomplish
One common misconception is that you are feeding your plants when you fertilize. But plants feed themselves through the process of photosynthesis. They draw the nutrients they need from the soil and air in their environment largely through their roots. You only need to fertilize when you suspect a nutrient deficiency in the soil is affecting your plant, when seasonal growth needs extra support, or if a soil test indicates a specific deficiency.
Fertilizers vs Amendments
When you fertilize you are adding nutrients to the environment of a specific plant that will help that plant’s development based on the season and its growth stage.
Amendments, unlike fertilizer, help all your plants indirectly. When they are worked into the top layer of the soil, amendments improve soil structure, drainage or water retention, and encourage microbial activity. Many things can be amendments, from coffee grounds to manure and compost. Amendments are generally worked into the top few inches of soil when you are creating a new landscape area or bed, after evaluating the quality of the soil.
Existing landscape and established beds should be amended each year by adding a layer of compost and mulch around plants on the soil’s surface to maintain the organic quality of the soil over time.
Choose your fertilizer content based on the growth needs of your plants.
Most complete fertilizers supply 14 essential nutrients, including the 3 critical macronutrients, nitrogen (N), phosphorus (P) and potassium (K). Nitrogen helps the plant get started by supporting the development of healthy foliage. Phosphorus contributes to the development of strong roots. Potassium encourages bloom and fruit. Fertilizers are labeled based on the guaranteed percentages of these three macronutrients. A 100-pound bag labeled 12-12-12 has 12 pounds each of nitrogen-phosphorous-potassium. The balance of 64 pounds is filler.
For annuals, vegetables, or young fruit trees, nitrogen and phosphorus are important at the beginning of the season when the plant is in a high growth stage, getting taller and strengthening its root system. Check the package label and for your first application you might choose a fertilizer with a nutrient ratio like 4-6-2. This makes sense as you want nitrogen to help the plant develop and phosphorus to build the root system, but you need less potassium. As the plants grow you can switch to something more balanced, say 5-5-5, which you can use throughout the season. Then as the plant gets ready to fruit, use a high PK fertilizer like 3-20-20 to support fruit production. You might also use this fertilizer to support a rosebush in a container.
Plants other than annuals, vegetables, or fruit trees will have different needs, and it’s important to understand each plant’s requirements. For example, many non-flowering landscape trees and shrubs don’t need fertilizer, even at planting, if they are healthy. Native California plants don’t need fertilizer at all, while plants in containers will need their nutrients replaced as water leeches them from the soil.
Don’t over-fertilize.
Too much fertilizer can damage plants.
• If your plant looks unhealthy, perhaps with yellowing leaves, try to figure out why. Nutrient deficiency symptoms such as yellowing leaves can also result from unhealthy roots or improper care.
• If fertilizer is required, consider a slow-release type, such as organic fertilizers, that will feed the plant throughout the season. Follow the directions on the label carefully.
• Fertilizer should be applied at the drip line of trees and shrubs and not directly on the trunk or crowns.
So many options
Organic or synthetic?
• Organic fertilizers are developed directly from plant or animal sources. They are slow acting, relying on soil microorganisms to break them down over time. They can be more expensive or harder to find.
• Synthetic fertilizers provide the same nutrients, but they go through a manufacturing process. To become available to the plant roots, they are soluble rather than relying on microbes to break them down. Because solubility makes the nutrients available to the plants right away, it’s very important to follow the label so you don’t over-fertilize and burn the plants.
Complete or Incomplete?
• Complete contains some or all three of NPK.
• Incomplete - just one or two of NPK.
Dry or Liquid?
• Dry fertilizers can be more cost effective, while liquid fertilizers may be more convenient.
Other Nutrient Sources
Many people will add various specialized products to their soil as a way to maintain nutrient health. For example, you can add nitrogen with blood meal, fish meal, or soybean meal.
Sources that we have been asked about include:
• Coffee grounds—these can be a source of nitrogen, although too much can cause problems.
• Wood ashes—excess ashes can cause both high pH and salinity
• Epsom salt—provides magnesium and sulfur. They are soluble, so will dissipate with irrigation and can lead to excess.
• Compost tea—can be beneficial, however it’s difficult to know how much or what nutrients are included. Use it sparingly.
• Banana peels—can provide potassium to the soil but requires nitrogen to break down. You’re better off putting them in the compost.
• Eggshells—need to be ground to a fine powder, otherwise the calcium stays intact.
For a more complete list of materials used as fertilizers, see https://ucanr.edu/sites/default/files/2021-01/342404.pdf .
More is not better
Remember to apply fertilizer only when you know it’s needed. Watch your plants and test your soil if you suspect a nutrient deficiency. Reasonably priced kits for testing NPK levels are available at most local nurseries and are relatively easy to use.
Use fertilizer in conjunction with composting, cover crops and other ways of promoting good soil nutrition, and key your use to the needs of the plant and the time of year and enjoy your healthier plants.
For a wonderful discussion of fertilizing and amending soil, see our video at https://youtu.be/BydAcQbMu_c .
Help Desk of the UC Master Gardeners of Contra Costa County (RDH)