Make the Best Use of Fertilizer in Small Spaces

Nurseries are beautiful places, but they can be very confusing when it comes to fertilizer. Here's how to make the best choices for the plants in your container garden.

by Jessica Walliser

By definition, a plant fertilizer is either a synthetic chemical or a natural material added to the soil or growing media to increase its fertility and aid in plant growth. Your mother may have drenched her potted plants with a water-soluble chemical fertilizer every week, but there’s been a major shift in thinking over the past decade. We’ve moved away from the idea of “feeding the plants” and toward the idea of “feeding the soil.”

When you use naturally derived fertilizers, your plants are provided with a much more balanced nutrient source—one that provides mineral nutrition for growing plants by feeding the soil’s living organisms. These microscopic organisms (most of which are fungi and bacteria) process these fertilizers, breaking them down into the nutrients plants use to grow. In addition, many of these microbes live in a mutually beneficial relationship with the roots of plants, providing the plants with certain mineral nutrients in exchange for small amounts of carbohydrates. When we feed the soil, our plants reap the benefits.

You may think all this doesn’t matter for container gardens, since the root system is contained in a small area, but if you use a 50/50 blend of compost and potting soil to fill your containers, quite the opposite is true. The compost in your containers is alive with beneficial soil organisms. Plus, compost contains myriad macro- and micronutrients essential for plant growth. Science has shown us that encouraging healthy, biologically active soil is the best way to promote optimum plant growth, even when plants are growing in containers.

Nevertheless, there are times when our container-grown plants need more nutrition, such as when the nutrients contained in the compost are depleted or unavailable. For those times, there are a number of easy-to-use natural fertilizers that do an excellent job of feeding the soil. These fertilizers are derived from assorted combinations of naturally sourced materials, and they can readily be added to containers throughout the growing season.

An added benefit of using these natural fertilizers is that many of them contain trace nutrients, vitamins, amino acids and plant hormones that aren’t usually noted on the label and are rarely found in chemical fertilizers. These compounds act as natural growth enhancers and play a vital role in the health and vigor of plants.

Reading the Label

When shopping for fertilizers, spend some time reading the labels. Natural fertilizers have four main ingredient sources.

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  1. Plant Materials: These are fertilizer ingredients derived from plants. A few examples include corn gluten meal, alfalfa meal, kelp meal and cottonseed meal.
  2. Manure Materials: You may also see pelletized poultry manure, dehydrated cow manure, cricket manure, bat guano and worm castings or worm “tea” on the label of a natural fertilizer.
  3. Animal Byproducts: Fertilizer components found in this category are often derived from the byproducts of our food industry. They include fish emulsion, bone meal, feather meal, blood meal and crab meal.
  4. Mined Minerals: Natural fertilizers for plants may also include mined minerals, such as greensand, rock phosphate, crushed limestone and sulfate of potash.

Using fertilizers containing a combination of these ingredients is a terrific way to feed your soil when nutrients become depleted and adding more compost isn’t an option.

Before choosing any type of fertilizer for your container garden, however, it’s important that you understand the numbers on the label. In addition to listing their ingredients, natural fertilizers also state their N-P-K ratio somewhere on the bag. This ratio exhibits the percentage by weight of three macronutrients—nitrogen, phosphorus and potassium.

fertilizer container gardening

For example, a bag of 10-5-10 holds 10 percent N, 5 percent P, and 10 percent K. The remaining 75 percent of the bag’s weight is filler products. For natural-based fertilizers, the numbers in the N-P-K ratio are often smaller (2-3-2 or 1-1-6, for example). This is because the label percentages reflect levels of immediately available nutrients, and many of the nutrients in natural fertilizers are not available immediately upon application; it takes some time for the soil microbes to process these nutrients and release them for plant use.

Although this may seem like a disadvantage, natural fertilizers release their nutrients gradually, serving as a slow-release fertilizer.

It’s also important to understand how plants use these different macronutrients.

  • Nitrogen: Nitrogen is a component of the chlorophyll molecule, and it promotes optimum shoot and leaf growth. Adding a fertilizer high in nitrogen (such as 6-2-1 or 10-5-5) to a fruiting or flowering plant, such a tomato or a petunia, will result in excessive green growth, often at the expense of flower and fruit production. But adding it to a green, leafy vegetable plant, such as spinach or lettuce, makes much more sense.
  • Phosphorus: Phosphorus, on the other hand, is used for cell division and to generate new plant tissue. It promotes good root growth and is used in fruit and flower production. Phosphorous is particularly important for root crops, such as beets, carrots and onions, as well as for encouraging flower and fruit production. That’s why fertilizers that contain bone meal and rock phosphate are often recommended for use on root crops: Both are rich in phosphorus.
  • Potassium: This helps trigger certain plant enzymes and regulates a plant’s carbon dioxide uptake by controlling the pores on a leaf’s surface, called stomata, through which gases pass. Potassium levels influence a plant’s heartiness and vigor.

fertilizer container gardening

Excerpted from Container Gardening Complete: Creative Projects for Growing Vegetables and Flowers in Small Spaces (Cool Springs Press, 2017) by horticulturist Jessica Walliser, an award-winning radio host on KDKA Radio’s “The Organic Gardeners” in Pittsburgh, Pennsylvania, and a longtime contributor to Hobby Farms.

This story originally appeared in the March/April 2018 issue of Hobby Farms.

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