Compost vs. Fertilizer: What's the Difference?

Tumbler compost bin near house wall.

The Spruce / David Beaulieu

Beginning gardeners sometimes confuse the terms "compost" and "fertilizer." This is understandable because there's some overlap between the two. More importantly, this confusion can lead to uncertainty about whether to use compost or fertilizer in a particular situation. To clear up the confusion, we'll take a bird's-eye view of the different types of fertilizer, which is a complex topic in itself. Compost is an even more complex topic since compost serves a number of different functions. After acquiring a basic understanding of how fertilizer and compost differ, you'll learn what situations call for fertilizer and when you should use compost, instead.

What Is Compost?

At the most basic level, compost is decomposed organic matter. However, many forms of organic matter are decomposing in many different locations all the time. When we say "compost," we are usually referring specifically to organic matter that has decomposed in a compost pile (often contained in a bin or tumbler). Many gardeners keep such a pile in their backyards to make their own compost. This is one of the differences between compost and fertilizer: While you can buy compost, you also have the option of saving money and just making it for yourself. Fertilizer, by contrast, is typically something you buy.

There are four basic steps to take when making compost:

  1. Include "green" elements, such as kitchen scraps.
  2. Include "brown" elements, such as the dead leaves you rake up in fall (shred first for best results).
  3. Provide water.
  4. Turn the pile over occasionally.

Bacteria eventually find your pile. They help it heat up, and this heat breaks down (decomposes) the organic matter. The resultant product looks like soil to the untrained eye but offers far more value than average soil.

Uses for Compost

Most beginners understand that you can apply compost around plants to help them grow. This adds to the confusion about the difference between compost and fertilizer. Compost is better thought of as a "soil amendment" than a fertilizer. By using this term, we recognize that compost not only improves plant health and growth but adds other benefits, too. It can improve soil structure and thereby achieve better drainage, aeration, and moisture retention. It also contains beneficial microorganisms and microbes that can aid in plant nutrition and make plants more disease-resistant.

What Is Fertilizer?

Fertilizer supplies your plants with nutrients. A fertilizer falls into one of two categories, chemical or organic.

A chemical fertilizer is an artificial product. This is the type of fertilizer you are most likely getting when you buy one of those big lawn fertilizer bags at a home improvement center. It's put together with great care to ensure that particular nutrients are available in particular quantities (this is where NPK ratios come in). That's the good news. The bad news is that these products can be harmful if used incorrectly. Too much chemical fertilizer can burn your plants and different plants require different types of fertilizer. For example, a fertilizer high in nitrogen will give your tomato plants lots of healthy green foliage, but too much nitrogen and not enough phosphorus could mean fewer fruits.

An organic fertilizer is a natural product. While you still have to be careful that you don't over-apply them, they're generally safer than chemical fertilizers. Examples include bloodmeal and bonemeal.

Uses for Fertilizer

When gardeners apply compost around plants for its nutritive properties, they're using it as an all-purpose product that can be applied at any time. But when applying a true fertilizer, you want to use it in a more targeted way, at a particular time, based on the specific nutrients it offers. Here are some examples:

  • Bone meal is high in calcium and phosphorus. Use it on your bulbs when you plant them in fall.
  • Blood meal is high in nitrogen. Use it on your bulbs when they come up in spring.
  • Lovers of lawns typically fertilize their grass with chemical fertilizers.

If you choose to apply a chemical fertilizer it is important to know the specific nutrient needs of the plant for which it is intended. Chemical fertilizers are available in a number of different NPK ratios which means one size, or one type of fertilizer, does not suit all types of plants.

Can You Use Compost and Fertilizer Interchangeably?

Compost and fertilizer should not be used in place of each other. Using compost achieves certain results that fertilizer cannot. For example, fertilizer won't improve the structure of your soil. On the other hand, some garden soils may lack a a certain nutrient that may not be present in compost. You can find out the condition of your soil by sending a sample to your county extension to have it tested. If a certain nutrient is missing or at lower than optimum levels, you can supplement the addition of compost with a appropriate fertilizer. Your county extension can tell you what fertilizer to use, as well as when and how much to apply.

When to Use Compost vs. When to Use Fertilizer

Again, addressing a specific nutrient deficiency is one case of when people choose to use an organic fertilizer, not compost. One example is when a tree or shrub fails to flower. Sometimes, the problem is a lack of phosphorus, which can be solved by applying bone meal.

Sometimes, chemical fertilizers are just more convenient. This is especially true for feeding the lawn. Many feed their lawns up to three times annually, based on schedules developed by fertilizer companies. The composition of the fertilizer can be different for each of these feedings (especially in the case of "weed and feed" products), based on your lawn's different needs as the year progresses. People with busy schedules often opt for professional lawn care to insure that their lawns receive the appropriate supplements at the correct times throughout the year.

One of the best benefits of compost is that it can be used with good results anytime. When you are starting a garden from scratch, you will have far more use for compost (at least in the beginning) than for fertilizer because your goal is to build up the soil. When you make your own, compost is free and it doesn't require careful measuring and mixing to apply. It can be tilled into the soil before planting and added as a side dressing or worked into the soil around established plants using a garden rake or hoe.