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Compost vs. Fertilizer: What’s the Difference?

The main difference between compost and fertilizer lies in how they work to improve the growing conditions for your plants. Fertilizer has the primary use of delivering concentrated doses of specific nutrients directly to plants; compost is instead more of a multipurpose substance that mainly benefits the structure and nutrient content of the soil. Serving different purposes, these two substances are not interchangeable with one another. It’s therefore best practice to use both fertilizer and compost in combination to reap the full benefits of both substances. 

What is Compost?

Compost is a natural, biologically-active substance made from decomposed organic material. It contains the nutrient-rich decayed remains of the organic matter, along with a host of beneficial microorganisms and microbes. You could more accurately describe compost as a soil amendment; when added to soil, compost works to enrich the planting area and boosts the growth of the surrounding plants.

dark brown compost

There are many different materials that compost can be made from; these materials are categorized as being either ‘green matter’ or ‘brown matter’. Green matter materials are those which are nitrogen-rich, including grass clippings, fruit and vegetable scraps, eggshells, and fresh plant clippings. Brown matter materials are instead those rich in carbon, such as dry leaves, wood, straw, sawdust, and pine needles. 

For a high-quality compost, it’s essential to have the correct balance of green materials and brown materials; the green-to-brown ratio should be roughly ⅓ greens to ⅔ browns. At this ratio, the organic matter will decompose most efficiently, resulting in a balanced, nutrient-rich compost.

Benefits of Using Compost

  • Compost is a totally organic, nutrient-rich amendment that enriches the soil and improves its structure.
  • After application, compost continues to decompose, thus providing a steady stream of nutrients over the following months to years.
  • Compost helps soil to retain optimal moisture levels by simultaneously improving drainage and water retention. 
  • Compost can benefit all soil types; in sandy soil, compost can improve the soil structure by assisting its moisture retention; in clay-heavy soil, compost can improve the soil structure by promoting better drainage.
  • The earthworms that live in compost promote better aeration of the soil by tunneling through it; they also increase the concentration of nutrients by digesting its organic matter before expelling nutrient-rich castings.
  • A layer of compost protects the soil against soil erosion and drought. 
  • The bacteria within compost can improve disease resistance in the surrounding plants; the plants create antibodies in response to the compost’s bacteria.
  • You can make compost for free; this has the additional benefit of reducing your household waste.
  • If you don’t have the means to make it yourself, you can purchase compost from other manufacturers.
  • A layer of compost can help to prevent weed growth in the underlying soil; it blocks sunlight from reaching any weed seeds lying on the soil surface.

Drawbacks of Using Compost

  • Making your own compost takes a lot of effort; you must gather the necessary materials, then tend to your compost pile or bin until it fully decomposes. 
  • It can take a long time for compost to fully decompose, and only at this point will it be usable. 
  • You need quite a lot of brown and green material to make enough compost, particularly if you have a larger area to cover.
  • Compost is dirty, messy, and gives off a strong odor.
  • If it hasn’t been properly processed, compost can contain disease-causing bacteria, weed seeds, and other undesirable and potentially harmful elements.
  • Compost is typically more expensive to purchase commercially than fertilizer. 

What is Fertilizer?

Fertilizer is a nutrient-rich substance made from either natural or inorganic materials. All fertilizers contain varying levels of the primary nutrients plants need to grow, i.e. nitrogen (N), phosphorus (P), and potassium (K). Organic fertilizers contain natural sources of these nutrients, while synthetic fertilizers contain the nutrients in synthetic chemical form.

white balls of granular fertilizer

Commercially-available fertilizers come in a wide variety of formulations to meet the nutritional needs of different plants and soils. The formulation of any given fertilizer is represented as an ‘NPK ratio’, typically found on the packaging of the product. With so many formulations available, you can pick a fertilizer that perfectly suits your individual requirements. 

It’s also possible to purchase fertilizers in either slow-release or quick-release formulations. Quick-release fertilizers are designed to release their nutrients in a fast, concentrated burst upon application; slow-release fertilizers instead release their nutrients gradually over a longer period of time beyond their application.

Benefits of Using Fertilizer

  • Fertilizer is a fast-acting solution when you need to add certain nutrients to your soil. 
  • There’s a wide range of different fertilizer formulations available; you can pick a product with the ideal NPK ratio for your needs.
  • Slow-release fertilizer will keep your plants continuously fertilized with a steady stream of nutrients long after application.
  • Quick-release fertilizer can provide an instant boost of nutrients to plants that need immediate fertilization.
  • Organic fertilizers are completely natural substances, making them an environmentally-friendly way to enrich soil with nutrients.
  • Fertilizer is typically lower-cost than compost to purchase commercially. 

Drawbacks of Using Fertilizer

  • Quick-release fertilizers provide little long-term nutritional benefits; multiple applications are necessary to maintain the nutrient levels over time.
  • The many different options for NPK ratios can be overwhelming; you must understand the nutritional requirements of your plants and select the correct fertilizer accordingly.
  • Beyond its nutrient delivery, fertilizer lacks the many additional benefits that come with using compost; for example, fertilizers aren’t able to improve soil structure or moisture retention.
  • Fertilizers can provide too high a concentration of nutrients; this leads to a condition called fertilizer burn in plants.
  • The harsh chemicals in synthetic fertilizers can be very harmful to the environment; they can leach away from your yard via water runoff, ending up in waterways and the wider local environment.

What’s the Difference Between Compost vs Fertilizer?

Although compost and fertilizer serve similar purposes, there are some differences between the two substances. The easiest way to differentiate between compost and fertilizer is to look at what exactly they benefit; compost works to enrich the soil, whereas fertilizer directly feeds the plants.

As soil additives, both compost and fertilizer deliver the nutrients that plants need to stay healthy and strong; they will both contain a number of primary and secondary nutrients in some quantity depending on their ingredients. However, what sets them apart is the way in which they deliver their nutrients. Fertilizer is a targeted product that you apply at a certain time to deliver specific nutrients straight to your plants. Compost is more of a multi-purpose substance that provides additional benefits beyond its nutrient content.

Can You Use Compost and Fertilizer in Place of One Another?

As they serve slightly different purposes, you cannot use compost and fertilizer interchangeably. Each product can provide its own benefits that will suit different circumstances. 

For instance, you may use a fertilizer to replace a specific nutrient missing from your soil; this may be an effective way to replace the nutrient, but it won’t do anything to improve the soil structure. Vice versa, you could use compost to enrich and improve the condition of your soil; however, the compost may lack a high concentration of a particular nutrient that the soil is missing. Whether it’s better to use compost or fertilizer will depend on your particular circumstances and what benefits you seek from your additive.

Can You Use Compost and Fertilizer At the Same Time?

Yes, you can use compost and fertilizer at the same time to reap the full benefits of both substances. This is the recommended practice when trying to optimize the quality and growing conditions of soil in a planting area.

Performing different functions, compost and fertilizer are complementary substances to each other. You can therefore significantly improve the quality of a planting area by adding both substances at the same time. Before adding any amendment, it’s best to test your soil first; a soil test will reveal the current nutrient content, i.e. levels of nitrogen, phosphorus, and potassium, in the soil. From this information, you can select the ideal combination of products that will replenish the nutrient content of the soil to optimal levels for your plants.

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