Compost and fertilizer can play an important role in every garden, regardless of its size.  But sometimes these garden-helpers are used interchangeably as if they are the same.  Although there are similarities between the two, they have significant differences as well.  In this post, we will discuss what they have in common as well as what makes them unique.  We will also find out which one, if either, is better for your garden.    

 Compost

Compost is a soil amendment comprised of decomposed organic matter, such as lawn and garden waste.  You can purchase it from plant nurseries and big box stores, or you can make your own.

Compost, or black gold as it is affectionately called in the garden community, has multiple purposes. When added to the soil, it changes the soil structure or texture, which improves drainage, aeration, and moisture retention.  In addition, the beneficial microbes in compost feed on and break down the organic matter, converting the micronutrients and macronutrients in the compost into a form that plants can use.  Compost, in essence, creates healthy soil. Healthy soil in turn produces healthy plants.

It takes time for compost to work its magic which is why many gardeners add one to three inches of compost to their garden in the fall months so that it has time to break down and feed the soil before spring planting.  Too much compost, however, can do more harm than good, in that it can increase the microorganisms too quickly and thus deplete the soil of available nitrogen, not to mention, alter the soil pH.  So, it is important not to add more than 25% to garden beds or containers.

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Fertilizer

Fertilizer is made from materials, either organic or synthetic, that primarily consist of three macronutrients: nitrogen, phosphorus, and potassium. These are often referred to as NPK. (You can learn more about NPK in this helpful article.)  Sometimes it will also include micronutrients like zinc, copper, iron, and boron, but this is not always the case.  While compost is a soil amendment used to feed the soil and improve its structure, fertilizer’s sole purpose is to feed plants which aid in plant growth and development.  

Fertilizer improves plant fertility by supplying nutrients to the soil that may not be present in adequate amounts.  Compost also provides some nutrients, both macronutrients and micronutrients, but on a smaller scale than fertilizer.

Unlike compost, which can be used in any situation, fertilizer is not a one-size-fits-all treatment.  That is because plants have different nutritional requirements.  Instead, fertilizer is used to target current nutritional deficiencies revealed by a soil test or for future nutritional requirements that the plant will have as it develops. 

Can Compost and Fertilizer Be Used Interchangeably?

In general, compost can be used as a type of fertilizer because it contains macronutrients and micronutrients, the latter of which are not always found in fertilizers.  But you will need to plan ahead because compost does not act as quickly as fertilizer does, especially liquid fertilizer. 

On the other hand, fertilizer is not a substitute for compost because it does not amend and improve the soil, something that is vital to both soil and plant health.  Instead, compost and fertilizer work together to help create an environment in which plants can thrive.  Compost acts like a sponge, soaking up and storing nutrients supplied by the fertilizer.  When the plant needs the nutrients, they are readily available.

Which is Better?

So, which one is better, compost or fertilizer? Ideally, the soil would have enough diverse beneficial microbes from compost so as not to need fertilizer.  But due in large part to non-regenerative practices, this is usually not the case.  So typically, most gardens require both compost and fertilizer to be healthy and productive.

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