Compost vs Fertilizer – 6 Key Differences & When To Use Each

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Nourishing your plants is an essential part of gardening and plant care. Properly nourished plants grow faster, greener, and provide a more optimal yield than those lacking essential nutrients. For this reason gardeners, farmers, and all types of plant lovers use compost or fertilizers to keep their plants healthy. But what exactly is the difference between compost and fertilizer? Are they the same thing?

Compost vs Fertilizer – Key Differences

As you probably know composts are a dark, spongy, and crumbly substance made from decomposed materials. Fertilizers on the other hand are concentrated forms of nutrients that typically come in the form of granules, liquids, or powders. In some contexts, compost could be applied to your soil as a form of fertilizer.

What Sets Compost Apart From Fertilizer?

Compost is made from decomposed organic materials like leaves, wood, manure, vegetable scraps, and other natural products. The large quantities of organic material in compost provide distinct physiochemical properties that are beneficial to soil health. 

Fertilizers originate from a wide variety of places including natural materials, synthetic sources, or inorganic materials like rock. Fertilizers largely serve as a nutrient source for plants. 

In general, composts are intended to nourish the soil and condition the biology and structure while fertilizers are used primarily to provide a specific set of nutrients to plants.

1. Compost Does More Than Provide Nutrients

Composts serve many benefits to the health of your soils and the plants that live within them. On top of providing nutrients, it serves to create air space, retain moisture, and stimulate the beneficial microorganisms in your soil’s food-web. 

These benefits are largely related to a compound known as “humus” that forms the majority of organic materials in the compost. Humus forms as organic materials break down in their finest and most stable form. The stability of humus allows it to naturally incorporate itself into the soil where it can last for years.

2. Compost Improves Soil Structure.

The organic component of compost transforms the physio-chemical properties of soil into something beneficial for plants and soil life. It holds over 90% of its weight in water and retransforms the soil structure to hold more air. These two properties alone make it an incredible and important conditioner to your soil.

3. Compost Holds Nutrients and Slowly Releases Them

Nutrients in compost are also much different than those held within fertilizers. In composts, nutrients are molecularly attached to organic material where they are accessible to plants but don’t leech away. This also means that the nutrients are slowly released as opposed to instantaneously available in large quantities like in many fertilizers. Composts also can hold nutrients so that when a nutrient-rich substance enters the soil it can be easily absorbed and retained. 

The nutrient holding capacity in compost also allows it to absorb harmful chemicals like metals or chlorine but don’t worry! Compost makes them inert and inaccessible to your plants!

4. Compost Holds Beneficial Microbes

One of the most important roles that compost plays in your soil is providing huge diversity and quantity of beneficial organisms that are important to plant and soil health. When applied to soil compost bacteria and fungi play crucial roles in making nutrients available to your plants while also playing an important role in deterring harmful pathogens.

5. Fertilizers Are Concentrated Nutrients

There are many different types of fertilizers including a wide array of organic products derived from natural materials. What all fertilizers have in common is that they are extremely nutrient-rich materials whose main focus is providing quick and accessible food to your plants. They generally contain no beneficial microorganisms and don’t contain organic molecules like hummus which naturally hold the nutrients. In many cases, they can be leached away easily from the soil.

6. Nitrogen-Phosphorus-Potassium

Most fertilizers are defined by their quantities of Nitrogen, Phosphorus, and Potassium or NPK. These are the most important nutrients needed by plants and are usually represented by 3 numbers in commercial fertilizer packages written like 15-5-10 (N-P-K). Many different mixtures can be found in your common garden center and each has its intended use. For example, some fertilizers are good for encouraging vegetative growth while others are intended for flowering or the production of fruit.

Fertilizers typically come as a liquid, powder, or in the form of small pellets called granules. They can be applied on top of the soil directly or mixed in before planting. Alternatively, they can be sprayed onto the plant to provide a “Foliar Feeding”. It is important to be careful when applying fertilizers to soils or plants because when excessively applied it can result in significant damages to soil health.

4 Organic Fertilizers Better Than Store-Bought

Organic fertilizers are those derived from natural materials like manure or vegetation. They typically contain a diverse array of nutrients and may interact more closely with your soil’s food web. Some examples of organic fertilizers are listed below.

1. Kelp Fertilizer

Kelp Fertilizers are essentially dried seaweed and contain a large number of mineral nutrients and micronutrients. They typically come in powders or as concentrated liquids. 

2. Fish Emulsion

Fish Emulsion is a nitrogen-rich fertilizer derived from broken-down fish. It is naturally smelly! It is most commonly diluted in water and used in irrigation but can be applied as a foliar spray

3. Fermented Plant Juices

Fermented Plant Juices or FPJ is a liquid fertilizer made from the fermentation of vegetation with molasses. These provide a wide array of nutrients depending on the material they are made from.

4. Activated Biochar

Biochar is a highly effective form of fertilization because it also contains organic material in the form of charcoal. This charcoal serves a purpose similar to hummus as it holds large quantities of nutrients, water, and air. Activated Biochar is made by infusing charcoal in a nutrient-rich medium.

4 Problems with Conventional Fertilizers

Conventional Fertilizers are usually synthetically produced and are commonly found in strong concentrated liquids or pellets. They most often only contain the 3 main macronutrients (NPK) and are void of other important micronutrients. While they may cause instantaneous growth in plants they can easily have long-term impacts on the health of your plant and soil when overused. Conventional fertilizers in the form of pellets or granules are typically released slower into soil whereas liquid fertilizers act quickly but may not last. 

1. Conventional Fertilizers Can Damage Your Soils Health

The use of conventional fertilizers can have devastating impacts on your soil’s health and biology. One of the most drastic effects comes from the use of Nitrogen-rich fertilizers which results in the loss of organic material and humus. This greatly reduces your soil’s ability to hold nutrients and makes it dependent on the application of fertilizer. 

2. Is Compost Tea a Fertilizer?

Compost Tea is a popular tool used by gardeners to improve the health of their plants and soils. While it does contain some important nutrients for plants it mostly contains beneficial microorganisms. These can protect your plants from disease but also makes locked up soil nutrients more available to your plant. 

3. When Should I Use Compost?

A well-made compost can be applied at any time of the year to give your plants a boost and ensure the health of your soil. It is most often recommended to apply compost when planting and can be applied throughout the season as a top dressing. A soil well amended with compost will have all the nutrients your plants need and provide protection against disease. Proper composting eliminates the need for conventional fertilizers.

4. When Should I Use Fertilizer?

Ideally, you won’t have to use fertilizer very often! If you give your soil adequate quantities of compost then your plants should have all the nutrients they need for healthy growth.

Certain fertilizers may be useful for certain plants at the end of the season to ensure proper production. Another use for fertilizers maybe if you’ve identified a specific nutrient deficiency in your soil that is better treated with a specific fertilizer. Unfortunately, the short-lived nature of fertilizers can make your plants dependent on this practice and less resilient to the natural environment!