Compost vs. Fertilizer, Explained

by Matt Gibson and Erin Marissa Russell

Compost or fertilizer? What’s the difference? Which one should you be using in your yard or garden? Or should you use both? Everyone who is interested in growing their own plants wants to create a healthy garden environment. To keep plants growing as vigorously as possible, many gardeners consider using compost and fertilizer. Compost is an all-natural organic material to improve the quality and tilth of your soil, while fertilizer can be either organic or chemical, and contains specific elements in exact ratios that plants can use to grow. We’ll explain this in detail.

Experienced gardeners have no problem deciding whether they will use compost, fertilizer, or both in their gardens. Meanwhile, those of us that are new to gardening may have a little bit more trouble deciding what to use to keep their garden producing at a high level.

In this article we will break down what compost is and what fertilizer is, respectively. We will break down the commonalities and differences between the two soil additives, and we also discuss the advantages and disadvantages of each.

What Is Compost?

Compost is basically a mixture of decaying natural substances, such as animals, vegetables, fruits, leaves, eggshells, coffee grounds and grasses. The decaying substances are then added into the soil to make the soil fertile and to boost the soil with lots of nutritional elements that plants need to grow and function at their best. Compost and other soil amendments are not rated with NPK (like fertilizer). With compost, you are generally trying to make your soil better and healthier overall by adding rich, organic material that improves the overall health and tilth of the soil.

Compost is just one of many soil amendments that alter the soil to improve its physical or chemical properties. Other soil amendments include lime, manure, leaf mold, ground bark, fertilizers, materials to change water retention levels, clay, gypsum, and more.

Benefits of Compost

The advantages of using compost are as follows:

• Works as a good food for the entire soil
• Keeps the natural health of the soil at high levels
• Helps for the proper growth of beneficial microbes
• Provides the soil with sufficient nutritional elements
• Helps the soil retain moisture
• Promotes proper growth of plants
• Helps the soil fight against disease
• Helps to control and limit weeds
• Works to feed soil directly
• Because you can make your own compost, it is much cheaper and environmentally friendly than fertilizer.

Compost sounds like a winner for multiple reasons, but using compost in your garden also has its drawbacks.

Disadvantages of Using Compost

The disadvantages of using compost are as follows:

• When using compost, you need to make sure that none of your garden plants have soil-borne pathogens
• Compost takes more time to create an effect in the soil
• Making compost can be a very time-consuming process
• Making compost requires more physical labor than most fertilizers
• As compost is made from decaying organic matter, sometimes compost spreads can be kind of smelly.
• Compost takes more space to store and process.
• An advantage to

What Is Fertilizer?

Fertilizer comes in organic and inorganic forms. Organic fertilizers, like manure, compost, or bone meal, are derived strictly from plant and animal sources. Inorganic fertilizers are made from minerals, gasses, and inorganic waste materials. Fertilizers made from organic sources feed and enrich the soil. Inorganic, or synthetic fertilizers are fast acting, but can burn and damage plants. 

All fertilizers are required by law to use the NPK ratio system to measure the levels of Nitrogen, Phosphorus, and Potassium (K on periodic chart) within them. So a fertilizer that is rated 12/12/12 has a balanced amount of each nutrient within it. Your soil or plants might be in need of one of the items versus the others, so you can choose a fertilizer based on what you need. If you’re growing something that needs a lot of nitrogen, you’d want one with a high ratio of it. 

Advantages of Using Fertilizer

The advantages of using fertilizer in the garden are as follows:

• Helps plants grow faster
• Adds nutritional elements to the soil
• Ensures the fastest growth of plants
• Helps eliminate certain soil deficiencies
• Fulfills the specific needs of a wide range of plants
• Provides macro and micro-nutrients
• Provides essential nutrients that every soil needs, including nitrogen, phosphorus, potassium, and magnesium 

Disadvantages of Using Fertilizer

There are many more disadvantages to using fertilizer in the soil than there are disadvantages of using compost. The disadvantages of using fertilizer are as follows:

• There is a risk of overloading the soil with nutrients when using fertilizer
• Can cause severe environmental pollution
• Can pollute groundwater
• Certain fertilizers can upset the symbiotic relationship of microbes in the soil
• Fertilizers can break the microbial balance in the soil
• Certain fertilizers are harmful to the environment as well as the human body
• Fertilizers can be more expensive to use than compost, which is often made from food trash, leftover leaves, and kitchen scraps. 

Common Traits of Compost and Fertilizer

Compost and fertilizers can work together really well when used properly. The organic matter in compost sponges up nutrients within fertilizer and stores them until they are needed by plants. Compost also provides many nutrients that plants need in very small amounts, like boron. You can use fertilizer without using compost buy why miss a chance to increase soil fertility and your soil’s ability to hold moisture. Soil that is amended regularly with compost becomes rich, dark, and crumbly, often needing much less fertilizer than soil that hasn’t been regularly treated with compost.

Differences Between Compost and Fertilizer

The easiest way to tell the difference between compost and fertilizer is that compost feeds the soil while fertilizer feeds the plants. Fertilizer adds to the soil’s nutrient levels. But instead of feeding the soil food web, the ingredients in fertilizers are intended to meet the needs of fast-growing plants. Sometimes the amount of compost gardeners are directed to add to the soil is very general, fertilizer application rates are based on the needs of plants. Either organic or conventional fertilizers work well for veggies, but organic fertilizers have been shown to be better for balancing the soil food web. Chemical fertilizer can also feed composting, but continual use may throw off your soil’s chemistry and discourage beneficial insects and microbes.

Common Questions and Answers About Compost and Fertilizer

Are eggshells good for compost?

You can add crushed or whole eggplants to your compost pile to increase the calcium in the finished compost. Crushing the eggshells before adding them to the pile will help them break down into the compost more quickly.

Can I put bread in compost?

Stale or moldy bread can be torn into small pieces and added to your compost pile. Be sure to turn the pile or mix the bread into it carefully after adding the pieces of bread to avoid attracting rats, mice, and other pests to your compost pile. Some types of bread count as green compost material (adding nitrogen to compost) and some count as brown compost material (adding carbon to compost)..

Can I put compost in potted plants?

You can add compost to your potted plants, but they don’t need much, and they don’t need it frequently. Add about an inch of compost to the top of the surface of your potted plants’ soil twice a year for best results. The same rule applies to using compost in window boxes. You can also make a potting soil out of two parts screened compost to one part sand or perlite.

Can I put moldy fruit in my compost?

Yes, you can use moldy fruit in compost as long as you meet a few conditions. Put the moldy food in the center of the pile on top of dry brown material (like sawdust, ashes, straw, or leaves). The reason the position matters is that the center of your compost pile gets hot quicker and stays hot longer than the rest of the pile. Cover the food with material like grass clippings, cornstalks, or shredded paper. Mix the fruit well into the pile by turning it frequently so pests aren’t attracted to food sitting on top of the compost pile.

Can I put potting soil in my compost?

Many gardeners don’t realize that plants use up the nutrients in potting soil after just one year of container gardening. However, you can add last year’s potting soil from your containers to your compost pile. Keep in mind that the bits of perlite in your potting soil won’t break down in the compost pile.

Can I put tomatoes in my compost?

If for some reason you don’t want to eat your tomatoes, you can add them to your compost pile. That said, be advised that the seeds inside the tomatoes may sprout next spring inside the pile or wherever the compost is placed. If this happens, you can pull them like weeds or transplant the tomato plants to the right part of your garden. Please note that if your tomatoes have contagious plant diseases, you should only add them to a hot compost pile. If your compost isn’t hot, burn or bury the diseased tomatoes.

Can too much compost hurt plants?

Although it’s possible to get too much compost in your garden soil, resulting in an overload of certain nutrients that’s bad for plants, this doesn’t usually happen. If you’re worried, though, limit yourself to one or two inches per foot of loosened soil when you’re planting a new bed and half an inch to an inch each fall on top of existing beds.

Can onions go in compost?

Cooked and raw onions can be used in compost piles. The only exclusion is onions that have soaked up lots of fatty or meaty juices, which shouldn’t be used because they can attract pests to your compost pile. If you’re adding in raw onions, you may wish to chop them into halves or quarters to discourage the onions sprouting into new plants. It also takes a good bit of moisture to start breaking down a whole raw onion, so mix these well into the center of your compost pile.

Can orange peels go in compost?

Unless your composting approach is vermicomposting, orange peels are an excellent addition to your compost pile. The orange peels will add phosphorus, nitrogen, and potassium to the completed compost. To reduce the amount of time it takes for orange peels to break down, you may wish to break them into small pieces before adding them to your compost heap.

Can you fill a raised bed with just compost?

It’s not a good idea to fill a raised bed or any are of your garden completely with compost. The best approach is for your garden’s soil to consist of five percent organic material. As a rule of thumb, you can add one or two inches of compost per foot of loosened soil when you’re creating new beds, or in the fall, you can spread half an inch to an inch of compost on top of existing beds.

Can you keep adding to a compost pile?

Yes, you can keep adding to your compost pile whenever you have new scraps that will work for it. This style of composting is called cold or passive composting. This type of composting takes a while to see results. You may need to wait between three and eight months before you have completed compost you can use. Start with about three inches of moistened coarse, strawlike material, like plant stems, leaves, twigs, hay, or straw, and do your best to add brown materials on top of green materials. Your finished compost that’s ready to use will be available from the bottom of the bin or pile once it’s ready.

Can you mix compost and manure?

You may add horse, cow, sheep, rabbit, or chicken manure to your compost mixture, and doing so actually reduces the risk of burning plants with fresh, unadulterated manure. Do not use manure from humans, dogs, or cats in composting. Composted manure may be used as a mulch for plants once it’s had time in the heap, bin, or pile to break down. Another option is to till manure into the soil long before spring planting, like in the fall or winter, so it has time to break down. Adding manure to your garden’s soil helps loosen too-firm, compacted dirt and adds moisture to sandy soils. Manure also increases the carbon and nitrogen available for your plants in the soil as well as lowering the risk of runoff or leaching nitrates from the soil.

Can you put banana peels in compost?

Banana peels are an excellent addition to compost. In addition to breaking down quickly, they’re full of nutrients to add to your soil, such as calcium, magnesium, phosphates, potassium, sodium, and sulphur. If you want your banana peels to break down even quicker, you can cut them into small pieces.

Can you put dead plants in compost?

All kinds of plant material is suitable for composting, including cut grass, leaves, weeds and other gardening trash, straw or hay, and dead plants. Only include debris from plants with contagious diseases if you use hot composting. Otherwise, this material should be burned or buried. Plant materials you should not compost include seed-containing invasive weeds, branches, wood chunks, and anything overly soggy.

Can you use too much organic fertilizer?

Yes, it’s possible for your plants to overdose on fertilizer, even when it’s organic. The results of getting too much fertilizer include burned roots, yellowing leaves, “leggy” or spindly growth, lots of foliage but no fruiting, and increased risk of pests and disease. For best results, follow the instructions provided by your fertilizer’s manufacturer, or you might decide to add fertilizer before the spring growing season begins to reduce the risk of burning your plants.

Do you need to fertilize if you use compost?

Whether you’ll need to use fertilizer along with your compost depends on what you’re growing and what your soil is like. The best way to know whether your soil will be able to provide the nutrients your plants need to thrive with a boost from compost now and then or whether additional fertilizer is needed is to do a soil test. Most of the time, these are available through your local Extension office, which you can find by choosing your state from this map on the National Pesticide Information Center website.

Does compost need sunshine?

You can set your compost pile, heap, or bin up either in the sun or in the shade, but be aware that letting your compost get plenty of sunlight will speed up the decomposition process. As the temperature increases, so does the activity of the bacteria and fungi in the compost. If you choose a location in full sun, you may need to moisten your compost heap occasionally.

How do I add compost to my garden?

To avoid adding too much compost to your garden at a time, follow this rule of thumb. For new garden beds, add one or two inches of compost per foot of loosened soil. For existing garden beds, add half an inch to an inch of compost in the fall.

How do I apply fertilizer to my garden?

For results specific to your fertilizer, you should follow the directions provided by the fertilizer manufacturer that are printed on the package. Most balanced fertilizers are added every three or four weeks during the growing season. Organic soils may require increased fertilizer, and clay soils may need fertilizer less often, like every four to six weeks. Because there are so many different types of fertilizer, make sure to check the instructions for dosage and application for the type you’re using.

How do you know when compost is ready?

You can tell when your compost is ready just by looking at it, smelling it, and touching it. When your compost looks like fertile, dark soil instead of a bunch of rotting food scraps, it’s ready to use. You won’t be able to recognize the ingredients making up the compost when it’s finished, and finished hot compost doesn’t heat up, even when it’s mixed. Compost that’s ready to use should not feel slimy to the touch. Depending on the style of composting you’re using, it will take anywhere from one month to two years to create finished compost. Allowing a few months after your compost looks done for it to “cure” helps ensure it’s chemically stable.

How do you mix compost into soil?

If you’re digging in an area to start a new garden bed, you can add one or two inches of compost per foot of loosened soil. For existing garden beds, spread half an inch to an inch of compost over the bed in the fall.

How long can you keep compost?

You can keep compost if you intend to use it the following season. Holding onto your finished compost for longer than that can result in lowered nutrition in the compost. To store your compost, you can cover the pile or heap with a tarp or put the finished compost into trash cans with lids or plastic bags. If you choose plastic bags or trash cans, open them up periodically to give things a stir and check moisture levels. The bottom will be damper than the top, so pull that bottom layer up to keep things uniform.

How long does it take for compost to turn into soil?

Depending on the type of composting you’re doing, it can take anywhere from one month to two years for the compost to be complete and ready to use. You know your compost is ready when it looks, smells, and feels like dark, fertile soil. You won’t be able to recognize its ingredients, and if you use hot compost, it will stop heating even when it’s mixed. Allow it to rest and to cure for a few months after this point to make sure it’s chemically stable. You can speed up the composting process by limiting your pile to one cubic yard, turning the materials once a week, and keeping your ratio of carbon to nitrogen around 20 to one. Keeping the pile moist and shredding or chopping the materials you add to your compost also makes things decompose faster.

How long does it take for organic fertilizer to work?

It will take microorganisms in the soil between two and six weeks (depending on soil temperature and moisture level) to break down organic fertilizers so the nutrients they hold are available for your plants. To ensure you’re using your fertilizer correctly, follow the instructions the manufacturer has provided. They’re usually printed on the box for your convenience.

How long should fertilizer be down before it rains?

Let your fertilizer rest on the soil for two days before there’s any rain forecasted. Rain before this time can wash the fertilizer away before it has a chance to become part of your soil. Also, if it rains right after you’ve applied fertilizer, your fertilizer may start running off into nearby lakes, rivers, and streams. This disrupts the nearby ecosystem and reduces available oxygen in the water.

How much fertilizer do I need?

To make sure you don’t give your plants too much or too little fertilizer, rely on the instructions provided by the manufacturer that should be printed on the package.

How often should I turn my compost pile?

Turn an active, hot compost pile every three days until the compost stops heating up. Failure to heat even when mixed is a sign that tells you the compost is almost ready. (Just give it a couple more weeks to cure.) If you don’t use hot compost, you can scale back to turning your compost once per week.

Is compost a fertilizer or soil amendment?

A soil amendment is anything you add to your garden soil to increase its quality and make it better suited for gardening. Examples of soil amendments include peat moss, wood chips, straw, manure, grass clippings, gypsum, wood ash, vermiculite, and perlite. Sometimes, the amendment you need to use for your soil might be fertilizer. A fertilizer is anything that provides the plants you grow with the nutrients they need to thrive, including compost. That means that compost is both a fertilizer and a soil amendment.

Is compost good for all plants?

Compost benefits all kinds of plants, as long as you don’t give them too much. A good rule of thumb is to use one or two inches per foot of loosened soil when you’re digging new garden beds and half an inch to one inch over existing beds in the fall.

Should I add worms to my compost?

There’s no need for you to add worms to your compost area, though it won’t hurt to throw them in. Most of the time, the worms in your yard will find your compost pile on their own. The exception to this rule is vermicomposting, which uses lots of worms that you’ll need to manage and care for yourself.

Should I water plants before fertilizing?

Granular fertilizers should be followed with plenty of water to prevent burning your plants. Of course, water-soluble fertilizers are applied while you water your plants. There is no need to water plants before fertilizing unless recommended by your fertilizer manufacturer.

Should you mix compost with soil?

For best results, your garden soil should be made up of about five percent compost. You can either mix the compost into the soil or apply it on top like you would mulch. A good rule is to apply one or two inches per foot of loosened dirt to the top of a new garden bed and half an inch to one inch over existing beds each fall.

What foods cannot be composted?

Foods you should not compost include meat, skin, bones, dairy products, greasy and fatty or spicy foods, sweets, salad with dressing. Unless your style is hot composting, you should also not include gardening debris or plant matter that has a contagious disease. Instead, burn or bury this trash. Non-food items that you should not place into your compost pile include human or pet manure, branches and wood chunks, coal fire ash, sawdust from treated wood, tea or coffee bags (grounds or leaves are OK), invasive weeds that have gone to seed, lots of soggy material, plastic, metal, glass, glossy or coated paper, wood ashes, lime, stickers/labels on produce, BBQ charcoal, synthetic fertilizer, and anything too heavy to decompose, like heavy cardboard.

What is the ratio of compost to soil?

The ratio of compost to soil varies depending on what you’re growing. For potted plants, use a mix that’s 20 to 50 percent compost. A vegetable garden, however, just needs one or two inches per foot of loosened earth for a new bed or, for existing beds, half an inch to an inch each fall.

When should fertilizer be applied to plants?

For instructions specific to the type of fertilizer you’re using, check the packaging for guidelines from the manufacturer. Different types of fertilizers will have different dosages and application instructions as well as schedules for application.

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