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Is Compost Fertilizer?

Are compost and fertilizer the same thing – or are they different? Let’s find out..

Compost is not the same as fertilizer. Fertilizer feeds the plant and compost feeds the soil. Compost is an organic mixture of decaying fruits, vegetables, grass, eggshells, paper, and coffee grounds. Fertilizers can be organic or chemical and are made of nitrogen, phosphorus, and potassium (NPK).

Let’s dive down into the subject of compost and fertilizer in more detail, and discover the main differences between the two and what they should be used for…

Is Compost Fertilizer?

Is Fertilizer The Same As Compost?

Nope. Not the same. Old time gardeners say – fertilizer feeds the plant but compost feeds the soil.

Catchy saying, but what does it mean? Fertilizers are used when plants need to grow fast, or they lack specific chemicals to keep them healthy. Fertilizers are all about the plant.

Compost is all about the soil – but also about the environment. Compost adds nutrients like fertilizer does, but it also bulks up the structure of the soil. Healthy soils lead to a healthy microorganism community and that leads to healthy plant roots. 

Healthy soils hold water, so water can be conserved. Soils with a healthy structure aren’t blown or washed away easily. Some experts say that well structured soils help with carbon sequestration, too.

Composting kitchen scraps and lawn clippings helps reduce landfill volumes. 

The Definition of Compost

Compost is a freestyle, organic mixture of many types of decaying household and yard substances like fruits, vegetables, grass, eggshells, paper, and coffee grounds. You can buy compost at your local gardening or hardware store, or you can make it yourself.

There are no hard and fast rules for compost composition. Instead, there are guidelines for “browns” and “greens”. The browns portion is woody and rich in carbon. Brown things include woody plant stems, paper, and wood chips. The greens portion is rich in nitrogen and includes things like leaves, grass, and most food scraps. 

Many experts say the ideal ratio of browns to greens (or carbon to nitrogen) for most suburban gardens is 25:1.

Compost can be made from many, many substances. You can add the manure of many animals – or human manure, from a composting toilet. The list of what to add to compost is long – straw, sawdust, dry leaves, eggshells, grass, plant clippings, coffee grounds, vegetable scraps and fruit scraps. 

The list of what NOT to put in compost is also long – no meat, no fat, grease, oil, weeds that go to seed, invasive plants, dairy products, dog, pig or cat poop. 

The Definition of Fertilizer

Fertilizers can be organic or chemical, but they always have an extremely specific ratio of three chemicals. The chemicals are nitrogen (N), phosphorus (P) and potassium (K). The ratio is typically abbreviated to NPK. Fertilizers are required by law to state their NPK ratio clearly – even the organic type.

Nitrogen, phosphorus, and potassium are part of a group known as macronutrients or primary nutrients. The primary nutrients that plants need are carbon, hydrogen, nitrogen, oxygen, phosphorus, and potassium. 

Secondary nutrients are calcium, magnesium, and sulfur. Trace nutrients are boron, chlorine, copper, iron, manganese, molybdenum and zinc. 

Should I Use Compost or Fertilizer? (Especially Chemical Fertilizer)

Compost and fertilizer are very different. In general, compost is better for the environment. Composting not only adds nutrients to gardens but reduces landfill volumes and helps conserve water. The general rule is, unless you know you have a specific nutrient deficiency, don’t add fertilizer.

Compost adds nutrients to your soil – in a non-exact amount – but compost also adds woody bulk – structure – to the soil that helps the soil retain moisture. If you have sandy soil, compost will help bulk it up with organic material. 

If the water drains too fast from your garden beds, compost will help. Here’s a link with information about compost and water drainage.

Compost feeds worms and other beneficial bugs. You make think microbes are gross, but they are essential to soil health. Microbes like bacteria and fungi are the lowest of the low in the food pyramid. 

Not only are they necessary food for many other things, but they often form healthy, symbiotic partnerships with plant roots. Healthy root systems help plants fend off diseases.

Compost provides micronutrients like boron, cobalt, copper, iodine, iron, manganese, molybdenum, and zinc. The micronutrients come from the wide variety of compost materials like fruit peels, eggshells, nut shells and old cereal.

Experts say compost helps with carbon sequestration. Carbon is stored in the top 3′ of soil. If the soil is poor quality, the carbon won’t be held. Composting also helps with soil erosion by improving soil structure. Compost makes the soil harder to blow or wash away.

You should use fertilizer when your plants are suffering from the lack of certain nutrients, or your soil has specific deficiencies. There are many plant or gardening websites that will help you diagnose nutrient deficiencies in plants. 

If your plants just aren’t growing, have dead tissue around the leaves or yellow leaves you may have a deficiency of one of the primary nutrients. But what should you add? There are many test kits available online and in stores. 

Many experts say that the kits you buy may be trustworthy, but most are not. Your best bet is to take a sample of your soil to a lab. Do some online research or call your local extension agent for expert advice.

Not all labs do the same tests, so check first. Also, labs can’t test for nitrogen, which is very soluble. Nitrogen levels don’t stay constant in the soil if you water a lot or live in a rainy climate. While it’s tough to measure nitrogen in your garden, you can go to a website or ask your extension agent what your area’s nitrogen level is. 

Once you check your area levels, you’ll know whether you need to amend your soil for nitrogen. But experts agree – if your flowers, grass and vegetables are happy, then there’s no need to amend. 

Excess NPK is generally bad for the planet, so avoid adding it if you don’t need to.

Here’s a great reference about the differences between fertilizer and compost.

Fertiliser on young tree

Can You Put Fertilizer in Compost?

Yup. Compost and fertilizer can work together. For a green solution, try an organic fertilizer.

Unless you have tested your soil and know exactly what type (the NPK ratio) of fertilizer to use, then experts recommend using at 12-12-12 one. They say to use 1 cup of fertilizer for every 25 square feet of compost.

Fertilizer can overwhelm many microorganisms, so be sparing. Add the fertilizer in several doses over time and be sure to turn the compost at the same time. 

Some experts recommend using a soluble fertilizer, dissolved in water, and then sprinkled onto the compost.

Is Compost Manure?

Nope. Compost is not manure, but manure can be one ingredient in compost. The rule of thumb about manure is that it’s OK to mix in vegetarian animal manure, and some omnivorous animal manure, but not wholly carnivorous animals. And sorry, no pig manure.

Remember that most experts recommend a “brown” to “green” or carbon to nitrogen (or C/N) ratio of 25:1 for the average suburban garden. The ratio of cattle manure is 19:1. Sheep manure is 16:1. So, you have to mix it in with something with a higher ratio, like sawdust, which is 442:1. There are several online spreadsheets to help with the math.

Meat eating animals’ – and pigs’ – manure sometimes have pathogens and parasites that can infect humans. Experts say to avoid them in your compost. 

Also, some compost experts say to consider the source of your manure if you can. For example – cows. Cows will eat anything. Cows that eat in a pasture filled with noxious weeds may have the seeds in their manure. Pretty soon, you may have noxious weeds growing in your flower garden. 

For a weed-free manure, experienced composters recommend bunny “berries”.

Final Thoughts

So there we have it – compost is definitely not the same thing as fertilizer, and they are used for different things too.

I love the old saying that ‘fertilizer feeds the plant but compost feeds the soil’, as that’s by far the most succinct way of understanding this.

And although they are not the same, compost and fertilizer can be used together – and if you use an organic fertilizer the whole mix can be eco-friendly too. 🙂

Mark H.

Homeowner and property investor Mark H. aspires to bring you the very best outdoor living content, based on his years of experience managing outside spaces. Read more >