Compost is a soil conditioner that adds organic matter, aerates the soil, feeds soil bacteria and worms and helps to improve the water holding capacity of the soil. Fertilizer is different to compost in that its main purpose is to add nutrients to the soil and feed the plants.
Some organic fertilizers like pelleted chicken manure contain both organic matter like compost and nutrients including nitrogen to feed plants.
Synthetic fertilizers are usually made for a specific purpose, are either high in nitrogen for good leaf growth or higher in potassium and phosphorus to promote flowering. This article will explore the top differences between compost and fertilizer and when you should use each one.
Key differences between compost and fertilizer
| Compost | Fertilizer | |
| Holds moisture | Improves moisture holding ability of soil | No improvement to water holding ability of the soil |
| Nutrients | Adds a small amount of nutrients | Able to deliver large amounts or specific nutrients quickly |
| Soil improver | Improves soil slowly over time | Increases nutrient levels rapidly |
| Feed worms | Yes | No |
| Soil bacteria | Improves soil bacteria populations | Small improvement |
| Drainage | Improves soil drainage to improve root growth | No effect on soil drainage |
| Time to apply | Apply compost when replanting vegetable beds or preparing soil for new plants or as a top dressing. | Apply routinely in spring and fall or apply to address a specific nutrients deficiency |
| Soil structure | Improves soil structure over time to improve drainage and friability | No effect |
| Sandy and clay soils | Improves drainage | No effect |
| Soil aeration | Improves aeration | No effect |
In this comparison I am considering fertilizers that are focused on nutrient delivery rather than fertilizer that also contain organic material such as aged manures or pelleted chicken manure.
Lots of fertilizers are synthetic or contain concentrated amounts of nutrients that are aimed to deliver specific nutrients for your plants.
Let’s explore the key differences between compost and concentrated fertilizers.
1. Moisture holding capacity of the soil
Compost contains lots of organic matter that helps to improve the water holding capacity of soil when it is mixed through. This is different to pure fertilizer which is added to boost the nutrient content of soil.
2. Nutrients
The main purpose of fertilizer is to deliver nutrients to your plants or lawn. This could be specific nutrients to address a deficiency such as magnesium or iron or it could be an all round fertilizer to support plant growth.
Fertilizers that have a high N-P-K ratio are great for lawns or to encourage fast leaf growth on trees and shrubs. Fertilizers with a higher potassium and phosphorus (K and P) ratio are great for promoting root health and flower growth. These are often referred to as fruit promoting fertilizers as they encourage the plant to set flowers and eventually fruit.
3. Soil improvers
The main purpose of compost is to improve the soil, to feed worms and soil bacteria, help to aerate the soil for plant roots and hold water. All of these improvements will help your plants to grow large and healthy.
Fertilizers have a different purpose and are mainly there to deliver nutrients to plants specific to their needs. Over time compost will add small amounts of nutrients to the soil but if a big amount is what you need then reach for a fertilizer.
Remember to get the best of both worlds and add pelleted chicken manure. It will still deliver some organic matter but delivers a concentrated range of nutrients at the same time.
4. Feed worms
Compost is great for feeding earthworms or to provide them with bedding in your soil. They will digest the compost and release any captured nutrients out into their castings so the plants can absorb it.
Worms will tend to avoid synthetic fertilizers however and instead will stick to eating organic matter.
5. Soil bacteria
Soil bacteria populations will increase with the addition of compost. Compost itself will contain a range of bacteria that are good for your soil and the organic matter will help to feed those naturally in the soil.
Fertilizers are not aimed to improve the soil bacteria populations but are there to deliver nutrients to plants. While soil bacteria will feed on these nutrients this is not the aim of fertilizer.
6. Drainage
Soil drainage will be improved if you dig compost through heavy clay soil. The organic matter will help to separate the clay particles and allow water to drain through. This is perfect to avoid plants from becoming waterlogged or developing root rot.
Fertilizers are not aimed to improve the drainage of your soils. The combination of compost and fertilizer is usually a great idea to get the soil reading for planting. When I am planting new fruit trees I will mix in aged manure, compost and pelleted chicken manure to gently fertilize before I plant.
7. Time to apply
Compost can be dug into the soil to prepare your garden beds for new plants, to repair an old vegetable bed or can be added at any time as a top dressing for mature trees. If you have a bare patch of soil to improve, dig some compost through and cover it with mulch.
The only time I wouldn’t add compost is through the frozen heart of winter. Leave the soil alone until it warms up in spring.
Fertilizers need to be added at a specific time and for a specific purpose depending on the plant, your soil and your goals. Citrus for example love a good feed at the start of each season with an all purpose fertilizer and then a top up with iron chelates and trace elements in spring.
Green vegetables will love a feed of nitrogen rich fertilizer 4 weeks after planting and growing trees love a dose in spring.
8. Soil structure
Compost will improve soil structure while specific fertilizers will not. Compost adds air, organic matter, and helps to break apart heavy clay soil to improve plant root growth. Remember to add compost to improve your soil and fertilizer to deliver specific nutrients to your plants.
9. Sandy soils
Compost will help sandy soils to hold water for longer. Mixing compost through helps to hold in the rain and prevent fast evaporation from the soil. Very sand soils will need a few applications of compost a few years in a row to get the best benefit.
Fertilizers will not improve sandy soils but will need compost to stop them from washing straight out. Nitrogen is lost from soil rapidly so mix in some compost before fertilizing.
10. Soil aeration
Compost will help to add oxygen to your soil which will help the aerobic bacteria to breed and grow. This will also encourage worms to dig up and digest the compost and release the nutrients to your plants. Digging compost through your soil or building up layers of compost and straw on top helps to create an amazing environment for your plants to grow.
Fertilizers in general do not improve the aeration of your soil but their nutrients will be delivered better if combined with compost.
When to use compost in your garden
Here are the key times to use compost in your garden.
- To improve soil drainage
- To improve the water holding capacity of the soil
- To improve soil and bacteria populations
- To build nutrients and improve soil over time
When to use fertilizer in your garden
Use fertilizer in your garden for the reasons below:
- To treat nutrient deficiencies
- To feed citrus at the start of spring
- To improve poor soils
- To boost leaf growth
- To encourage flower growth
Common fertilizer types
Here are the key fertilizers that we can add to our gardens. Check out my descriptions below, examples and the key times you should use each one.
| Fertilizer Type | Examples | When to Use |
| Fertilizers that also contain organic matter | Concentrated or aged manures that have been compressed, pelleted chicken manure, bone meal, blood meal | To dig through and imrpve soil for vegetable gardens When planting new trees Fertilize fruit trees |
| Inorganic or synthetic fertilizers | Lawn fertilizer, all purpose fertilizers, indoor fertilizers, fertilizers specific to a species ie. rose fertilizer, azalea fertilizer, succulent fertilizer etc. | For lawns, vegetables, indoor plants, roses, camelias or any specific plant you have. |
| Iron chelates | Powdered iron chelates Liquid iron chelates | Feed to citrus once per year in spring, to address and iron deficiency (Dark veins, light leaves) |
| Trace elements | Powdered trace elements Liquid trace elements | Feed to citrus in spring Address a trace element deficiency ie. Magnesium |
| Garden lime | Powdered garden lime | To reduce the acidity of the soil for plants that like alkaline soils |
| Epsom salts | Epsom salts as granules from grocery stores, health food stores or your garden center | To deliver magnesium to plants to treat or prevent a deficiency |
| Sulphate of potash | Powder | To improve the taste of fruit and increase flowering |
| Liquid fertilizers | Liquid fertilizers can be all-purpose, high in nitrogen, flower and fruit promoting or specific for a purpose ie. For roses | For the fast delivery of a specific nutrient. Liquid fertilizers can quickly address a nutrient deficiency or add nitrogen for fast growth. |
Compost vs Fertilizer | Summary
Compost and fertilizer are different and have different benefits for your garden. Compost will improve your soil over time, add air, hold moisture and feed worms and soil bacteria. Fertilizers are great to encourage plants to grow fast or to deliver specific nutrients to a particular type of plant. Lawns and citrus for example need fertilizer regularly to keep them growing strong and producing fruit.
You can get the best of both worlds by looking out for an organic, concentrated animal manure pellet. My favorite is pelleted chicken manure which adds both organic matter and nutrients to my soil.
Happy growing.
I am an accredited practicing dietitian, experienced gardener and a dedicated cook. I love writing and sharing my experience so you can learn from my successes and mistakes.