Compost can be used instead of potting soil in small pots to raise seedings in the short term but to grow plants longer the compost needs to be improved. Potting soil contains other ingredients such as minerals from ground rocks or sand which adds structure to support good root growth. Potting soil has been designed to drain well, have a consistent pH, and to support plant roots to grow strong.
Compost is a great ingredient to add to potting mix to add extra organic matter, soil bacteria and for good drainage. Adding compost to potting soil so that is around ¼ of the mix is a great way to improve soil in raised garden beds and pots.
This article will explore how to use compost in combination with potting soil, what you need to do to the soil to keep your plants growing well over the long term.
Can I use only compost as potting soil?
You can only use compost as potting soil but it is best to use it in combination with potting soil or to add extra perlite and river sand for structure. Compost can vary and each mix will have a different pH, nutrients, minerals and will drain differently. Potting mix is specially designed for good plant growth but compost can boost the mix.
What to know about using compost instead of potting soil
Here are a few things to know if you choose to use compost instead of potting soil.
1. It can have variable pH
Compost can have an unknow pH which will differ for every mix. It will depend on what it is was made from including the range of food scraps, animal manures, coffee grounds and fall leaves. You can test your compost but it can vary throughout the mix.
Remember that some plants love acidity like blueberries and azaleas and other don’t. Test the pH to be sure before you plant straight into compost.
2. It will not have the structure of potting soil
Compost is made from organic materials which can include shredded bark, lawn clippings food scraps and coffee grounds but it will not contain sand. For long term plant health and to support their root growth potting soils will often add a structural element.
Potting soils will contain a mix of peat moss, compost, sand, perlite, vermiculite and coconut coir. Potting soils will differ depending on the brand but they are designed to hold their structure for longer than just compost alone.
Potting soil will not drop in level as quickly as straight compost. It will hold the roots firmly and help the plant stay upright in strong winds or if it is knocked.
3. It will support plant health on its own in the short term
Plants can grow successfully in compost for a season or for a few months. Compost is made up of pure organic material it will continue to break down and drop in level over time.
While this does happen with potting soil, it will also have other structural components that will break down slower. This includes sand, peat moss or coconut coir and perlite. This allows plants to grow in the potting soil for longer than pure compost.
4. Water will drain through faster
Compost is great for helping soil to hold water for longer it can dry out quicker if it is used as potting soil. Compost made from lots of brown materials like hay and bark mulch will have particles that are further apart. Water can drain through fast and the pot can dry out if it is not watered regularly.
5. Compost can stay too moist
Compost that is very dense or broken down further can actually hold water for longer, staying moist for too long and causing roots to rot. I planted a strawberry plant in straight compost and after around 1 month the compost was still very wet after the first watering. This compost was dense and well broken down and was not supporting good plant growth on its own.
How to improve compost to use as potting soil
If you want to use compost as potting soil then adding a few vital ingredients will help it to sustain plant growth for longer.
1. Coarse sand
Mixing through up to ¼ of the mix with coarse river sand will help the compost to have the structure to support plant root growth. Sand will help to stabilize the mix and will prevent the level from dropping as quickly.
Perlite is a great way to add more oxygen and improve drainage when using compost as potting soil. Perlite is basically baked rock minerals which puff up in the heating process to allow good drainage and to hold more air.
3. Slow release fertilizer
Add a handful or two of pelleted chicken manure to slowly release nutrients including nitrogen to your plants. Pelleted chicken manure is a gentle fertilizer that will help your plants to grow well in a compost mix and feed them for around 3 months.
4. Top up with potting mix
If you have extra or old potting mix laying around you can mix this in with the compost to improve the mix. Even old potting soil will still contain organic matter and sand which will allow the structure of the soil mix to be improved.
Can you plant directly in compost?
Some plants will naturally pop up in your compost pile and thrive. Pumpkin, cucumbers and tomato are common plants that can grow from seeds leftover in compost. Pumpkins can grow well over summer in the top of your compost because it is moist and nutrient dense.
Plants that grow for longer than a season will grow much better in a soil and compost mix. Potting soil has the structure that helps roots to grow solidly in the ground. This will stabilize the plant and stop it from blowing over.
Potting soil will also contain minerals that support plant growth over the long term. Mix compost in the mix in a ratio of 1:4 so it can boost the nutrients and soil bacteria levels and extra nutrients.
Can you use compost instead of potting soil? | Summary
Compost works best with mixed with other ingredients to improve its drainage, oxygen levels and structure before using it straight as a potting mix. Potting mixes will usually contain peat, coir or sand to help them to stay structurally sound as the plant grows and stop the level from dropping rapidly. Improve homemade compost with river sand and perlite to make a stable mix for your plants to grow in.
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.