When it comes to nurturing indoor plants, one of the first considerations is the soil. Many gardeners wonder, “Can I use outdoor soil for indoor plants?”. While outdoor soil may seem like a convenient option, it’s not a good idea to use it. In this question and answer article we will explore and understand the differences in the needs of indoor plants. We will look at the characteristics of outdoor soil so that you can see why they are incompatible.
Can I use garden soil for my indoor plants?
In short, it’s not a good idea for the following reasons:
1. Moisture Retention
Outdoor soil, or garden soil, is typically rich in organic matter and organic materials. It’s tailored to the needs of outdoor plants, providing the necessary nutrients for healthy growth and supporting a diverse ecosystem of organisms. It is often referred to as ‘heavy’, this means that the soil can contain a lot of clay particles. Clay particles are small and flat compared to other soil particles.
Surprisingly typical soil is made up of around 50% air and water. How the spaces for air and water are formed is crucial to understanding what happens when using outdoor soil for indoor plants. A heavy, outdoor soil tends to have small air spaces or pockets. This is fine because the water can be retained in the soil and the roots can grow far and wide.
On the other hand, indoor plants have specific requirements that differ from those of outdoor plants. One of the most crucial factors is moisture retention. While outdoor soil may excel at retaining water, indoor plants are susceptible to root rot if exposed to too much moisture.
This means that an indoor soil needs to have bigger air spaces held open by bigger particles within the soil. Hence why materials like vermiculite and coco coir are used in indoor potting mixes. And outdoor soil which contains the smaller clay particles is not a good idea to use.
Conversely some outdoor soils may be sandy, this type of soil has large air and water spaces and is free draining. However it is still not suitable for indoor plants because of the balance between moisture retention and drainage. Whilst drainage is good for indoor plants, there is still a requirement for some moisture retention.
2. Pests and disease
Furthermore, outdoor soil may contain weed seeds, fungus gnats, and other pests that can harm indoor plants. All of these things may not be visible to the naked eye. And whilst there are those who would promote the idea of sterilising outdoor soil for use in indoor plants to remove disease I am not in favour of this. The process of sterilising will destroy the soil structure. So whilst there may not be any disease in the sterilised soil, the moisture retaining capabilities would be poor. Overall it would be a waste of time and effort.
However, when it comes to indoor plants, the story can be quite different, whether they are tropical plants, succulents, snake plants, or fiddle-leaf figs, these plants have never been exposed to the pests and diseases from the outdoors. Outdoor plants have some resistance to these plants which indoor plants simply don’t have. For this reason it is important to protect the indoor plants from exposure to such pests and disease.
So, what’s the best soil for indoor potted plants?
The answer lies in creating or selecting the right potting mix. A good indoor potting soil should have a balance of organic matter to provide essential nutrients while promoting good drainage and aeration. Here are some examples:
- Growing medium
A growing medium balances moisture retention and drainage, coconut coir is a great example
2. Soil aeration
An inert material that helps with soil aeration by making spaces available within the soil. It allows the roots to have access to oxygen within the soil. A good soil aerator like perlite will also help with moisture retention
3. Essential nutrients
Worm castings are a great source of essential nutrients. They provide slow release nutrients as well as providing beneficial microorganisms to the soil. Pine bark is another example of a material that provides slow release nutrients to your houseplants it also helps to create a well drained aerated soil, which is great for the health of plant roots.
A word on sphagnum peat moss
I would NEVER buy a product containing peat moss. The reason that it is used in potting mixes is because its structure provides an excellent balance between moisture retention and drainage, it manages to retain water without causing root rot. It is also lightweight making it cheap to transport. However extracting peat, a finite resource, from peat bogs is an environmental disaster. Peat bogs are vital for storing CO2. Extracting peat releases that CO2 into the atmosphere and prevents the restoration of the bogs which could trap and hold more CO2 if left in place.
Given that there are alternative organic materials with good water retention properties, such as coconut coir, which are renewable resources that are as good as peat, it should not be used. I make use of my consumer buying power by not supporting the products that contain peat.
Do you need different soil for indoor plants?
When selecting or creating a potting mix for indoor plants, consider factors such as the type of plant, its moisture needs, and its preference for well-drained soil. For example, succulents require a well-draining mix to prevent root rot, while tropical plants may prefer a mix that retains more moisture.
In some cases, you may choose to customise your potting mix by blending different ingredients to meet the specific needs of your plants. Just remember to prioritise good soil structure, moisture retention, and drainage to ensure the health and vitality of your indoor plants.
What happens if you use outdoor potting soil for indoor plants?
Outdoor potting soils will not have the pests and diseases that outdoor garden soil might have. However their soil structure is not optimised for the drainage moisture retention balance that indoor plants require. Having said that, there are some mixes that have been blended to be suitable for both indoor and outdoor use. If you want to be able to use a potting soil for both then look out for these.
Is there a difference between indoor and outdoor potting soil?
Yes there is, although some mixes are designed to be suitable for both. The main difference for indoor potting soil is the balance between moisture retention and drainage that is designed and built into the potting mix. This is to ensure that the plant gets sufficient water without getting too much water and root rot occurring.
Avoid using outdoor soil directly in indoor pots, as it may lead to poor drainage and waterlogged roots. Instead, opt for specialty mixes tailored to the needs of indoor plants. These mixes are often available at local garden centers or online retailers and may include ingredients such as pine bark, perlite, or vermiculite for added aeration and drainage.
How do you prepare potting soil for indoor plants?
One of the simplest ways is to buy a suitable indoor potting mix for the type of plant. It is a good idea to repot indoor plants every couple of years with a new potting mix. This will give the plant new vigour and energy from the fresh nutrients within the organic matter. If you want to mix your own potting mix consider carefully all of the elements outlined above; growing medium, aeration and nutrients. Gather your ingredients and mix them together in a bucket or container until they are well mixed. Then use to pot or repot indoor plants. Always make sure to use a pot that is a little bigger than the plant itself, without being too much bigger. The roots need to be able to grow, but if there is too much space this will not help to promote good growth.
In conclusion, while outdoor soil may be suitable for outdoor gardening, it’s not the best choice for indoor plants. Creating or selecting the right potting mix is essential for providing the necessary nutrients, moisture retention, and drainage that indoor plants need to thrive. By understanding the unique needs of indoor plants and investing in the right soil mix, you can enjoy healthy and vibrant greenery in your home year-round.