What type of soil should I use for my indoor plants?

There are lots of different variables to consider when creating an optimal environment for your house plants. One of these variables is the type of soil you use.

When it comes time to repot your plant, you'll need to make a decision about the type of soil you use. This article will help you to make an informed one.

Indoor Plant Soil 101

There are three important functions of soil to keep in mind. This first is that soil provides your plant with nutrients. The second is that it's where your plant gets its water from. 

Indoor plants need to have their soil replaced once the potting mix has been depleted. When considering which potting mix to use, consider the nutrients of the soil, the ph and how well it drains water.

How do you know when the soil needs to be replaced?
Re potting house plants

It depends. Often the need to re-pot your plant will come from the plant outgrowing its current pot. The way to tell when this is needed is when the roots start to grow out of the pot, or it generally looks overcrowded. Repotting is a good time to replace soil. However, if you want to keep your plant in its current pot, you will still need to replace the soil every ~1year or so, depending on the plant. If your plant is looking lackluster, it has been a while since you changed the soil and you are doing everything else right, this may be a sign that it's time for a soil change.


Potting mixes, the type that you'll find at your local garden centres, will likely contain a variety of soil types. They composition of the soil will vary depending on the purpose of each potting mix, for example, indoor plant potting mix will be different than outdoor potting mix.

 

What are the common types of soil?

Below are a list of the well know types of house plant soil.

1.Peat Moss

Peat moss is a popular soil type because of it's ability to absorb water (it can absorb 10-20 times its weight in water), it lightens the soil to create good drainage and it is packed with beneficial beneficial bacteria and natural fungicide.

Peat moss is sourced from peat bogs. It has taken ~1,000 years to produce the peat moss reservoirs that this soil is sourced from, so it's not exactly an easily renewable resource! Because of this, the use of peat moss is not considered a sustainable growing practice and it is encouraged to try to use find other similar sources of soil, or at least to use peat moss in moderation.

What are some alternatives to peat moss? Peat moss alternative can provide the drainage benefits of peat moss, but unfortunately they do not provide the same disease preventing qualities as peat moss. However this value can be achieved through different means.

Coir (pronounced koy-er)

Also known as coco-peat. This is made from the hairy outside layer of the husk on coconut shells. Coir lightens soil and retains water, similar to peat moss. 

Rice hulls

Rice hulls are a waste product in the production of rice for human consumption. They are thin and light, providing lighter soil, good drainage and good water absorption when added to other soil mixes. It's usually cheaper than Peat Moss and Coir, and a sustainable option

Rockwool

Produced from a mixture of volcanic rock, limestone and baked coal which is heated to high temperatures creating time fibres creating tiny fine fibres that are spun into what is know an rockwool. WE DO NOT RECOMMEND USING ROCKWOOL. Why? Because it is formed by creating near irreversible chemical bonds which means it is not biodegradable. It is suggested to have negative effects on the environment and human health. You can easily find more information on this topic on google.

Sphagnum Moss.

Bear with me as this may get a little confusing. Peat Moss, which we have referred to as Peat Moss in this article (which is the commonly know term), comes from a plant called Sphagnum Moss. Sphagnum peat moss (aka peat moss) is actually the dead decayed plant matter of sphagnum moss that settles at the bottom of sphagnum bogs (combined with other organic decayed matter).

Sphagnum moss is the living plant that grows on top of the bog. Unlike peat moss, it is harvested when it is alive. Sphagnum Moss can hold large quantities of water (up to 25 times as much water as their dry weight) which makes it a great soilless material for certain houseplants. Once dried, it can be mixed with peat to help with drainage and to bring down the ph level of the soil. It has a pH of about 4.0 so is great for making the soil more acidic.

Read more at Gardening Know How: Sphagnum Moss Vs. Sphagnum Peat Moss: Are Sphagnum Moss And Peat Moss The Same 

2. Perlite

Perlite is a form of volcanic glass that occurs naturally. You can pick it out because of its white colour. It is valued because of it's porous qualities that allows water to drain more readily and provides good aeration for plants. This is useful for plants that thrive in well drained soil, for example those that require soil to dry out completely between watering (e.g. cacti and succulents).

It also absorbs moisture on its exterior which makes it great for facilitating moisture to plant roots. It is naturally sterile and has a fairly neutral pH.

Note: you should use eye and mouth protection when dealing with Pearlite as it can aggravate existing respiratory conditions and cause eye irritation.

3. Vermiculite

Vermiculite is made from compressed dry flakes of a silicate material which is absorptive and spongy. It's colour is golden brown to dark brown. Unlike perlite, vermiculite is best used for plants that require soil to stay damp and to not dry out. It can absorb 3-4 times its volume when water is added. Because vermiculite acts like a sponge and absorbs water, it does not aerate the soil as well as perlite. Use a potting mix blend with Vermiculite for plants that link damp soil.

You can learn more about the difference between perlite and vermiculite in this article by Epic Gardening, Perlite vs Vermiculite: What’s the Difference?

4. Sand

I'm not going to go into this one in much detail. Sand helps to aid water drainage so can be useful for plants that don't like wet soil.

5. Compost

Compost provides your plant with nutrients to live off. Most potting mixes that you buy will likely contain slow release fertiliser or compost to feed your plant over a period of time. Peat Moss, for example, is a form of compost. If you want to try to make your own compost, I suggest you spend some time researching how to do this for indoor plants. Specifically, consider the preferred pH levels of the plant you are planning on potting or re-potting and make decisions about the compost you use accordingly.

6. Wood chips or bark

Wood chips and bark can provide a slow release of nutrients for plants and acts as a dense sponge. Before using wood chips or bank, please do your research as too much mulch can cause root rot.

Choosing your soil mix

It's common for houseplant growers to create their own soil mixes using a composition of the soil types used above, depending on the conditions in which the plants are being grown and the type of house plants. We'll be creating more resources to help you make good decisions about the type of soil you use for the houseplants we have listed on our store, specific for the NZ climate. In the meantime, there are lots of useful content on this topic on youtube and google.

Happy house planting!