Until you start gardening, “soil” and “dirt” are interchangeable. Once you’ve tried to grow anything by just digging a hole in the ground, you’ll notice that the ground is probably full of clay or sand, and doesn’t break up into nice crumbly bits. That’s because soil health and the content of the soil matters, when you’re growing plants. Good soil needs organic material that will break up the clay, and healthy fungi like mycellium, nutrients from earth minerals and compost like nitrogen and calcium, and air pockets that will create help allow water to reach roots.
This is important in the garden, but it’s even more important in potted plants, where our green friends have access to a limited amount of soil. To create good soil conditions, there are a wealth of products and conditioners that gardeners apply. There are a whole class of additives that can help create those air pockets, and they include products like perlite, vermiculite, peat moss and coco coir. Knowing which to choose, and when to use them, is critical—because like fertilizer, these are more expensive products than compost, and using too much of them is just as problematic as not using them at all.
What are perlite, vermiculite, peat moss and coco coir?
Perlite and vermiculite are volcanic materials that have been crushed and heated until they are small granules. They’re both porous, and they hold water well, although vermiculite is capable of holding more than perlite. Peat is a spongy moss that is harvested specifically from bogs, which have digested the Sphagnum moss, forming a bottom layer of dead moss that holds a lot of moisture perfect as a soil amendment. Since harvesting peat is unsustainable, there’s a great movement to use alternatives, like coco coir, which is a renewable product of the coconut fiber industry, but has similar properties to peat.
While each of these products can create air pockets and hold onto water, they behave differently in your soil. Perlite and vermiculite are granular, while peat and coir are more fibrous, and will hold your soil together more. Every one of these items can be found at your local nursery, and bringing in a sample of your soil will allow the garden center to help figure out which best fits your needs.
Using perlite and other amendments in potted plants
Plant roots are the most important part of any plant—this is how they obtain nutrients, hydration, and stability. Indoor plants have access to limited resources in a pot. While plants outside would stretch their roots in search of what they need, indoor plants, for the most part, can’t do that. If you’ve ever wondered why plants form aerial roots, that’s the plant trying to find those additional resources, outside the pot.
In other words, the soil in the pot is extremely important. It needs to include all the nutrients for the plant. Pots are also quite drying, because we use drainage holes in them, and the soil is exposed to air moving around it, and there’s just less soil altogether to hold onto moisture. Those air pockets are even more important here, which is why potting soil usually contains not just perlite or vermiculite, but also peat or coir.
If you’re going to make your own potting soil, the addition of these products, in the right amount, can help ensure your plant’s roots have better moisture levels. The right amount is going to be determined by your plants. For those more likely to experience root rot, you want more perlite—go as high as 50%. But for most of your plants, you’ll start with a basic recipe: one part perlite or vermiculite, one part peat or coir, and two parts compost mixed with a slow-release fertilizer like Osmocote, per the instructions on the fertilizer package.
Raised beds can use help, too
In your garden, using these amendments becomes a more costly endeavor. It also isn’t as necessary—a large mass of soil can hold onto moisture more easily. Outside, fungal structures like mycelium will form within your soil, creating those air pockets, and incorporating in large amounts of compost or compostable goods like leaves or chips will help create soil that has the right texture and composition. Compost invites worms, which will eat their way through the composting goods, also helping to create air pockets.
Still, if you have particularly clay-filled soil and want to break it up, adding perlite or vermiculite can help, if it is incorporated into the soil well, in amounts varying from 10-50%. That’s a huge swing, but the amount of soil you have in your garden is vast, and any amount you add is going to help to create fluffier soil. Start small and work your way up, season by season, to see what works, and also based on what you’re growing. Generally, you don’t see the addition of peat and coir in raised beds due to cost at this scale, but you may find them in commercial soil mixes.
If you have planters outside, they subscribe to the same idea as indoor potted plants and should get similar amendments, including perlite, vermiculite, peat or coir.
Understanding and improving the content of your soil is a never-ending process. Even if you start with perfect bagged soil, plants drain your soil of resources, causing the soil to need new inputs all the time. What works one year may not work the next due to environmental influences or new plants with different needs. While ensuring you have air pockets and hydration in your soil by using these four components are important, it’s equally important to consider all the other inputs you need. The best way to do that is to have your soil tested, and/or bring a sample of it to your local farm store or garden center and ask for help.
