How to Add Perlite to Soil (and When to Do It)
Until you start gardening, “soil” and “dirt” are used interchangeably. If you try to grow something by simply digging a hole in the ground, you will notice that the soil is probably full of clay or sand and does not break down into nice crumbly pieces. This is because soil health and soil composition matter when you grow plants. Good soil needs organic material that breaks up clay and healthy fungi such as mycelium, nutrients from soil minerals and compost such as nitrogen and calcium, and air pockets that help water reach the roots.
This is important in the garden, but even more important for potted plants, where our green friends have access to a limited amount of soil. To create good soil conditions, gardeners use a variety of products and conditioners. There is a whole class of additives that can help create these air pockets, and these include products such as perlite, vermiculite, peat moss and coir. Knowing what to choose and when to use them is important because, like fertilizers, they 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 and coir?
Perlite and vermiculite are volcanic materials that have been crushed and heated into fine granules. They are both porous and hold water well, although vermiculite can hold more water than perlite. Peat is a spongy moss that is specially collected from bogs that have digested sphagnum moss, forming a bottom layer of dead moss that holds a lot of moisture and is ideal as a soil fertilizer. Since peat harvesting is unsustainable, there is a large movement to use alternatives such as coir , which is a renewable product of coir production but has properties similar to peat.
While each of these products can create air pockets and hold water, they behave differently in soil. Perlite and vermiculite have a granular structure, while peat and coir are more fibrous and hold the soil together better. Each of these items can be found at your local nursery, and a sample of your soil will allow the garden center to determine which one will best suit your needs.
Using Perlite and Other Additives in Potted Plants
Plant roots are the most important part of any plant – this is how they get nutrients, moisture and stability. Houseplants have access to limited pot resources. While outdoor plants stretch their roots to find what they need, indoor plants, for the most part, cannot do this. If you’ve ever wondered why plants form aerial roots, it’s the plant trying to find additional resources outside of the pot.
In other words, the soil in your pot is extremely important. It must include all the nutrients for the plant. Pots also dry out a lot because we use drainage holes in them and the soil is exposed to air moving around it and overall there is less soil to hold moisture. These air pockets are even more important here, so potting soil usually contains not only perlite or vermiculite, but also peat or coir.
If you’re going to make your own potting mix, adding these products in the right amounts can help provide higher levels of moisture to your plant’s roots. The correct amount will be determined by your plants. For those more prone to root rot, you’ll need more perlite – up to 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 such as Osmocote, according to the instructions on the fertilizer package.
Raised beds could use some help too.
In your garden, using these amendments becomes a more expensive proposition. This is also not necessary: a larger mass of soil retains moisture more easily. On the outside, fungal structures such as mycelium will form in your soil, creating air pockets, and adding plenty of compost or compostable materials such as leaves or wood chips will help create soil with the right texture and composition. Compost attracts worms, which eat through the composted materials and also help create air pockets.
However, if you have particularly clay soil and want to loosen it, adding perlite or vermiculite can help if it is well incorporated into the soil, in amounts ranging from 10 to 50%. This is a huge scope, but the amount of soil in your garden is huge, and any amount you add will help create looser soil. Start small and work your way up, season by season, to see what works and what you grow. Typically, you won’t see peat moss and coir added to raised beds due to their cost at this scale, but you can find them in commercial potting mixes.
If you have outdoor pots, they follow the same idea as indoor potted plants and should be given similar amendments, including perlite, vermiculite, peat moss or coir.
Understanding and improving the composition of your soil is a never-ending process. Even if you start with perfect bagged soil, plants will deplete your soil’s resources, causing the soil to continually require new inputs. What works one year may not work the next due to environmental influences or the emergence of new businesses with different needs. While it is important to ensure there are air pockets and soil moisture with these four components, it is equally important to consider all the other materials you need. The best way to do this is to test the soil and/or bring a sample to your local farm store or garden center and ask for help.