With millions of plants growing in soil, you may occasionally question what is in your soil media, and if you need to add anything to your soil for optimal plant growth. Soil additives are materials like vermiculite, perlite, sand, mulch, or others that amend the soil for preferred media conditions for the plant (well-drained, dry, moist).
Of the soil additives, vermiculite and perlite are the two that instigate the most questions from gardeners, and we have to remember that vermiculite and perlite are not the same substance! They also are both commonly used as a soilless medium.
Mixing up and confusing these two substances will severely impact how your plants will grow due to their extreme differences.
Whether it be origin, appearance, or usage, these two soil amendments possess numerous differences.
In the late 19th century, the state of Montana was transformed for many decades after boatloads of vermiculite were discovered in the land.
If you’re looking at the periodic table, vermiculite is mostly composed of aluminum, iron, and magnesium and contains vital water molecules, and does not contain asbestos.
Scorching temperatures force water to convert to steam, and this is the product you see when you open that fresh bag of vermiculite for your plants. In general, I use vermiculite for seedlings for it is known for its water-retaining abilities; a main process of seed germination is imbibition, which is when the seed absorbs water to sprout.
The micro and macronutrients plants need like potassium, magnesium, calcium and phosphorus are also contained and absorbed in the media through the properties that vermiculite has.
Vermiculite’s water-retaining properties allow for ideal seed germination like pictured above.
Vermiculite is used in most potting mixes because ideal soil contains 25% water. When you see this substance for the first time, it may look like it belongs at the bottom of your fish tank. Normally this mineral has a color range of gray to brown and is a tiny particle that resembles small-scale gravel. Vermiculite is smaller than perlite.
When to Use Vermiculite
Common reasons to use vermiculite include pursuing seed germination, propagation, or non-drought tolerant plants. If you have problems with your soil drying out to the point that your plant is continuously wilty, adding vermiculite may bring resolution.
It does aid in aeration as well, but that is more of perlite’s job if you’re looking to reach that ideal 25% of air in the soil.
If you are looking to accomplish the 25% of water needed in the soil, this hydrated phyllosilicate is what you may be searching for. You may want to be cautious with this mineral if you are prone to overwater your plants.
This mineral is generally gray to brown colored and has a small particle size.
Excessive water retention can result in irreversible damage from organisms such as the oomycete, Pythium (root rot).
This mineral can be helpful when added to a soil mix you are using to propagate cuttings, especially stem and leaf ones. The composition of vermiculite aids in young root growth, and allows formation to be easier for the plant.
Since the plant needs to form adventitious tissue when asexually propagated, assuring ample water until the tissues form is crucial.
How to Utilize Vermiculite
Similarly to perlite, the amount of vermiculite you will need in order to benefit from its properties depends on your plant, soil texture, and the environment. The more you add, the longer water will be retained in the micropores of the soil.
Also, based on your purpose, you can buy vermiculite in different sizes (fine, medium, coarse).
Sprinkling some of this mineral over soon-to-germinate seeds also provides them a fine layer that can be penetrated with sufficient water. With that, the mineral also provides significant aid in retaining nutrients for the plants, so they don’t get washed out as quickly.
The lemon balm pictured above loves water and in the summer the soil dries quickly.
Which plants do you have that might benefit from vermiculite?
- Ferns (Polypodiaceae family)
- Spider plants (Chlorophytum comosum, Asparagaceae family)
- Aloe plants (Asphodelaceae family)
- Pothos (Epipremnum aureum, Araceae)
- Lemon balm (Melissa officinalis, Lamiaceae family)
- Tomato plants (Solanum lycopersicum, Solanaceae)
All About Perlite
Has your cactus turned mushy, or did your spider plant develop brown, flimsy tips lately? This may be due to excess moisture building up in your soil media.
In my garden shed, there is never a moment when a bag of this mineral is not sitting in the corner of the room, accompanied by plastic plant pots. Perlite’s versatility and characteristics are helpful to any gardener or houseplant connoisseur.
When looking at secondary minerals such as perlite and vermiculite, you will notice that both of them are successful due to their built-in water capacity. When you hear ‘volcanic glass’, it may seem like that has nothing to do with your potting media. Although this is what perlite is, and this mineral also expands when exposed to extremely hot temperatures.
This mineral has a puffed look and feels like styrofoam. The white specs pictured above in the soil media is perlite, and is common in soil mixes.
As far as secondary minerals, it is originally formed from the primary mineral, black obsidian (the volcanic glass). It is considered secondary because it underwent a weathering process and formed a new substance.
When to Add Perlite
Imagine you have finally arrived at your home after a long day of plant shopping, and are ready to repot your favorite cactus. Many stores sell cacti-succulent potting mixes, which contain additional perlite, but you went ahead and bought pure perlite.
The puffed-up material is used for aeration of the soil and to manipulate the soil structure to be less compact. As you open the bag of the white material that has inflated over 20 times its original size, be sure to keep your face away! The bag may produce white dust when you open it and is toxic if inhaled.
Although sharing similar appearances, take care not to mistake perlite for styrofoam, for styrofoam is toxic to plants and soil.
This soil amendment is added to the soil when plants are suffering from saturation due to a mix of poor soil textures (like excessive clay content). Moreover, plants generally like alkaline soil so this additive’s pH of 6.6 to 7.5 may bring soil benefits as well.
When watering your houseplants, does it ever seem like your fern plant never dries out? Long-term saturated soils can result in plant rotting and death, but perlite additions often can mend that issue.
Perlite serves a purpose similar to macropores in soil or soil aggregates; it allows for air to enter the media and have the ability to percolate water to the micropores efficiently.
Pictured above are some soil aggregates, which helps with aeration in the soil.
This is also why you may notice that perlite is a bit larger than vermiculite; this characteristic enforces ideal aeration.
Often potted plants will form a layer of crust on top of the soil, and perlite can eliminate this. Soil temperature is more important to the plants than air temperature, and perlite can bring stability to this factor as well.
How to Apply Perlite
You may wonder how it is applied, considering every soil is different. Every name-brand potting mix has various amounts of nutrients, aeration, and water retention materials. Also, it depends on what type of plant you are growing to determine how much perlite to apply.
It may benefit you to ask, does my plant need a lot of water or does it look like my soil is compact and not well-drained? Add more if your plant likes it dry and if your soil does not drain well. If you want a specific ratio for a particular purpose (versus eyeballing it), information like that can be easily researched.
Most cacti do not like saturated soil, but this secondary mineral provides aeration and well-drained soil.
Which plants do you have that might benefit from perlite?
- Cacti (Cactaceae family)
- Succulents (Aizoaceae, Crassulaceae family)
- Lavender (Lavandula, Lamiaceae family)
- Snake plants (Dracaena trifasciata, Asparagaceae family)
- ZZ plant (Zamioculcas, Araceae family)
- Geraniums (Pelargonium, Geraniaceae family)
Soil Medias and the Perching Effect
When discussing soil amendments (like perlite and vermiculite) and adding minerals to obtain ideal soil characteristics for our plants, we have to consider soil science.
Potting mix is mass-produced and every company sells different combinations of soils for plants.
You may have heard your friends tell you when you got a new cactus, to add a layer of pebbles to the bottom and top of your pot. Contrary to popular belief, this has high potential of creating problems for your plants.
Depending on what you may add to your soil, a change in media is a disruption. If the media was thoroughly mixed, there is more potential to soak up remaining water because of the evenly dispersed materials; if you use a layer of rocks, it ends up forming a pool of water at the bottom, which is asking for disease.
Pictured above is a common misconception about putting a layer of rocks on the top or bottom of the soil. It actually does not benefit the plant.
The water goes to the bottom because not all the water is sucked up by the media due to the barrier of a layer.
It is important in the horticulture world to know when to use different minerals for soil amendments. So, keep in mind how blended your soil textures are in your medias, especially vermiculite and perlite!