Gardeners know how important soil amendments and drainage mediums are, but which is best? Let’s take a look at perlite vs vermiculite to determine which is better for your growing needs.
The Importance of Well-Draining Growing Mediums
There’s one element we can’t do without when it comes to growing plants. I’m not talking about water, heat, sunshine, or nutrient feeds, but the all-important soil and compost element.
Without a structured, nutritious and well-draining growing medium, many of us green-fingered maestros would be twiddling our thumbs, instead of tending our blooming gardens and promising allotments. Good soil can take years of hard graft to achieve, but you reap what you sow. All the hours, effort and enthusiasm are well and truly worth it in the end.
Today I’m going to talk about what makes a good soil. We’ll take a look at nutrients, structure, and why it’s important to add extra drainage to your garden soil or greenhouse potting compost. We’ll also specifically look into perlite vs vermiculite for aeration, drainage, and water retention.
Typical garden soil is formed over thousands of years. This is how long it takes for rocks to break down into small mineral particles of sand, clay and silt. Interestingly, these particles only make up around half of soil’s entire volume. The remaining half is comprised of water, air and organic matter, as well as organisms in the soil and plant roots.
Soil “type” is determined by the percentages of sand, silt, and clay particles in it. We all strive for a nutritious, light, well-draining soil that has pretty equal amounts of all three mineral particles. For all newbie gardeners out there, we call this perfect mix a “loam”.
Each soil type has its own characteristics which makes it easy to identify. For example, take a handful of soil and feed it through your fingers.
- If the soil feels quite gritty, then it has a higher sand percentage. Sandy soil is crumbly and non-clogging when you roll it into a ball.
- If the soil feels quite sticky and clogging, then it has a higher clay percentage. When rolled between your palms, your ball will be a sticky mess, as will your hands.
- Soil that feels rather smooth and silky, with less clogging, is silt-based. These soils produce a smooth ball that’s quite shiny and less sticky.
Now that we’ve established what to look for, let’s move onto soil structure and why it’s so important.
All of the particles in soil are held together in “crumbs” with spaces between them. The size of these spaces define the soil’s structure and determine how easily water can drain through it. Grainy and sandy oils have large mineral particles, with larger spaces between the “crumbs”. As a result, it drains better and allows air to move within the soil.
In contrast, those with smaller mineral particles, (such as clay and to some extent silt), have smaller spaces between the “crumbs”. Therefore, less water can drain from the soil and less air can move within it.
We all hear about plants requiring “well-draining” or “free-draining” soil mixes, which really refers to the soil structure. Well-draining soils are higher in large mineral particles, such as sand and aggregates. Well-draining compost mixes have a greater amount of sand, perlite, and vermiculite added to the basic peat and loam mix.
Overall, good drainage is an important part of a healthy soil’s structure. It allows excess water to drain away from plant roots, whilst allowing air and essential organisms to move around.
Knowing about your soil will help you to provide the ideal growing conditions for your plants. Each type has its own advantages and disadvantages, as listed below:
These tend to be high in plant nutrients, which aren’t easily washed away. Clay soils are hard when dry and sticky when wet. This causes a problem with poor topsoil, waterlogging, and taking longer to warm up in springtime.
Typically made up of fine particles of silt and clay, becoming like sediment with high fertility levels and good water-holding capacities. Unfortunately, silt soils have little structure and compact easily, becoming airless. This results in cold, sticky soil when wet, and dusty when dry.
Easy to work and cultivate with good water-draining properties, and quick to warm up in the spring time. The disadvantages of sandy soils include their tendency to quickly dry out whilst being naturally less fertile. Due to its free-draining make-up essential plant nutrients can easily be washed away.
Improving Drainage in Soil
The structure of your soil and composts can be altered by using soil improvers, aggregates and organic matter. All of these materials will help to improve a soil’s water-draining property, whilst the addition of organic matter will also increase the soil’s nutritional value.
Suitable mediums for adding drainage to your soil include:
- Course grit
- Sharp sand
- Well-rotted farmyard manure
- Composted bracken and bark
- Garden compost/leafmold
- Peat moss
- Growing a cover crop such as vetches, crimson clover and rye, then digging them into the soil
Improving drainage in Composts
Ready-made composts usually have a high percentage of peat, less loam, and maybe some coir fibre. Unfortunately, unless you buy a specific cacti and succulent mix, (which is meant to be free-draining), there will be very little drainage medium added.
In this case, it’s best to add one or some of these suitable soil improvers:
- Sharp or course sand
Let’s talk about each of these mediums to gain a better understanding of exactly what they are, and how your soil will benefit.
Sharp sand, or horticultural sand, is widely used throughout the horticultural industry as a loyal and trusty free-draining soil conditioner. Unlike traditional building sand with fine, regular grains, sharp sand is rough sand with a high percentage of larger grain sizes.
Not only does the addition of sharp sand increase its drainage properties, its large and irregular particle sizes also change soil’s actual structure. Sharp sand creates larger channels (or macropores) within soil for the water to drain through. This aerates it and makes it easier for plant roots to penetrate.
If this isn’t enough of a reason to add a little sand to your free-draining compost mix, let’s take the science bit into account too.
Gas diffusion within soil is essential for healthy plant growth. This is a means of spreading good gases—like oxygen and carbon dioxide—more widely through the soil pores. Other gases that are harmful to plants are also diffused in exactly the same way, through these channels.
Toxic gases such as methane and hydrogen sulphide are successfully moved away from a plant’s roots in soils which have larger channels (or macropores). These gases can quickly reduce plant vigour and even cause plant death.
In soils with a higher percentage of small particles and in turn, smaller air channels (also known as micropores), the diffusion of important gases is retarded. Overall, this means that poor soil aeration not only limits oxygen diffusion and plant respiration, but can result in the build-up of toxic gases around a plants roots or rhizomes, which will in time cause plant injury and death.
Perlite vs Vermiculite
This is a specific form of volcanic glass, which is mined and heated at high temperatures until it pops. It expands up to 13 times its original size into incredibly lightweight, small, roundish white balls.
It’s a naturally occurring medium, though not considered an organic product. The heating process creates a product whose outer shell is made up of millions of tiny air compartments, which readily absorb water and allow it to drain away. As a result, perlite can be used as a soil and compost improver. It helping soil to retain more water, and creates a lighter, more aerated soil structure.
Perlite is generally more suited to plants that need a drier growing medium that drains quickly. You’ll find that many succulent and cacti composts tend to have a percentage of perlite in them. This is just perfect for their needs, but not for every plant family.
Let’s take a closer look at how your can use perlite in your garden and what benefits can bring:
- Premium container soil: 1 part peat, 1 part loam, and 1 part perlite
- Rooting cuttings: much better results achieved than water alone, plus stronger root system formation
- Improves soil structure, adding air and defying compaction
- Great for growing plants in a soilless medium
- Good for plants needing a high humidity environment: particles have a greater surface area, resulting in greater water evaporation and therefore, higher humidity levels
- Lightweight medium: easy to transport and work with
- Easily accessible moisture: tiny exterior particles that absorb moisture, yet facilitate necessary amounts of moisture to plant roots
- Has naturally a slightly alkaline pH level
Horticultural vermiculite is the name given to a group of aluminium-iron and magnesium silicates. It’s comprised of multiple layers of very thin plates, or flakes. Like perlite, it’s processed under massive heat, causing it to expand. Unlike perlite, however, it’s grey-brown in colour.
Vermiculite is commonly added to garden soil and compost to increase its water and nutrient retention properties, while aiding aeration. This product is available in different sizes, ranging from very small plates to medium and large. These smaller sizes are a useful medium for seed germination, the medium grade for root cuttings and the rest are primarily used as a soil improver.
It’s an adaptable and completely sterile medium, and many gardeners swear by its soft and spongy form. As a result, when it comes to perlite vs vermiculite, the latter tends to be more popular. It does have higher water retention properties than perlite, but not all plants are water-loving. Some prefer better soil aeration, for which perlite cannot be beaten.
Let’s take a look at a few ways you can use Vermiculite in your garden, to get a better idea.
- Great for root cuttings: use alone or mix with peat or compost to accelerate root growth and anchorage
- Seed germination: use alone or mixed with soil or compost (no damping-off problems)
- Mulching plant: 3″ provides great water retention for tomatoes, roses, and dahlias
- Newly seeded lawns: speeds up seed germination, whilst protecting seeds from drying out
- Flower arranging: oak the vermiculite in water, then drain for a medium that will keep your blooms fresh for days
- Naturally neutral pH level, making it perfect for use with all plants
Numerous Benefits of Perlite vs Vermiculite
There are many benefits to be gained from adding drainage mediums to your soils and composts, as you can see.
Use sand as an amenable medium in most soils, as it’ll aid drainage and aerate your soil. If you think of tropical plants, for example, many are grown in a high sand mix with little loam. This soil doesn’t have great nutritional values, but these plants have adapted to growing in this environment.
When cultivating soil, we need to use mediums that are suited to individual purposes and plants alike. Now that you have a little more knowledge of all that’s available, especially about perlite vs vermiculite, experiment with these drainage mediums to see which suit your needs best!