What is the Best Soil for Greenhouse Growing?
If you are new to gardening and think growing is just a matter of adding water, you're in for a surprise. With terms like super soil, triple mix, natural soil, and even soil with no soil—it's no wonder new gardeners feel overwhelemed. So how do you choose the right soil for strong, healthy growth? And how do you get the most flavorful food and the brightest blooms?
Whether you grow in the ground or in pots in your greenhouse this 2-part blog is for you. In Part 1 (this post), we’ll explore different soil types. In Part 2, we’ll dig into how to replenish nutrients and amendments as your plants pull them from the soil.
The role of soil isn’t just to hold up your plants. It must retain water, supply nutrients, and provide oxygen for both plant roots and the beneficial microbes that help them thrive. If your goal is a productive and beautiful garden, start with a strong foundation—your soil.
Understanding Natural Soil
The soil you find beneath your feet in a forest or field is called natural soil. But natural soil varies depending on location. It might be mineral-rich prairie loam or nutrient-poor Amazonian dirt. The quality depends on age, geography, and past use.
While you might see bags labeled “topsoil” in garden centers, most don’t contain actual natural soil. In fact, selling it is often restricted. Bulk landscape suppliers may sell natural soil from development sites, but quality varies widely—it could be sticky, sandy, low in nutrients, or mixed with poor-quality subsoil. Natural soil isn't usually ideal for home gardening.
What to Know About Bagged Topsoil
Topsoil is technically the top 6–8 inches (15–20 cm) of earth in a natural setting. Good or bad, it doesn’t come in bags. Farmers hold onto their best topsoil because it’s too valuable to sell.
Some bagged “topsoils,” like Roots Organic, contain blends of organic materials—everything from perlite and compost to bone meal and worm castings—but no actual topsoil. See the “super soils” section below for more on these high-performance mixes.
Why Loam Is Ideal for Growing
Loam is a naturally balanced mix of sand, silt, and clay. It’s a top-tier soil for growing—but true loam is rare and, again, not something farmers part with easily.
Many bagged products are labeled “loam” but might actually be clay, bark, compost, or a mishmash of materials. These are fine for seedlings or transplants, but too lightweight for large garden beds or pots, where nutrient deficiencies and tipping plants are common.
Pros and Cons of Soil-less Mixes
With true soil hard to come by, many gardeners use soil-less mixes. These combine materials like peat moss, coconut coir, vermiculite, and perlite—designed to mimic soil while offering good drainage and seed-starting potential.
These mixes are perfect for getting seeds going, but on their own, they’re too light for larger, long-term crops like tomatoes or citrus. Use them as a base or starter, but not as your only growing medium.
Using Amended Soils for Gardening
To stretch a small amount of natural soil, suppliers often mix it with compost, bark, peat, sand, and other soil-like elements. These blends can be purchased in bulk or big bags and are a good option—though still not as rich as pure loam. For many growers, blending your own is the best way forward.
How to Make and Use Triple Mix
You can create your own version of triple mix: one-third soil (from your garden or landscape supplier), one-third compost, and one-third peat. This blend drains well, supports strong growth in year one, and works well in most garden settings.
Just remember: every time you harvest a crop, you’re removing minerals. Over time, your soil will need compost or fertilizers added back in to stay productive. Compost piles, worm castings, and slow-release amendments can all help keep your mix balanced.
What Is Super Soil and Should You Use It?
Some growers go a step further and build “super soils.” These start with soil-less or triple mix, then get a boost from ingredients like:
- Biochar
- Worm Castings
- Seed meals
- Gypsum (calcium sulfate)
- Ag Lime (calcium carbonate)
- Pumice
- Kelp
- Glacial rock dust
- Humic and fulvic acids
These custom blends provide a full spectrum of nutrients, reduce dependency on synthetic fertilizers, and support consistent growth. It takes time to fine-tune your mix, but the payoff is worth it.
P.S. The best flavors and brightest blooms come from soils with the right mix of minerals. You can add some directly—like zinc sulfate or Epsom salts (magnesium sulfate)—or use balanced commercial fertilizers. In the next post, I’ll share more about which amendments to use and how to apply them for your best growing season yet.
