The Five Basic Types of Soil, Explained
There are at least five kinds of soil at the big box store. What are the differences between them? And which ones can be used to start seeds?
Let’s take a quick look at the five basic types of “soils” commonly found at garden centers.
First, what is soil? Technically, soil is earth that has within it a mixture of organic matter, minerals, gases, liquids, and organisms. I’m using “soil” casually here to include many other things – peat moss, coconut coir, bark, wood chips, perlite, compost, etc.
Top Soil - A combination of sand, silt, clay, and broken down organic matter. It contains humus, which in Latin means “earth” and is made up of decomposed dead plants, leaves, insects, and other carbon lifeforms. Top soil has high levels of organic matter and microorganisms that support plant growth. It’s the heaviest of the soils listed here, and can become swampy if watered too much – but it’s the cheapest by far and a great amendment to existing garden beds.
Potting Soil or “Potting Mix” - A mixture of organic and inorganic materials such as peat, coconut fiber, tree bark, wood fiber, perlite, recycled paper, and soil. Potting soils usually contain larger particles such as wood chips and bark that provide aeration and drainage. It’s meant for smaller potted plants and needs to be watered more frequently than other soils due to its high drainage.
Raised Bed Soil – Similar in composition to potting mix, but contains fewer large particles. Essentially, it’s a denser and heavier form of potting mix that retains more moisture. It has better drainage than top soil, but less drainage than potting mix – perfect for raised or in-ground garden beds.
Seed Starting Mix - Usually comprised exclusively of peat moss, coconut fibers, perlite, vermiculite, and sometimes compost. This medium is light and fluffy and contains no large particles, so roots can travel freely and grow quickly. It drains well, holds moisture, and has excellent aeration/porosity. Varieties without compost will contain little to no nutrients, so seed starting mixes cannot support fully fledged plants. You’ll need to eventually up-pot your seedlings into a soil that has nutrients.
Dirt - Plain old dirt is not a viable growing medium for a couple of reasons. Since it is made almost entirely of sand and/or clay, it will either drain too much (sand) or not drain at all (clay). In either case, dirt cannot support the ecosystem that plants need to grow. (You know what dirt looks like, so I didn’t take a picture of it - sorry!)
What kind of soil(s) work for seed starting?
Seed sprouting requires soil that has a certain structure to it: porosity, drainage, and moisture retention. What you’re looking for is a soil medium that allows the roots to grow freely in a moist, aerated environment.
Porosity. The soil needs to have air pockets in it so the roots have access to oxygen. If the soil is too swampy, it cannot release carbon dioxide (CO2) created by organisms within it. If too much CO2 accumulates in the soil, it will become toxic to your plants, and they may deteriorate, become stagnant, or die.
Drainage. Drainage allows for two things. It provides a way for plant waste to be carried away by water, and it also aids in root respiration. Roots get a minor but significant portion of their oxygen directly from water.
Moisture retention. All plants need moist soil to grow and absorb water and nutrients. Too much saturation will kill roots; too little will dry them out and also kill them. Moist to the touch, but not soggy, is what you’re looking for. Seedlings require very little water since their roots are so small. A common way to kill seedlings is by watering too much, so if you are using a high moisture-retaining soil, then restrain your watering accordingly.
In theory, any kind of soil medium will work so long as it has the above properties.
Can I use any of these soils to start my seeds?
I’m glad you asked! I did a controlled experiment comparing seedling growth in various soils. You can see the full results here in this video.
Below is a brief summary of the results, using our eco-friendly Sili-Seedlings 10-cell trays.
Potting Mix:
After 3.5 weeks:
Large, beautiful plants ready for transplanting.
Pros:
Plants grew the biggest, quickly.
Cons:
Large particles need to be removed before starting seeds.
Seed Starting Mix:
After 3.5 weeks:
Beautiful, healthy. Not as large as the potting mix seedlings, but root growth more elaborate.
Pros:
Root development excellent, plant growth decent.
Cons:
Needs to be transplanted around the 2 week mark or before. Few nutrients in the soil make plant growth not viable shortly after true leaves sprout.
Note:
I have a few hypotheses (but nothing conclusive) for why the seed starting soil didn’t work as well as the potting mix. One hypothesis is that seed starting soil lacks nutrients: the potting mix has fertilizer added while the seed starting mix doesn’t. Another hypothesis is that seed starting soil isn’t meant to grow big seedlings - it’s just meant to get them started so they can be transplanted early. The root growth in the seed starting soil was more complex than that in the potting mix, even though the leaves were smaller.
Raised Bed Soil:
After 3.5 weeks:
Green leaves, but stunted growth after 1.5 weeks. Root growth was substandard.
Pros:
Seeds germinated quickly, but then grew very slowly thereafter.
Cons:
Soil was too dense for proper root growth.
Top Soil:
After 3.5 weeks:
Seeds sprouted early, but then didn’t grow after the 4-day mark. Roots rotted.
Pros:
It’s cheap.
Cons:
Way too dense, with low porosity. Not enough drainage for root growth. Seedlings sprouted but never grew true leaves.
Note: I didn’t test dirt - though in hindsight, for the sake of science, I should have! Next time…