Selecting the Best Soil for Starting Your Seeds

It’s that first warm day after a long cold spell that makes you believe spring is finally on its way. You head out to the mailbox and there they are—the seed packets you’ve been waiting for. Excitement hits, and before long you’re off to the garden center to pick up soil for starting your seeds.

But the moment you walk in, you’re faced with an entire wall of options: potting soil, potting mix, seed-starting mix, garden soil, container mix. One bag is labeled organic, another promises moisture control. And what on earth is coir? It’s overwhelming, and far too easy to grab the bag with the prettiest picture and hope for the best.

The truth is, there are real differences among these soil mixes—and understanding them can mean the difference between strong, healthy seedlings and a discouraging start to your gardening season. This article will walk you through the basics: the key properties of a good seed-starting medium, the ingredients commonly used, and how each one affects the overall quality of your soil mix.

Soil Factors That Impact Seedling Growth

Before we talk about what goes into a good seed-starting mix, it helps to understand a few simple soil qualities that make all the difference for young plants. You don’t need a horticulture degree for this—just a general idea of what each factor means and why it matters.

Water Retention: Staying Moist, Not Muddy

Seedlings in containers can only use the water held in that little bit of soil, so a good mix needs to walk a fine line:

• It should stay moist between waterings
• But it should never stay soggy

When soil holds too much water, roots can’t breathe, growth stalls, and diseases take hold quickly.  When it dries out too fast, seedlings become stressed.  The best mixes land somewhere in the middle.

Porosity: Letting Roots Breathe

Porosity is just a fancy word for the amount of air space in the mix. Those tiny pockets of air are incredibly important.

Roots need:

• Oxygen to grow
• A way for carbon dioxide to escape
• Enough space for extra water to drain out

Tight, compacted mixes trap water and encourage fungal problems—the kind of issues that wipe out seedlings almost overnight.

Nutrient Retention (CEC): Holding Onto the Good Stuff

CEC (cation-exchange capacity) tells us how well a soil can hold nutrients instead of letting them wash away.

• High CEC → nutrients stick around
• Low CEC → nutrients rinse out quickly

If you’ve ever had seedlings that look pale or weak even though you fertilized them, low CEC might be the reason.

Shrinkage: When Soil Pulls Away from the Container

If you’ve used peat-based mixes, you may have seen the soil shrink back from the sides as it dries. This makes watering tricky because water runs down the gap instead of soaking in.

• Shrinkage can happen when:
• Peat dries out
• Low-quality materials like wood chips break down
• The mix contains particles of wildly different sizes that settle over time

Shrunken soil = uneven moisture, and uneven moisture = unhappy seedlings.

Sterility: Starting With a Clean Slate

Seed-starting mixes don’t need to be perfectly sterile, but they should be free of common pathogens—especially those that cause damping-off. Most ingredients, like peat or vermiculite, are naturally low-risk.  But if the mix contains compost or manure, it should be pasteurized first to avoid trouble.

Soil pH: Keeping Nutrients Available

Most seedlings prefer a slightly acidic environment—usually between pH 5.2 and 6.2. When pH drifts too high or too low, plants can’t access certain nutrients, even if those nutrients are present.

Gardeners or manufacturers usually use:

• Dolomitic lime (most common)
• Calcitic lime
• Gypsum (adds calcium without changing pH)

to keep pH in the right range.

Bulk Density: Light Doesn’t Always Mean Better

Bulk density refers to how heavy or light the mix feels. Many commercial mixes aim for lightweight, which is fine—unless fillers like wood chips or Styrofoam beads are used.

Warning signs:

• Big chunks of wood
• Large, inconsistent particles
• Mix looks fluffy but collapses when wet

These blends often compact, hold water poorly, and break down quickly—none of which is ideal for starting seeds.

Common Bagged Soil Components

Now that you know some of the things to look for in a good growing mix, let's examine some of the most common components of bagged soil.

Sphagnum and Peat Moss

Nearly every bagged soil you pick up will contain sphagnum moss or peat moss. Both come from Sphagnum, a genus of water-loving plants found in bogs, swamps, and other wetlands.

Sphagnum is used in potting mixes for several reasons:

• Exceptional water retention—Sphagnum can hold 500% or more of its weight in water. This ability comes from its porous hyaline cells, which make up as much as 80% of the plant’s stems and leaves. In other words, it acts like a natural sponge—perfect for keeping seedlings evenly moist.
• Improved air space—Despite holding so much water, sphagnum also has excellent porosity, meaning it creates plenty of air pockets in the mix. This helps roots breathe and improves gas exchange, which is critical for healthy seedling growth.
• Resist shrinkage—Sphagnum contains a naturally occurring phenolic polymer called sphagnic acid, which slows decomposition and keeps its structure intact. That makes it far less prone to shrinkage than many other organic ingredients—an advantage for seed-starting mixes.

Coconut Coir

Coconut coir is becoming increasingly popular as an alternative to peat. Coir is the reddish-brown fiber found on the husk of a coconut—you’ve probably seen it as the coarse, hairy material used to line hanging baskets. Since it’s a natural byproduct of the coconut industry, many environmentally conscious gardeners prefer it over peat.

Coir has a few other advantages as well:

• Its pH is closer to neutral, which is often more suitable for seedlings.
• It is much easier to re-wet than peat, which can become water-repellent when dry.

While coconut coir isn’t yet as common as peat, you’ll often find it in higher-end or organic soil mixes.

Photo by Dani Buckle on Unsplash

Bark

Bark is another common ingredient in many bagged soils. It’s inexpensive—thanks to being a byproduct of the lumber industry—and naturally lightweight. It can also be hammer-milled and screened to create uniform particle sizes, which makes it more predictable in a potting mix.

When chosen carefully, bark can make an excellent growing medium. Pine bark is especially well-regarded because it breaks down slowly and doesn’t rob nitrogen from the soil as it decomposes. Hardwood bark, on the other hand, breaks down much faster and has a higher carbon-to-nitrogen ratio, which can tie up nutrients. For this reason, hardwood bark should be fully composted before being used in a growing mix.

In terms of performance, a bark particle size of less than ¼ inch provides a good balance of water retention and porosity.

A Note About Wood Chips

In recent years, some potting mixes have begun incorporating wood chips or shredded wood as a filler. Unless this wood has been composted for several years, it makes a very poor growing medium. Fresh wood:

• has low water-holding capacity
• decomposes quickly
• consumes nitrogen during breakdown
• settles and compacts
• is unpleasant to work with

And let’s be honest—those sharp little pieces hurt. They poke fingers, wedge themselves under fingernails (ask me how I know), and get stuck in containers as you try to spread the soil. While wood-heavy mixes are often cheaper, the frustration they cause—and the poorer plant performance—usually isn’t worth the savings.

Photo by University of New Hampshire Extension

Vermiculite

Also known as aluminum-iron-magnesium silicate, vermiculite is a naturally occurring mineral, with deposits found in the United States and other parts of the world. The vermiculite used in potting mix has been superheated to reduce its density and expand it's cation-exchange capacity. It also contains significant amounts of potassium and magnesium, which are needed for plant growth.

Perlite

Perlite is a volcanic rock that, like vermiculite, has been expanded by heating. Perlite offers little in terms of water and nutrient retention, however its highly porous nature makes it great for improving the aeration of potting mixes. Nowadays, some inexpensive growing mixes will substitute polystyrene (Styrofoam) instead of perlite. Styrofoam is used to reduce the density of the soil making it less expensive to ship. Compared to perlite, Styrofoam has 42% less pore space and retains 90% less water. As far as soil components go, Styrofoam is in my opinion, worth its weight.

Lime

Both sphagnum moss and pine bark have slightly acidic pH values, therefore many bagged soil manufacturers will add lime to neutralize the soil. The most commonly used lime for bagged soil mix is dolomite, which is just lime (calcium carbonate CaCO3) mined from a deposit that contained significant amounts of magnesium. On rare occasion, you may also see gypsum (CaSO4) used as a liming agent. Note that mixes that contain uncomposted hardwood bark should not be limed as the pH of the medium increases naturally with decomposition (Bilderback, 1982).

Wetting Agents

Most growing mixes contain significant amounts of sphagnum, peat, and/or bark. As these materials dry out, they become increasingly hydrophobic, such that it can become very difficult to rehydrate them. To compensate for this, some manufacturers will add surfactants to improve water penetration. Surfactants are detergent-like substances that reduce the surface-tension of the media, thereby encouraging water to run through the media rather than around it.

Worm Castings

Many high-end mixes, and especially organic ones, will contain worm castings. Earthworm casting are made when the worm, well, does its business. Castings contain significant amounts of ammonia, urea, and body tissues that are rapidly mineralized.

Earthworm castings have been shown to improve germination rate and enhance shoot and root growth in developing seedlings. It is believed that this is due to the increased nutrient availability in soil that has passed through the earthworm gut. For example, field studies have reported that earthworm castings contain 2-3 times more available potassium when compared to the surrounding soil (Basker et al., 1993).

Other Ingredients

Each manufacturer has their own proprietary blend of ingredients. It is not uncommon, for example, to see silica, moisture-retaining gels, fertilizers, growth enhancers, or a host of other marketable amendments advertised on the bag. If you are just using the soil to start your seeds, it probably isn't worth splurging on these mixes. After all, the seed contains everything it needs to get the seedling out of the soil (with the exception of water, of course).

Fertilizing at this early stage likely is of more benefit to pathogens than it is to the seedling. However, if you are going to be direct seeding into a larger container that will be moved outside, you may want to consider a soil that has more nutrients to support the plant later in life.

Frequently Asked Questions:

Q:  Can I make my own seed starting mix?

A: Absolutely! However, formulating a seed starting mix that everyone likes is like trying to find universally loved potato salad recipe—it doesn't exist. If you are interested in making your own mix, I would recommend first determining what ingredients you want to include, those you want to avoid, and then researching to find recipe that has the right ingredients in proportions that will support plant health.

Q: Can I use garden soil for starting my seeds?

A: I wouldn't recommend it. While garden soil can grow perfectly healthy plants in its natural environment, when placed in containers it exhibits something called the container soil effect. Consequently, a well-draining, highly aerated garden soil will become dense and saturated when placed in a container. The type of the container doesn't matter, because the container effect is caused by a change in the length of the column affected by gravitational pull (Mastalerz, 1977).

Q: Can I add compost to my mix?

A: If you are considering adding compost to your mix, I'll ask just one question: Are you the type who carefully controls the inputs, temperature, and age of your compost pile? Or are you the type that occasionally trudges out to sling some banana peels on top of the partially decomposed heap (finger pointing back at myself here)?

Properly produced compost can be a wonderful addition to soil starting mix, but poorly produced compost can be an invitation for fungal problems. If you want to use your compost, but aren't sure it was produced with scientific precision, try mixing a pint of it into the bottom of the hole when you transplant. That way your plants can still harness all of the wonderful benefits of compost, but only after they've become strong enough to resist common pathogens.

Q: Is organic potting mix better than conventional?

A: Whether or not you purchase organic seed starting mix is a matter of personal preference. Organic mix can be a bit more expensive, but many gardeners consider it essential. On our farm, we use a soil mix that contains ingredients for which an organic certification shouldn't matter (sphagnum, bark, perlite, lime). We have, however, used Fertilome's Organic Potting Mix with great success.

Q: Do I need to pre-wet the mix?

A: Most potting mixes come premoistened to make them easier to spread out in the pots. However, we recommend re-wetting the media after you have filled your pots or trays and then letting it drain thoroughly before planting. We often repeat this step just to be sure it is moistened throughout prior to sowing.

Q: Will adding fertilizer make my seeds grow faster?

A: Initially, probably not. We do not recommend adding fertilizer until the seedlings have at least one set of true leaves. Over-fertilizing your seed starting mix will only cause problems. Once the plants are up and going, it should be fine to fertilize, however, only supply it as liquid form and only at one-third to one-half the recommended rate.

Picking Out the Best Bagged Soil

Okay, you've done your homework and you're ready to purchase some soil. When you get there, check out the various bags to determine their ingredients. If those look good, feel the bag to try to get an idea of what the media inside is like. Here are a few hints to help you find the best soil by feel.

1. Feel the outside of the bag to see if it has any large chunks of material in it. If you feel any pointy objects, it likely contains woodchips or worse yet, ground up pallets. Move on to the next brand.
2. Make sure the bag is not saturated with water. If your garden center keeps soil outside, you may have to dig down a layer to find a dry bag.
3. Pick up the dry bag of soil to gauge its density. If it feels overly light compared to other brands, the soil is either too dry or contains a lot of fillers.
4. Press on the outside of the bag. If it is brick-hard, it will be difficult to wet (and also work with). If it feels soft and moves a bit when you pick up the bag, you have a winner.

In Summary: Choosing the Best Soil for Your Seeds

There are a number of factors to consider when choosing the best soil for starting your seedlings. Moisture-retention, porosity, nutrient-retention, and pH are all important variables that affect plant health. Therefore, choosing a mix that has the right soil components will go a long ways in getting your seedlings off to a great start. Finally, once at the garden center, taking a bit of time to pick up the bag and feel its contents will provide some helpful clues about the quality of the soil inside.