Potting Soil Selection can seem overwhelming when you are eying the rows of bags at the store (or in the catalogue), but hopefully this week we can help you narrow down what options fit best for your growing situation!

As you decide which path to take, keep in mind your end goal: healthy, hardy seedlings. It’s not worth cutting corners if those shortcuts weaken or damage your seedling viability (and could end up costing more money in then end than you might save with a shortcut)!

Choice #1: Store Bought or Mix Your Own?

Back in the day, I worked on several farms that mixed their own soil, using a ratio of garden topsoil, compost, peat moss, and perlite. If they got fancy or had time, they’d let the topsoil heat up in clear plastic bags to cook out some of the possible pathogens. Across the internet, there are hundreds of recipes to make your own potting soil like this at home, from the classic “Cornell Horticulture” mix up through complicated 20-ingredient blends with all sorts of rock powders.

Alternately, you can buy a pre-made mix, sold in bags, bales, or supersacks.

Some folks find making their own soil fun (don’t let me stop you if you do). It gets you thinking gardening or farming at a slow, dark time of the year, can potentially save money, and you can tailor a mix for your particular needs. However, homemade mixes can lead to spotty results in germination or plant health and are harder to control variables around.

I personally prefer buying potting soil. For larger or farm scales, it is a bear to accurately mix thousands of pounds of soil. I also appreciate the security of buying in a vetted and quality tested mix from a business that works hard to specialize in their one product.

 

What’s in your potting soil and what makes it different from regular soil?

Potting soil is pretty important in the lives of young plants, as it’s job is to get those babies off to a quick, healthy start. That means it needs to be:

• Dense enough to support a seedling and all their roots – bulk and density can come from soil, compost, peat moss, coir, or other bulky substances (I’ve seen mixes with leaf mold and paper even!)
• Fluffy and light enough to let air and water circulate through its pores – lightness and ability to interchange air and water can come from perlite, vermiculite, and peat moss/coconut coir
• Free of diseases, weeds, and pests that could harm the delicate baby plants
• Nitrogen and other nutrients can be added through compost, minerals, or chemical sources

It’s a tall order in one product (part of why I prefer buying mix than trying to make my own). One thing we do, however, is sometimes add ingredients to modify our store-bought mix to better fit particular crop needs…

If you do want to start experimenting with amending potting soil, here’s a good early guide: https://extension.psu.edu/homemade-potting-media

For farmers, ATTRA has a great handout that goes in depth on potting media: https://attradev.ncat.org/wp-content/uploads/2022/06/pottingmixes.pdf

Choice #2: Sterile versus Compost-Based?

One big choice around potting soil is if you want to go sterile/soilless, or use a compost-based soil.

Sterile soils generally center around peat moss or coir, with some added rock minerals or nutrient charges. Their advantage is that they tend to be lighter, which can increase early germination, and they absolutely should not have any harmful plant diseases in them.

The downside of a sterile mix is that they don’t have anything innately in them that provides fertility to feed the plants, so you will need to factor in adding fertilizer as you water and grow.

Compost or soil based mixes have as their advantage generally more fertility to start with, and often enough slow release fertility that you won’t need additional fertilization. However, they are a bit trickier to manage watering correctly, and can harbor or support the growth of plant and root pathogens.

Personally, while we occasionally use sterile mixes for things like germinating grafting tomatoes where you might be tossing them around a lot and opening up potential wounds for plant pathogens, I prefer the compost based mixes because of their higher overall nutrient levels. Additionally, with spring planting dates swinging so widely now, we find that compost based soil is a little more forgiving if you have to transplant the crops a week or two early or late!

Choice #3: Fertility Management Options?

I confess that part of why I really like compost based mixes is that as spring planting approaches and the greenhouse (and our to-do lists) fills up, it takes one major task – fertilization, off that list. A quality compost based potting mix should be able to carry your seedlings from starting to transplant without added fertilizer applications.

Soilless mixes often come with some fertility added. If you see the words “Starter Charge,” that usually indicates that a little fertilizer was added that will start the plants and carry through 2 to 4 waterings of those flats. “Continuous Charge” indicates a slower release fertilizer (think of those little round balls) that will carry the seedling a few weeks longer. If using soilless mix, make sure to read and understand what is or isn’t included in terms of fertility, so you can be prepared to amend it before the little seedlings get sad!

“Wetting Agents” are often added to mixes with a lot of peat moss. Peat is hydrophobic, or sheds water off, so in order to get it to lower the surface tension so that it can absorb water for the first time, a wetting agent is added. (This is also the case for organic mixes—usually those use yucca extract.)

There isn’t really one neat answer on the potting media selection process, as so much depends on your budget, space, local availability, and needs. As you are choosing soil, make sure you read the label and descriptions on the soil bags and catalogues to start developing a sense of what each product works best for. Obviously a succulent mix of puffed volcanic rock won’t work as well for your vegetables as a mixed aimed to increase germination.

When you are working with a new product, keep note of what you do and don’t like. What grows good seedlings in your setup, and how do those seedlings transition to being out in the fields or garden beds? Do they seem like you need more fertility, or better drainage, or a finer screen? Take notes and adjust for the next round or next year.

 

Other common questions about potting media:

Should I pre-wet my soil before filling the trays? Most of the time, yes. Especially if your mix has a lot of coir or peat moss, their hydrophobic nature can make that initial wetting more challenging as all the water beads up on the surface and can wash your seeds away. You ideally want soil that is moist, but not sopping wet either.

Freshness does matter/Can you reuse potting soil? We don’t recommend reusing soil, as it is likely run down on available nutrients to your seedlings, and could harbor plant pathogens. You can recycle the soil in the compost pile or around other garden areas, however. Potting soil is best used fresh, as some components of the soil could age or get a bit funky after more than a year of storage (especially in plastic bags or sitting in the sun). If you don’t think you can use all the soil you have on hand in the current year, it will help keep it fresher and active if you store it out of the sun in a closed container in a dry space. Soil that gets rained on can leach out nutrients over time.

Can I just use garden soil to start seeds? Some home gardeners do this and it works out fine for them, but we don’t recommend this as there is risk of plant pathogens carrying over. There are guidelines to sterilize soil here: https://extension.psu.edu/how-to-pasteurize-medium-and-sterilize-containers-and-tools, but for most folks it’s easier to just buy good soil. If you are growing on a commercial scale, I would not recommend just using garden soil for your seed starting. So much of your farm’s potential production (and income!) depends on those seedlings being healthy and hardy that it doesn’t seem worth the risk!

Hot Topic: Peat Moss… One of the biggest environmental issues in the gardening world centers around this miracle ingredient at the heart of most potting soil mixes. You might have seen the press about this https://www.nytimes.com/2022/02/02/realestate/gardening-peat.html or heard how the UK is phasing out the use of peat moss over there by 2024.

If you want to catch a Master Gardener view of the controversy, along with a link to peat alternatives, this is a good explainer: https://piedmontmastergardeners.org/a-growing-controversy-should-we-stop-using-peat/