Starting from seed: Grow more plants, save some green
Making homemade planting medium can be more economical than buying a sterile mix at the store, said Brooke Edmunds, a horticulturist with Oregon State University’s Extension Service. Use a seed-starting or soil-less potting mix – not heavy garden soil.
While you’re battling the winter blues, make your own seed-starting mix and plan for the gardening days ahead.
Home gardeners can start vegetable and flower seedlings indoors from four to 12 weeks before the last average spring frost in their area, which means it’s time to get started. Making homemade planting medium can be more economical than buying a sterile mix at the store, said Brooke Edmunds, a horticulturist with Oregon State University’s Extension Service.
A good soil mix for germinating seeds should be fine and uniform, yet well-aerated, loose and free of pests, diseases and weed seeds.
A good germinating mix must be fine and uniform, yet well-aerated, loose and free of pests, diseases and weed seeds, Edmunds said. It also should be low in fertility and total soluble salts, yet capable of holding and moving moisture.
But beware, she warned. Soil straight from your backyard just won’t do the job.
Typical backyard soil is too compacted, full of weed seeds and it is not pasteurized, causing seedling diseases and death. Native soil often does not drain as well as seedling mixes. And it can develop a crust that prevents seedlings from pushing though.
Edmund’s recipe for a good basic pasteurized medium for growing seedlings is a mixture of one-third pasteurized soil or compost, one-third sand, vermiculite or perlite, and one-third peat moss.
“Many people just use half peat moss and half perlite, vermiculite or sand, and this combination seems to work well, too,” she said.
To pasteurize a small quantity of soil or compost in an oven, put the slightly moist soil or compost in a heat-resistant container or pan. Cover with a lid or foil. Place in a 250-degree oven; check the temperature periodically using a candy or meat thermometer. When the mix reaches 180 degrees, cook for an additional 30 minutes. Avoid overheating it, as the structure of the soil may be damaged, rendering it useless as a seedling soil ingredient.
Mix pasteurized soil or compost with peat moss. Add sand, vermiculite or perlite. All ingredients are available at most nurseries and garden stores.
Another task to complete before the start of seed-sowing is to clean your pots, trays and flats. After washing, rinse the containers in a solution of 1 part chlorine bleach to 10 parts water to kill remaining plant disease microorganisms that could weaken or kill your tender young seedlings.
For information about starting seeds, see Extension’s publication Propagating Plants from Seeds.
— Kym Pokorny
About Gardening News From the OSU Extension Service: The Extension Service provides a variety of gardening information on its website. Resources include gardening tips, videos, podcasts, monthly calendars of outdoor chores, how-to publications, and information about the Master Gardener program.
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Seed starting in containers, whether indoors or outdoors, is a very popular and common gardening practice, but it also comes with a lot of questions. It is tempting for people to assume that topsoil should be used for seed starting. However, read on, as this may not be the case.
So, can topsoil be used to start seeds? No, it is not recommended to use topsoil for starting seeds in containers. Seeds need the right amount of oxygen and moisture for the best chance of germination, and topsoil does not provide the best growing medium that suits these needs.
It is an odd paradox that the best soil for seed starting actually contains no soil at all! I can definitely see how it can be confusing that topsoil should not be used to start seeds, even though plants and seeds grow outdoors in the soil in nature all the time.
- What’s Wrong With Topsoil for Seed Starting?
- What Is the Best Seed Starting Mix?
- What is the Purpose of Topsoil?
- Related Questions
- Video Guide
- Answer One: Seed Germination Rates
- Answer Two: Seedling Selection
- Exceptions to The Rule
- The Right Mix for Starting Seeds
What’s Wrong With Topsoil for Seed Starting?
In order to understand why topsoil does not make the best growing medium for seed starting, let’s first get an understanding of what qualities seeds need in order to germinate and grow in the best way possible.
Basic Soil Needs for Seed Starting
- Air. Once a seed germinates, the roots of the plant need oxygen in order to grow. This is a basic need of all plants and is especially important for a young plant trying to establish its roots.
- Water. Seeds need moisture in order to germinate. The growing medium should have good enough drainage so that once the seed sprouts, the roots are not drowning. It also needs to be able to hold water well – like a sponge – in order to provide enough moisture for those roots.
- Space for easy growth. Once a seed sprouts, the roots need a nice loose mixture to be able to easily grow. The roots need to be able to spread easily. Too much compaction would prohibit this root growth.
- Freedom from predators. Just like an infant baby, seedlings are not very strong and will do best if you provide them with the best environment possible. Having the growing medium be free of disease, insects and mold will give seedlings a better chance to grow healthy and strong.
Topsoil Misses the Cut
Even if topsoil from the garden is top notch, it can still cause issues when putting it into containers; especially, if starting seeds indoors.
- Topsoil is too heavy and compact. This causes issues with the first three needs for seed starting listed above. Heavy and compact soil does not drain well and water can choke out your plants. At the same time, it does not provide enough air flow to the roots and the roots cannot push through the heavy and compact soil easily enough to grow and flourish.
- Topsoil can contain diseases. Too many pathogens and disease organisms are out there that can harm seedlings at their early stage of life when they are not strong enough to fight off these issues.
- Topsoil can contain harmful insects. If topsoil is dug up out of a garden and brought inside to start seeds, it will undoubtedly have some insects in it as well. While this may upset a spouse, it can also be a killer for seedlings if those insects like to feed on roots or young leaves.
As you can see, while it may be tempting to use soil from the garden to start seeds in containers, it is best to find a soil-less mix for any seed starting needs. Continue reading to find out what type of mixes are recommended for seed starting.
What Is the Best Seed Starting Mix?
Well, if topsoil should not be used to start seeds, what is the best choice? As listed above, certain qualities are necessary for a seed starting mix to have in order to give plants the best chance for success.
Seed starting mixes generally consist of peat moss or coco coir, vermiculite and perlite. Peat moss or coco coir is a great choice for the base of the mix because it is an organic material that holds water well while providing many channels for air. Vermiculite and perlite are lightweight minerals that improve air circulation and drainage when used in a soil mix.
Seed starting mixes can be bought at the store. It can be confusing as to which product to buy because unfortunately, companies are not always very consistent with their labeling. Some products labeled as potting mix would work perfectly fine for seed starting, while others would not. Other names used are potting soil, container mix, and seed starting mix. In general, look for the terms seed starting or germinating and you should be good to go.
Keep in mind that seed starting mix does not need to contain fertilizer. After germination, a seedling gets all the nutrients it needs from the original seed from which it sprouted. Adding fertilizer, especially synthetic types, can actually do more harm than good to seedlings. They are too powerful and can burn your plants. For more information on fertilizer for containers, check out this article – Does Potting Soil Need Fertilizer?
For no one can lay any foundation other than the one already laid, which is Jesus Christ.
1 Corinthians 3:11
Make your own mix!
To save money, I buy peat moss, vermiculite and perlite in bulk and make my own mix. If you search on the internet for homemade seed starting and potting mixes, you will undoubtedly find at least ten different recipes. I think they all probably work just fine, so do not get too hung up on the amounts of each item.
I use a bowl from the kitchen and combine four parts peat moss, one part vermiculite and one part perlite. Since I use the same mix as my potting mix, I also add one part of my homemade compost I make from shredded leaves and coffee grounds and one part of my homemade worm castings.
Remember, seedlings do not need this extra fertilizer, but because these amendments are organic, it will not hurt the plant in any way. They will start taking up the nutrients from the compost and worm castings when they need it.
When mixing these ingredients together, be sure to water it well before using it in any containers. If you grab a handful of the mix and squeeze, a few drops of water coming out is the correct amount of moisture.
If you want more details about making your own seed starting mix, check out our full article DIY Potting Soil and Seed Starting Mix to Save Money.
What is the Purpose of Topsoil?
You may be wondering that if topsoil cannot be used for seed starting, what are ways it can be put to good use? You are likely able to obtain it for free or buy it at the store very cheap. The following are a few ways topsoil can be put to good use.
- Use it to fill raised gardening beds. Other amendments should also be mixed with it, such as compost, but good topsoil does contain a good amount of good minerals and nutrients that plants will love.
- Use it to amend poor soil. Again, other amendments should also be mixed with it. While topsoil does contain some nutrients and minerals, it is quite heavy and can compact easily, so other organic material needs to be added for additional nutrients and better drainage.
- Use it in the lawn to fill in bare and low spots. Spread the topsoil and then apply grass seed.
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How is topsoil formed? Topsoil is formed slowly over time from the breakdown of minerals in rocks and decaying matter from dead plants. It is claimed to take one hundred years to form one inch of topsoil!
Can you use topsoil instead of potting soil? In general, this is not recommended because topsoil is too heavy and can compact easily in containers. Check out our article Can You Use Garden Soil for Potted Plants for more information.
What is the difference between garden soil and topsoil? While these terms are commonly interchanged and can mean the same thing, technically speaking, topsoil is the natural soil that would be removed from the top few inches of untouched ground, while garden soil is topsoil that has been amended with compost or other organic material.
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I recently got an email from Sally with a familiar question. It’s the same exact question that I had when I was a beginner gardener and wondered how to start seeds:
“I’m sure this is a silly question, but I always see it recommended to plant more than one seed per hole. But why? I just got a seed starting kit with some seeds and want to make sure I’m using them efficiently. Can you help me out?”
It’s a great question, Sally! Understanding the answer to this question will improve your understanding of gardening and seed starting in general, because the answer hinges on an important concept: seed germination.
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Answer One: Seed Germination Rates
Not all seeds are created equal. Some plant species have higher germination rates than others. Even within a single plant type, some of the seeds are older than others, causing the germination rate to go down.
Imagine you’re growing arugula and the average germination rate is 90%. If you plant a 72 plant starter tray with one arugula seed per insert, you can expect only 65 of those plant inserts to actually germinate (72 x 90%).
Now imagine you plant three arugula seeds per insert. Each of these seeds has a 10% chance of failing, so the probability of them all failing is 10% x 10% x 10% = 0.1%. This means that you are 99.9% likely to have the seeds in that cell germinate. So in a tray of 72 inserts, it would be extremely unlikely you would have any seeds not germinate — barring other factors that affect seed germination.
In short: Planting more seeds per hole increases chance you have perfect germination rates.
Answer Two: Seedling Selection
Just like not all seeds are created equal from a germination standpoint, not all seeds germinate equally. Sometimes you have a seed that shoots off like a rocket and becomes too leggy. If this was the only seed in your insert, you’d be forced to use it.
By planting 2-3 seeds per cell, you allow yourself to luxury of choosing the seedlings that look the strongest. All you have to do is determine which one you like the most, then snip off the other seedlings to kill them.
Exceptions to The Rule
Like most things in gardening, there are always exceptions to this rule of 2-3 seeds per hole.
If you’re planting large seeds like cucumbers, melons, or pumpkins, you should only use one seed per hole. However, you can still plant seeds close together and then thin them out once they’ve established themselves. You just want to avoid crowding these large seeds together so you don’t mess up the germination process.
If you’re growing certain herbs (cilantro, dill, basil), you can get away with planting multiple seeds per hole and leaving them all there as they germinate. These plants can handle being planted right next to each other and basically become one larger, bushier plant.
Now that you know how many seeds to plant per pot, you have a deeper understanding of seed germination in general. For more on seed starting, please check out the simple seed starting for hydroponics guide.
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Seed starting mixes can be confusing – there are so many at different price points; who’s to know what’s good and what’s junk? Is the extra expense worth it, or is the cheapest mix just as good?
We looked at a few examples on the market to see what is available from a range of prices and suppliers. Starting with the big box stores and going to a local hardware store with a good gardening selection and then to a garden center, we chose a representative sample to give you a good starting point in choosing what to use for your seed starting.
This is in no way meant to be an exhaustive or comprehensive review of all available seed starting mixes. From what we can tell, there are many different brands and types of seed starting mixes. We suspect that a few companies are producing many of the re-bagged or re-branded mixes, much like there are exactly two car battery manufacturers in the US, selling dozens of brands of batteries through different outlets. Our goal is to provide you with an educated overview of some middle of the road seed starting mixes so you will be better informed when confronted with the “Great Wall” of seed starting mixes at your local garden center.
Along the way, we found seed starting mixes are seemingly designed to be confusing – especially when they are stacked next to each other in the garden section of the big box store or in the garden center. None of them were the same size, so easily comparing costs while in the store was difficult, requiring converting different prices to the same volume. We chose a quart size as the standard for the seed starting mixes, as most home gardeners will be using smaller amounts than market growers.
Another interesting thing we found was the labeling on each bag and the claims made. Some had the word “Organic” prominently displayed or used as part of the name, with no verification that the contents were, in fact, organic. “Natural” was also strongly used, with no substantiation.
In addition to the seed starting mixes, we have included a potting mix we’ve started seeds in for several years and have recommended to our customers. This is included as a reference point, as gardeners in some areas won’t be able to use a potting soil to start seeds due to high humidity and the associated fungi and mold challenges. Others, like us, will be able to use it to their advantage.
What are seed starting mixes and do I need one?
Seed starting mixes are not soil, they are blended to create a good environment to get the seed germinated and into the seedling stage, where it is transplanted into a potting soil or garden soil. For a closer look at the components that typically make up seed starting mixes, read Seed Starting Media for the Home Gardener.
Not everyone needs seed starting mixes, some gardeners do very well starting their seeds in potting soil or a rich garden soil. This often saves the work and stress of transplanting, but if you need a sterile soil because of mold or fungi pressures, then seed starting mixes will really help. Other gardeners just trust a sterile seed starting mix and have had good results for their garden.
Let’s take a look!
For all of the photos, click to enlarge them for more detail.
Jiffy Seed Starting Mix
First up is Jiffy “Natural & Organic” Seed Starting Mix. We bought this at our local True Value garden center, so it should be available at independent garden centers and possibly the big box stores. There are several interesting things about this seed starting mix. First off, there is no ingredients listed, either on the bag or on Jiffy’s website. Second, this is an OMRI listed product, meaning it is approved through the Organic Materials Review Institute (OMRI).
OMRI is an international nonprofit organization which certifies which input (fertilizer, herbicide, pesticide, etc.) products are allowed for use in certified organic agriculture. OMRI Listed® products are allowed for use in certified organic operations under the USDA National Organic Program. This is a big deal, there is no way to buy or sneak a product in and most companies will prominently display the OMRI listing on their bags, as you’ll see below.
Jiffy doesn’t mention it, or list the ingredients. With the OMRI listing, we feel pretty comfortable in using this, but would like to see what makes up the seed starting mix.
Here is what the bag looks like. On first read the “Natural & Organic” label is somewhat misleading, especially with no mention of the OMRI listing. The bag has 12 quarts for $6.99, so not a bad price, especially for a smaller home gardener who will use less than one bag to start their tomato, pepper and eggplant seeds.
Jiffy Seed Starting Mix Closeup
An initial look at the soil shows what looks to be some coconut coir, possibly some peat moss, perlite and maybe some compost, but it’s hard to be sure. It had a good, rich soil aroma and was finely textured with no large lumps or woody chunks in it.
Jiffy Seed Starting Mix
Here’s a closer inspection.
Overall, we were pleased at what we saw, with the caveat of wishing to know what the ingredients were making up the mix. On the plus side, OMRI listing is very good – indicating it is accepted for certified organic agriculture.
Miracle Gro Seed Starting Mix
Miracle Gro Seed Starting Potting Mix was next up. The labeling seems to be trying to straddle the seed starting mix/ potting soil applications without specifying if it is suitable for both. The bag says it is enriched with Miracle Gro, which is no surprise, and the mix is excellent for starting cuttings. The bag has 8 quarts and is $4.77 at Lowe’s or Home Depot, which is enough for a small gardener to start seeds with. This mix is not OMRI listed. Here is Miracle Gro’s online listing.
Our concern with Miracle Gro is their blend of plant food emphasizes vegetative growth and strong flowering, without the nutrients for fruit or food production. We have had numerous customers call or email asking why their tomato plants had such lush, dark green leaves and were covered in flowers but not producing a single tomato, or very few. When we asked if they were using Miracle Gro, they seemed shocked that we could know what they were using!
Miracle Gro Seed Starting Mix Ingredients
The ingredients are listed – peat moss, perlite, Miracle Gro fertilizer and a wetting agent. Not terrible, but not the greatest either. If you’ve read Starting Seeds at Home – a Deeper Look, you remember that during germination, seeds have no need for fertilizers as they carry everything they need to sprout and establish a seedling inside the seed coat. The fertilizer might be beneficial during the seedling phase, but more soil nutrients are definitely needed to sustain a healthy plant, so this is not a seed to garden transplant mix. The wetting agent will be beneficial in the germination stage, but could contribute to mold and fungal issues for the seedling if the gardener isn’t aware and careful in not over watering and inadvertently saturating the soil, which the wetting agents will make worse.
There is valid concerns raised about the harvesting of sphagnum peat moss from Canada as being sustainable or not. It takes peat moss bogs several hundred to a thousand years to mature, depending on the conditions, so it is hard to “sustainably” manage them.
Miracle Gro Seed Starting Mix Closuep
Looking at the soil it was fine and fairly light with some perlite easily seen among the peat moss. It looks a bit chunky, but the feel wasn’t. It did seem to have a few larger chunks of material, but that shouldn’t matter in starting seeds.
Miracle Gro Seed Starting Mix
Not a bad seed starting mix, and we would take this over several store brands we’ve seen.
Black Gold Seed Starting Mix
Black Gold Seedling Mix is another of the seed starting mixes listed by OMRI, and it says so on the top right side of the bag. Black Gold is a well known supplier for hydroponic growers and would be considered one of the premium or super-premium seed starting mixes.
We sourced this from True Value, so it should be available from an independent garden center but probably not from the big box stores. The bag has 16 quarts for $6.99, so is a good value with enough to start seedlings for a very large garden or for sharing between a couple of neighbors or a community garden. Black Gold’s online listing.
Black Gold Seed Starting Mix OMRI Listing
In addition to the OMRI listing, the label shows this seed starting mix to be enriched with silicon for thicker stems and improved root mass. While it is true that silica and silicon are important components to strong cell, stem and root growth, there is no mention of the silica in the ingredients. This might be a marketing approach, or it could be something worthwhile, but the bag doesn’t expand on the benefits.
Black Gold Seed Starting Mix Ingredients
Black Gold’s ingredients are pretty standard for seed starting mixes; peat moss, perlite, dolomite lime and Yucca extract as the organic wetting agent. Dolomite lime is calcium magnesium carbonate and increases the pH of soil, but also aids in organic decomposition. It is high in magnesium and calcium, which could be good for acidic soils but could be trouble for alkaline soils like we have in the West. The amount of dolomite lime is probably small and wouldn’t cause much of an effect in the amounts used in seed starting mixes, but might not be the best use as a garden or container amendment.
It is good to see what is used as the wetting agent – Yucca extract.
Black Gold Seed Starting Mix Closeup
The appearance confirms what we would expect to see from the ingredients – lots of white perlite specks on a very dark peat moss background. It is light and fluffy, allowing good root penetration and establishment.
Black Gold Seed Starting Mix
Because this is OMRI listed, we would choose it over a non-OMRI listed seed starting mix.
Square Foot Potting Soil
The final comparison is included even though it is labeled as a potting soil and isn’t one of the seed starting mixes. Garden Time’s Square Foot Gardening Potting Soil is about as close as you can come to a complete garden soil in a bag, and we’ve used it for a number of years to start seeds, transplant seedlings into and have been quite happy with the results.
The “Natural & Organic” label is prominent on the bag, even though this is not OMRI listed. Some of the other claims are “Proven Formula for Optimum Growth” and “Great for Vegetables”.
We sourced this from Lowe’s and have bought it at Home Depot in years past. We also saw it with a different bag in True Value. The company name is Garden Time and is a part of Gro-Well brands in Tempe, AZ. The bag is 1.3 cubic feet, which translates to 38.9 quarts for $8.98, making it by far the most cost effective seed starting medium. Garden Time’s Square Foot Gardening online listing.
Square Foot Potting Soil Ingredients
The ingredients are compost, peat moss, coconut coir, vermiculite, bloodmeal, bone meal, kelp meal, cottonseed meal, alfalfa meal and worm castings. The peat moss, coir and vermiculite are classic seed starting mix ingredients, with the compost, different meals and worm castings adding nutrients to the soil for the plants growth and health. We use this as both a seed starting mix and potting soil or transplant soil because of the nutrients, which are lacking in straight seed starting mixes.
Square Foot Potting Soil Closeup
Looking at the mix it’s easy to see the vermiculite granules, which are gold colored specks instead of the white of perlite. It has a lot darker look due to the compost and additional nutrients.
Vermiculite is mica expanded by intense heat and is used much like perlite to retain water, decrease soil compaction and improve water retention.
Square Foot Potting Soil
Here’s the closeup look.
Seed Starting Mix Cost Chart
The breakdown of the costs per quart tell a story – they are in the ballpark of each other, with the exception of our rogue potting soil thrown in. Most of them have similar ingredients, with a few different additions.
If we had to buy a dedicated seed starting mix and couldn’t use the potting soil we do, our choice would be one that lists the ingredients and is OMRI listed. Having said that, any of the seed starting mixes with the primary ingredients of coconut coir (our preference) or peat moss with perlite or vermiculite would make a satisfactory seed starting mix.
What you choose will depend largely on the availability for you, how much you need and the cost.
As part of the test, we are starting seeds in all four mixes and have documented our results in Seed Germination Observations. Our Potting Soils article gives you some details on what to look for in good soils to transplant your young, tender seedlings into.
The Right Mix for Starting Seeds
The Right Mix for Starting Seeds
Q. I’m really interested in starting my own vegetables from seed this year using soil blocks. But I’m finding quite a disparity in the basic “recipes” for soil block seed starting. They generally involve peat moss, soil from the existing garden, perlite, lime, compost and a fertilizer mix of bloodmeal, greensand and rock phosphate; but they vary. Some omit the garden soil, some omit the lime, and the amount of the fertilizer mix can vary greatly from recipe to recipe. I’ve also been told to find another source of potassium because greensand takes too long to break down to be effective for the young plant’s needs, but I am doubtful of this information. Can you give me any advice?
—Dave in Carlisle PA
A. I would never ever ever include garden soil in a seed-starting mix. But yes to milled peat moss; and yes, you then need a tiny bit of lime or hardwood ash from a wood stove or fireplace to balance the pH, as peat moss is acidic, while lime and wood ash are alkaline. (But you only need a tiny bit; say a teaspoon of lime or ash per quart of peat; “a little dab’ll do ya”).
Perlite—a mined volcanic mineral that’s popped in big ovens until it looks little Styrofoam balls—is absolutely essential in a seed-starting mix. It allows more air around the roots of the developing plants, provides less resistance to the growth of those roots than almost anything else you could use, and greatly promotes drainage.
But that ‘fertilizer mix’ just sounds weird. Your source was correct; greensand takes quite a while to become available to plants; same with rock phosphate. In the outdoor garden they’re typically added to the soil six months to a year in advance to give soil microorganisms time to release the power of their (respectively) potassium and phosphorus. Bloodmeal is a slaughterhouse by-product that does provide Nitrogen, but it’s fairly ‘hot’ and could burn tender young plants; and it certainly wouldn’t be the first choice of plant food for vegetarians.
Instead, use compost. A high quality compost (or worm castings) will supply all the nutrition that baby plants require and provide it gently. (You’re not supposed to feed young plants more specific fertilizers until the plants have developed a couple sets of true leaves.)
And I personally don’t get the idea of soil blocks—hunks of dirt pressed into a blocky shape with a specialized device—at all. So my ideal seed-starting mixture is for use in containers—recycled garden center six packs or similar vessels that retain water, provide excellent drainage and are long and deep enough to allow maximum root growth.
Now, when I make my own seed-starter, I mix equal amounts of milled peat moss (with a little bit of ash from my wood stove to balance the pH), perlite and screened compost. But I start a LOT of seeds, and often use packaged mixes to save time. These can be called ‘soil-free mix’, ‘professional mix’, ‘sterile medium’, ‘seed starting mix’ or something similar. Some companies substitute coir—shredded coconut fiber—for the peat moss in their mixes and that’s fine. Some premium mixes will include small amounts of gentle natural fertilizers like worm castings, and that’s excellent. Just don’t use mixes that contain chemical fertilizers like Miracle Grow or Osmocote; or ones that have added so-called ‘water-holding crystals’. (Note: These tainted mixes may be the only choice at big box stores).
Q. Dave replies: “OK Mike—tell me how you really feel; don’t hold back! (LOL!) Now, I really want to understand the WHY of a few of these things. I believe the garden soil is meant to prevent some form of shock when transplanting. You seemed very passionate in this answer, what can go wrong by adding a little soil? It’s the same soil the plants will go into in two months; right? And why are seed blocks bad? Last year I used toilet paper cores to start my seeds and planted them right in the ground. That worked fine, but I’d hate to not use the $100 block starter I already paid for!
A. Yes, garden soil is where the plants are going to go eventually, but that soil can contain weed seeds and harbor disease organisms—both of which are much bigger threats to baby plants than big ones. Most garden soils are also heavy with clay, which may help soil blocks hold together better, but sure seems to defeat the root-enhancing purpose of a light, soil-free mix. And having garden soil in the mix won’t lessen transplant shock one bit. The best way to avoid shock is to not plant outside too early in the season, to ‘harden the plants’ off by gradually acclimating them to the great outdoors for a week before actually putting them in the ground, and to install them in the late afternoon instead of early in the morning, so they can get settled in before they have to endure their first long, sunny day.
And finally, I never ‘got’ soil blocks because by definition they compress the soil, which is the opposite of what’s recommended for helping baby plants develop deep roots. However, I DO like your toilet paper core trick. Those little rolls of cardboard have enough depth for excellent root growth, won’t impede that root growth when planted outside, and will decompose over the course of a season. Slide them up a bit at planting time and you have instant cutworm protection as well!
Is the block maker attractive? Maybe you could pose it on a piece of marble and call it art!