Now try another little test: Tell me what all those different ingredients do, why they are used in potting soil. Even if you passed the first test with flying colors, chances are slim that you can pass this one.
NOT all potting soil is the same. Far from it. Most people think it is and pay no more nevermind to it once bought, as is evidenced by the standard answer I get when asking a customer what kind of potting soil their plants are growing in: "Regular potting soil." There are so many brands of potting soil made by so many different companies in so many different places that "regular potting soil" doesn't really mean anything. It's like saying you had food for lunch. There are too many variables to determine what that really is. What's the main ingredient? What kind of plants it formulated to grow? Does it have fertilizer in it? Is it all quality ingredients or does it contain cheap fillers?
Firstly, the "soil" in "potting soil" is a misnomer. According to Wikipedia, soil is an American metal band formed in Chicago, Illinois in 19... Wait. What the hell?! Wikipedia is ALWAYS WRONG! Those freaking morons! They're always ... Huh?! There's more than one entry for "soil"? Well ... uh, um ...
|Cross section of soil
Pots are non-natural environment we have created, so we have to take on the role of Mother Nature. Most plants have evolved to grow in particular soils She gave them, soils with certain attributes like moisture retention or fertility, and we have to match the soil to the plant just like Ma Nature does. Unless we just want to grow plants that are native to our area (which leaves out ninety-nine percent of all vegetables, flowering annuals, tropical houseplants, and other miscellaneous ornamental plants), we can't just go out in our yards, dig up some "skin of the Earth", plunk it in a pot, and expect it to work. You can use some in a potting soil mix, sure. A few handfuls to add in some good minerals and seed the new soil with beneficial organisms, especially if it's taken from that top layer, the dark rich looking one in the photo above. But pure dirt? Very much most often, no.
Instead of telling you which soil is best and sending you on a sometimes-fruitless search for "The One" that ends up being "The Not-So-Good-One-For-Me", I'd like to teach you how to pick a great potting soil by reading the ingredients list on the label and knowing what those ingredients do. Also, once you learn that, you can easily pick another one when your usual brand isn't available.
Better yet, once I get done with you, you can buy those ingredients separately and custom mix your own potting soil that's perfect for the particular plants you want to grow. Talk about freedom! No longer will you be constrained by the meager choices at the garden center! No more will you spend good money on cheap crap! Never again will you be held hostage by not knowing, having to settle for whatever someone else thinks you want! You can grow ANYTHING YOUR LITTLE HEART DESIRES, AND GROW IT WELL! WOOT!!
But I'm getting ahead of myself (... !melodramatically! ...). After the list of common potting soil ingredients I'll give a basic rundown on how to make your own. But first...
The most common potting soil ingredients and what they do
(in "sort of" order of how often you'll encounter them in potting mixes, most common first)
Pros: Is acidic, so is great to use with our alkaline water. And it holds that water well when hydrated (holds 16+ times it's weight). Is widely available most anywhere.
Cons: Sometimes holds water TOO well, becoming so throughly soaked and soggy that it takes forever to dry out. Conversely, sometimes it gets so dry it pulls away from the sides of the pot and can't be rewetted easily, the water just beading up on the surface and not soaking in at all. When this happens, it runs "around" the root ball - across the top, down the sides, then out the bottom - with little or no moisture soaking in. For these reasons, and since our climate is feast or famine when it comes to rain, it's best to mix in a healthy amount of other things with that peat, making your mix something like less-than-half peat. (Hint: When peat or any potting soil gets too dry, either use an olla-type waterer or bottom water overnight to rewet it.)
Environmental concerns: The US gets 80% of it's peat from Canada. Canada has a vast area of peat bogs, over 200 million acres, and peat is harvested from less than one percent of that. Apparently learning from the Europeans who in turn learned from the past damage to and destruction of so many of their peat bogs, the Canadians harvest their peat in a sustainable, managed way - they leave enough moss underneath and at the sides of the harvested bog to grow back relatively quickly. The North American Wetlands Conservation Council estimates that a bog managed this way can return to an "ecologically balanced system" in 5 to 20 years. That doesn't necessarily mean "full-blown, healthy, peat bog", but it's a really good start. Couple that with the estimate that new moss growth in all of Canada's peat bogs is 60 times as much as what is harvested and it does appear that using peat is not the great evil it once was. Just check the label to ensure you're getting Canadian, eh?
Pros: Doesn't get waterlogged or quite as dry/hydrophobic as peat (doesn't pull away from the sides of the pot like peat does), yet holds eight or nine times it's weight in water. Has a high nutrient-absorption capability, so holds on to those liquid ferts well and doesn't let them run out the bottom as much as other things do. Due to it's high lignin content (tough, indigestible fiber), it lasts a long time before it breaks down, but doesn't tie up nitrogen like uncomposted wood (see below for more on that). Very renewable and a good use of a former waste product.
Cons: Not as easily available as peat, though it's getting easier to find as time goes on. May contain too many salts - look on the package for "washed" or "desalinated".
Further Environmental concerns: It literally grows on trees, so in that way it's a sustainable resource, being a waste product of an industry that has been around for hundreds of years. Coconuts are grown for edible coconut, oil, and fiber that is used to make brushes, rope, and padding for automobile seats and matresses. The coir we use in horticulture is the fine dust left over after the larger, longer fibers have been harvested. This dust previously had no widespread use and took twenty years to break down into compost, a fact that makes it good in potting soil but problematic for the producers.
Most coir is produced in India and Sri Lanka, so there is a concern with the environmental cost of having it shipped to our side of the world. It's relative light weight and compression into bricks to take up less space during shipping lessens that somewhat, but it's still a very real concern. Also, the retting process (soaking in water for long periods of time to soften it) uses copious amounts of water, much of it being fresh water that has had salt or other substances added, so there is a pollution concern there as well.
Pros: Easily available. Does not break down over time.
Cons: Since it's takes hundreds of thousands of years to form, it may not be sustainable. For now though, our use of it isn't threatening to use up all the world's stores.
Further Environmental Concerns: Today's vermiculite does NOT contain asbestos. For much of the 20th century, the majority of vermiculite came from a mine in Libby, Montana, which had a deposit of toxic asbestos (also a naturally-occuring mineral product) intermingled, contaminating the vermiculite. Since vermiculite is commonly used as loose-fill insulation, and since, during it's years of operation, as much as 80% of all vermiculite insulation produced in the US came from this mine (along with vast amounts used worldwide), this was a massive danger to public health, finally showing up when miners and others that worked around it became seriously ill and the mortality rate rose quite high. This finally led to the Libby mine being shut down in 1990. Today we get our vermiculite from other mines that continually do asbestos testing to ensure it is asbestos free.
The science nerd in me has to geek out for a minute about how it really does pop like popcorn. The tiny bits of water trapped inside it expand when heated and finally explode, just like in a kernel of corn. The water gets there when the molten lava flows into a body of water like the ocean, then slowly cools, trapping it inside tiny little "cells". How COOL is THAT?!
Pros: Fairly readily available. Does not break down over time. Much lighter in weight than other things used to increase drainage in pots, such as sand, gravel, or decomposed granite, and also has much better air-holding capacity than those more dense products. That air-holding ability makes it really great for use in peat-heavy potting mixes. It's so good at that that a mix of peat and perlite is one of the, if not THE, most popular potting media choice for commercial greenhouse growers. It may be (arguably) sustainable since it is a volcanic product and volcanic processes are ongoing, and it isn't mined in the traditional tear-up-and-devastate-vast-amounts-of-land sense, but "harvested" by scraping from the top of the ground where it is deposited by previous volcanic activity.
Cons: Can float to the top of potting mixes when they are continually flooded, but doesn't do it as badly as the styrofoam pellets some potting soil makers use instead. (To find out which is in your soil, pick out one of the big pieces and squeeze it. If it flattens, it's styro - if it feels more like a crunchy grain of sand, it's perlite.)
Pros: I can't say enough good things about compost. IT'S ALIVE! IT'S NATURAL! IT'S SUSTAINABLE! And it's SO VERY healthy for your plants. Fresh compost contains beneficial bacteria and fungi needed by most plants to live, just like how we need the beneficial bacteria in our guts. Compost has excellent moisture-holding capacity, and contains a decent amount of needed nutrients (Dead and decayed organic matter is what plants have evolved to use for food. Makes me think of vultures in the animal kingdom. Plants are the buzzards of the plant kingdom! They're Arizona Chickens! YAHAHAHAHA!!).
Cons: It's fairly easy to get bagged compost that is old and has dried out, killing the beneficial organisms in it, though that's easily remedied by re-inoculating it with the beneficial organisms (adding a few handfuls of fresh compost or pouring on some aerobically brewed compost tea) and watering that in with molasses (1/4 cup per gallon of water) to bring life back to it.
Also, manure compost can contain broad-leaf herbicides that can still be present in large enough concentrations to damage plants. Herbicides are commonly sprayed on hay fields to kill weeds, and there are herbicides today that can survive the hay being cut, baled, stored, fed to livestock, and come out in the manure still able to kill broad-leaved plants. That's some scary shit, huh? It can take up to two years of composting for these herbicides to break down enough to not be a threat. To test for this, simply plant some pinto beans or other dried beans from your kitchen cabinet in some of it, planting some in regular soil as a comparison, then look to see if the beans growing in the compost/manure grow deformed. If so, that compost is probably tainted.
|"Raw" rice hulls, before boiling or composting
Pros: Like coir fiber, it's a good use of an agricultural industry waste product.
Cons: Can dry out quickly.
Further Environmental Concerns: Many people erroneously believe all rice and it's associated products like rice hulls are imported from far away when in fact over ten percent of the world's rice is grown in the US. Certainly some is still imported since our climate isn't amenable to growing all types of rice, but it's doubtful that rice hulls are imported in as large amounts, so the environmental threat from shipping isn't as large of one as may seem from first glance. Many people take issue with rice farming's massive use of water. In rice-growing areas such as Texas and California where water is in short supply, this is a valid concern.
Some makers, like Fox Farms, use "aged forest products", something very different from wood. This is mostly leaf mold (rotted leaves), which is a really good thing. It's Mother Nature's compost, what all forests have evolved to use as the basis for their own food. In a forest it's slowly broken down, colonized by beneficial bacteria and fungi, and forms the very cornerstone of the forest itself. More trees and plants take root in it, insects move in, small animals prey on them, everyone poops and dies and, along with more leaves, gets transformed into nutrients that trees and other plants use, and on it goes.
Some unscrupulous manufacturers will use sawdust or fresh shredded wood as filler since it's cheap. Again, read your labels - "pine bark fines" = fine (see what I did there?), "sawdust" = avoid, "composted sawdust" or "composted shredded wood/landscape refuse" = maybe, but only if it's low on the list and is WELL composted, not leaving big chunks of wood still in the mix.
Pros: Can be sustainable if obtained from a reputable source who composts it well and mixes it with other good things. For instance, Austin company Organics By Gosh takes in landscape trimmings from professional landscape companies as well as homeowners, diverting it from landfills. They compost all that stuff correctly and mix it with other good things for use in their retail products, including good-quality potting soil and compost.
Cons: Since they make such a cheap filler, it's easy to get potting soils and composts with a lot of uncomposted wood products in them, especially when they are bought at "big box" stores. Again, uncomposted wood is NOT good for any kind of horticultural use except as mulch ON TOP of the soil, NOT IN IT. Here's why: When wood decomposes, it ties up nitrogen in the soil. Wood contains much more tough fiber (such as cellulose and lignin) than other soft plant matter like leaves and compost, so the beneficial bacteria in your soil will be so busy working so hard on that wood that they won't be breaking down those other easier-digestible things. Bacteria breaking things down is what feeds the plants, so if that process is compromised, slowed down, those bacteria aren't making as many nutrients and the soil can become nitrogen-poor, starving your plants. To sum it all up: Fully-composted wood products that are rotted to the point that you can't tell they're wood anymore are fine to use in potting soil, but if you can still tell it's wood, avoid it.
One note: You may notice that a lot of commercial plant growers use wood or bark chunks in their growing mixes. Usually it's pine bark and for some reason pine isn't as bad as other wood, plus commercial growers fertilize a lot more often than we do with chemical fertilizers that have nutrients that are immediately available to the plants and don't have to be broken down by bacteria and fungi. That's how they can get away with it - they have push-button fertilization that they use often.
Another good thing to add to increase drainage is expanded shale, aka haydite. Expanded shale is mined from the naturally occurring Midway Shale deposit that runs not far underground diagonally across must of the state of Texas. Then it's crushed to an inch or smaller, and kiln fired so it expands. During cooling, voids are formed which gives expanded shale it's ability to soak up water, then release it later.
If you're not growing xeric things like succulents or cactus, and don't need to add weight to the pot of a top-heavy plant, you may not need much if any of these. Drainage can be better accomplished by other things in the above list since they also increase the soil aeration and actually catch some of that moisture as it goes by, releasing it later - same with nutrients. Since they're not porous in any way, sand and gravel just let those pass on by.
Pros: Adds needed minerals. Can be cheap. Increases drainage dramatically, so is good for cactus and succulent mixes.
Cons: Heavy. Can be TOO free-draining and doesn't contribute to aeration of the soil, so is best to avoid in large quantities except in cactus and succulent mixes.
|Fine sand: DON'T USE this
And a note about "gravel in the bottom for drainage": That's a myth and can actually be bad. There's a normal thing going on in your pots called a "perched water table", a constant-level layer of moisture at the bottom of all bodies of appropriately-hydrated soil that contains more moisture than the rest of the soil due to gravity. That perched water table is present in ALL potting soils that aren't parched-dry, and putting a layer of gravel at the bottom does nothing except move that perched water table up higher and take up valuable real estate in the pot. If your potting soil doesn't drain well enough, you need to mix that gravel, or better yet perlite, into the soil, not put it at the bottom. And if your shallow pot always seems to stay too wet, then you need either the aforementioned something-else-mixed-into-the-soil or a bigger, deeper pot - not gravel in the bottom. Here's an article that explains it in much more detail, with pictures. And here's another page with some good tidbits on this subject.
|Lithops, a type of succulent, in soil heavily amended with coarse sand.
|And more lithops because they're just so damn cute.
If you think it'd be good to buy a soil with fert in it, be aware that doing that can limit your use of the soil later. If a soil has a phosphorous fertilizer in it for flowers, your herbs will be more apt to bloom if you use it for them. If there's a high nitrogen fert in there and you want to use it for flowering annuals, be careful adding a flowering fert since you can overdose on nitrogen.
Plus, what kind of fertilizer is it? Organic? Commercial chemical? Time-release? Sometimes it'll say on the package, but not always clearly. Once ferts are added, you're committed since you can't really get them out again, at least not easily. So I prefer a potting soil without fertilizer in it so I can have more control.
Now that you know what the most common potting soil ingredients are and what they do, you are well on your way to finding that perfect potting soil that's really worth what you pay for it. Now you can walk into a garden center with confidence, read those labels, and pick out the one that you know will do what you want it to. And if you want to make your own but would prefer a recipe to follow, you can go look through the myriad potting soil recipes on the 'net and make an educated decision.
Wouldn't it be better if you could make up your own recipe and custom-blend your own soil that fits you perfectly? To never have to go without because you're cooking from scratch with easy-to-fnd ingredients? Why, yes it would! And here we go...
Making your own potting soil
Generally speaking, most things we grow in pots - vegetables, annuals, and tropicals - are heavy feeders that like a moisture-rententive soil that doesn't dry out quickly and hangs on to that water for a good while, staying rather moist but still fluffy with air. That doesn't mean constantly soggy wet without much air in it. The only things that like that are water lilies and bog plants. It means somewhere in the middle of soggy wet and popcorn fart dry - "like a well wrung-out sponge" is a good goal to aim for.
Think of it this way: "Soggy wet" = all water and no air. "Popcorn fart dry" = all air and no water. Plants' roots need air and water both, so aim for the middle. Use the ingredients that will give you a nice mix of both air and water, and you'll do well. From that point you can adjust up or down for plants that need more or less of either, things like cactus or ferns.
From the list above pick out which ingredients sound like they'll give you that good mix of air and water, ones your Inner Environmentalist feels comfortable using, and go get some of them. We carry most if not all the ingredients I mentioned above at The Natural Gardener, a lot of them in smallish sizes as well as large so you don't need to buy a Number 8 washtub full if you don't need that much. If you still need a little help, come see me or one of my coworkers at the Info Desk and we'll be glad to help you find them. If you live too far from us, check out your local nursery to see what they have - chances are good they'll have most if not all of them. And if you just don't have a local nursery, most of the big box stores should have this stuff, though sometimes coir fiber is hard to find there, and do remember the caution about using their bagged manure composts.
Use those memories and findings as a guide. Since they will be the bulk of your mix, pour any peat, coir, or compost you'll be using into your mixing tub first. Next add in small amounts of vermiculite, perlite, and/or other ammendments, mixing as you go, until you get something that looks like the good potting soil you've used before. If you're growing herbs, succulents, or cactus, now's the time to add a bunch of coarse sand or mineral mix - a little for herbs, more for succulents, and a lot for cactus. Don't worry too much about getting too much or too little of something. You probably won't, and if it ends up that you did, just fix it later, unpotting that plant and mixing in more of what you think it needs. You'll learn from that and can adjust next time you mix up more.
Measure out the amounts and write it down for the future if you want. Or not - I usually "mix by the seat of my pants", adding things until it looks and feels about right, and it usually ends up just fine.
Voila! You just made your own potting soil! Yes, it really is that easy. Over time, just like cooking, you'll refine your "recipe" until it's absolutely perfect. You'll get to know the properties of the mixes you make intimately and can customize them for any type of plant you want to grow, perfecting it all until your plants thrive, growing bigger and healthier and more gorgeous than before. The container gardening world really will be your oyster then, and that's when you'll start finding a lot of pretty pearls a lot more often.
Here are a few more things I've found helpful, and hopefully you will, too.
If you're a buy-small-bags-of-ingredients kind of person, just one of the big thirty-or-so-gallon ones will hold all of them and probably even have room left over for a bag of extra home-mixed soil. But buy two anyway. You'll thank me later when you use the second to store things vermin like to get into, like corn meal and bird seed. Man, that stuff's rat crack, so you'll definitely want to keep it in something they can't chew through. Get extras of different sizes for pine straw, empty pots, tools, used potting soil, extra compost, and yes, even trash. They're not expensive. Not as cheap as plastic, but plastic doesn't last anywhere near as long as galvanized does so you'll be way ahead in the long run. Plus, plastic cans aren't rat proof.
You can save an empty mulch bag and turn it inside out to store your home-mixed soil in. They're usually white on the inside, which would then be the outside, so you can write what's inside it outside it with a magic marker, even writing out the recipe you used to mix it. Nifty! Those mulch bags are made to be TOUGH, so they'll last longer than a garbage bag. And it keeps another one out of the landfill.
A note on sifting: Some coir fiber and compost can be a bit chunky for even regular potting soil, so it helps to sift out those big chunks first. You can buy pre-made compost sifters or make a simple one yourself out of boards and small-holed mesh wire like hardware cloth.
The way you use it is pour in some coir fiber and push it around on the wire, back and forth, shaking it some so that they fines fall through into the wheelbarrow. You can use it like the people in the picture are doing, too, though that works better for sifting compost or regular old dirt - lean it against something, pour your compost or soil at the top so that the fines will fall through and the large chunks and rocks will roll to the bottom of the sifter. If you need to catch the sifted stuff, put a tarp or shallow pan under it.
Match up the soil to the type of plants you're growing and to the kind of waterer you are.
"Wait. What? What kind of waterer I am?" Yep. If you're a chronic underwaterer like me, then you're gonna' need a more moisture-retentive soil, maybe something with extra vermiculite in it to keep it from drying out before you get around to watering again. However, if you're a chronic overwaterer who just lives to visit their plants every day and can't refrain from loving on them with water every time, you'll probably do better with something with more drainage, like with extra perlite or rice hulls in it.
Not sure which one you are? Think about all the plants you've killed. (Don't feel bad. We've all done it. Besides, like I always say: it's okay 'cause they're not puppies.) Think back to see if you saw leathery yellow leaves or brown crispy ones - yellow means too much water while brown means not enough. DON'T GO BY WILTING. Wilting is a sign of stress, NOT DRYNESS. A plant will wilt just as often from too much water as not enough. It'll also wilt from stress from transplanting, or even just being too hot. My peppers do this every summer and even though I KNOW better, I still get the urge to water them every time I see that. So I just don't look at them between 1 and 9pm. *snicker*
Something else to take into consideration: Indoors or out? If this potting soil will be for pots kept indoors, then just make one that's fairly moisture-retentive so the water doesn't often leak out onto your floor and you're done. If it's for pots that will be outdoors, you've got one more thing to think of: the vagaries of weather. Add in enough drainage-inducing ingredients so that flooding rains don't waterlog your pots, and enough moisture-rententive things so that you're not watering twice a day in summer. Finding the mix that does that well takes some practice and experimentation (and a lot more plant killing - remember, NOT puppies - NOT puppies), but you'll figure it out.
On using "used" potting soil: Do it! If the plant that used to be in it didn't die of some contagious pest or disease, go for it. No sense in letting it go to waste. If it makes you feel better, you can bake it (yes, in the oven) for half an hour at 200 degrees F. Do be warned that it can stink up the house, so do it on a nice day when you can open the windows.
The plants that grew in it before did take out some nutrients that'll need putting back, so amend appropriately. And if that old soil looks like it contains too much of something like peat, add in something else to counteract that. If you have your own stash of compost, vermiculite, et al, it'll be a cakewalk. I add compost to enliven it as a matter of course, and don't use it for planting anything rare or sentimental, just in case.