Misadventures in Making Potting Soil: It's All Fun and Games Until a Plant Loses Its Nitrogen!

Well, it’s been a while since my last post, and as you are about to read, it’s been a busy few weeks!

Let me start from the top. Container gardening, like any hobby, has its levels of dedication. I began two years ago at the bare minimum: empty Tidy Cats containers filled with the cheapest potting soil I could find (read about that here). Having learned some lessons from that, I moved on to larger, nicer containers and a much better potting soil last summer. The results were good, but I was bothered by the end rot and some other issues with the Sta Green mix. This year, I decided to up the ante a bit more and make my own potting soil.

Supplies ready to go! How could this go wrong?

It seemed simple enough. The 5-1-1 formula (pine bark-perlite-sphagnum moss) is fairly well known in the container garden world, and it seemed like a cheaper, higher quality, and all around better mix for my beloved flowers and veggies. And it would be, if the main ingredient was readily available.

This brings me to the “busy couple of weeks” part. The formula often cited calls for “pine bark fines.” I soon learned two things. The first is that they are impossible to find as a finished product (at least here in Western New York). The second is that most garden associates will look at you like you’re asking for lunar topsoil if you ask them whether the store carries pine bark fines. In short, you probably are not going to find them in a bag ready to go.

And yet there are legends… Rumors of a garden center that has bins full, ready for the scooping (at least, in years past). Images of pine bark mulch bags that seem to exist only in the imagination (or on the West Coast). People who can get you in touch with a guy who knows a guy who works for a lumber mill. You get the picture... The long and short of it is that pine bark fines are the white whale of many a would-be potting soil maker. The other two ingredients (sphagnum moss and perlite) are readily available in large quantities at pretty much any garden center you can imagine. The container garden gods were taunting me!

So what could I do about this? Surely I wasn’t about to let the garden gods sit there on Mt. Olympus all haughty and triumphant. Instead, I applied some logic. If “pine bark fines” would work, then why wouldn’t pine bark mulch? True, there were some bigger chunks, but those would just make a nice, fluffy mix. Plus, I decided to re-use my potting soil from last season and add it to the mix. That should mitigate the water losses from the chunks being too big. With this plan in mind, I went to the local orange hardware superstore (not the blue one), and bought 6 bags of the finest, non-died pine bark mulch I could find, a bale of the moss, and a huge bag of perlite. Hold the ambrosia: I was ready to mix!

The original mix... Lots of large chunks

And mix I did—by hand. For hours on end. The result was a fluffy, good-looking mix that seemed to fit the bill, aside from some large chunks of pine. I proudly took some photos, and then went online just to reconfirm my brilliant work by Googling images of the pine bark mulch that others successfully used in their mixes. I soon found a mulch that looked similar to the one I used, so I clicked the associated Garden Web forum link and read 6 chilling words that someone had posted in response to the original author (who had asked about the suitability of his—and by extension my—pine mulch). Those six words were “it will shut your peppers down!”

See those yellowish wood chips? EVIL! Evil I tell ya!

But why? I then began several hours of reading up on the topic. Not liking the answers I was finding, I did what any good internet searcher would do: I searched some more, hoping to find someone who said “awww shucks, don’t worry! I used that stuff and everything was terrific! Here are the pictures of all my big peppers!” No such luck. It seemed that my hours of mixing were about to become more hours of something else. I wasn’t sure what yet.

It turns out that the problem with having so much sapwood (the non-bark portion of the mulch) is that it will compost in the container. As it composts, bacteria break it down. Those bacteria enjoy a diet rich in sapwood, and unfortunately for whatever plant happens to be planted in the pot, nitrogen. Theoretically you could continuously put a nitrogen source like urea in the pot to compensate, but the bacteria tend to view large influxes of nitrogen as an invitation to multiply and do their work even faster.

Suddenly, I had a good hundred gallons or so of this disaster waiting to happen on my hands, and I needed to find a solution. I really had three choices, two of which were not viable. One was to use it as is. As outlined above, not doable. I love peppers and tomatoes, so I wasn’t about to have them “shut down!” The other non-viable choice would be to trash all the dirt and just buy new stuff. Too cost prohibitive, for the number of containers I have.

I chose a third, time consuming and labor intensive option. I knew that I could not remove all of the sapwood. Luckily, I wouldn’t really need to. What I did need to do was to get the large chunks out of the dirt. I bought a ½ inch hardware cloth (which is like a pre-cut rectangle of chicken wire) and used it to screen every single container full of the previously homemade dirt mixture. This was a hot, dirty, and very time consuming job. However, at the end, I had a fluffy, light, very good looking mix that only contained small shreds of sapwood. I ended up losing about 50% of my original mix.

I did save some of the original mix to grow certain herbs and flowers in. I figured that I could mix it 50/50 with my new sifted mix for the flowers, fertilize as needed and see what happened with it.

I was still a bit worried about my new sifted mix, so I bought 4 large bags of Miracle Grow for the tomatoes, and filled their pots with completely new mix. Tomatoes, being the VIP, are getting the VIP treatment! However, that got expensive quickly, so I took the remaining MG and topped off a few of the pots with the screened mix.

 

On the left: my mix after sifting...on the right: Miracle Grow

Interestingly enough, my 2 step, 8-9 hour mistake mix looks a lot like the Miracle Grow, so it can’t be too bad! I did add a little garden lime (for calcium and ph) and a scoop of Osmocote controlled release capsules (the little “balls” of fertilizer that are always in potting soil) to my frankenmix, and so far the broccoli seems to enjoy it, as does the chamomile that popped up everywhere and got transplanted to its own pot. I’m thinking that the peppers, eggplant, and squash will also enjoy it, especially when they get their dose of Tomato Tone, which I absolutely LOVE after it vanquished my blossom end rot issues last summer.

To make a long story short, this was an adventure I do not intend on repeating. I do intend on saving all or most of my potting mix from this summer (whatever sapwood there is now will be long composted by next year) and just screening it and fortifying it next spring. I look at it this way: I got a really nice workout scooping and mixing by hand, an education on potting soil, and it only cost me about $30 in wasted materials. The other $30 I spent on my mix is still out in the garden to use.

If you’re lucky enough to find the pine bark fines, it might be an experiment worth trying. It is a lot cheaper-- especially for a large container garden--than buying an equivalent amount of decent quality potting soil. The savings get even more significant as your container garden gets larger. But if you cannot find them, save yourself the aggravation and just buy a decent potting mix. You’ll be glad you did!