Can I reuse potting soil infused with old root hairs?

April 15, 2020 11:43 AM

I have a patio garden with plants in pots/containers. Some of those pots (that used to have plants in them) have been idle for a while and I would like to return them to service, but in many cases the potting soil is matted together with fine root hairs from previous occupants (these hairs are generally of diameter well under a millimeter and are not circling the edges, but evenly distributed throughout). I can remove the soil all in one pot-shaped clump. It's easy to tear it up into smaller clumps, and I would guess the root hairs make up just a few percent of the soil mass, but they seem too fine and numerous to remove altogether. Is it possible to reuse this soil, and if so, what should I do to prepare it?

(I assume I should add compost/nutrients; this question is more about whether the old roots in the soil will impede new plants from growing and what I can do about it.)

posted by aws17576 to Home & Garden (14 answers total)

Best answer: I reuse soil like this in my window boxes by mixing it 50:50 with new potting soil. I haven't had any issues with the old roots stopping what I've planted from growing.
posted by minsies at 11:49 AM on April 15, 2020 [3 favorites]

Best answer: The soil should be fine just try not to use it for the same kind of plant that grew in it before.
posted by any portmanteau in a storm at 11:52 AM on April 15, 2020 [2 favorites]

Best answer: Tip: I use a cheap wire woven trash can to act as a sifter for this type of soil re-use, catches the fibers of the old roots, gravel of a certain size, twigs etc. Dollar store item it was I think.
posted by Freedomboy at 12:08 PM on April 15, 2020 [3 favorites]

Best answer: Just remove the thicker bits and do as much as you can to break up the lumps, and then mix what's left with new potting soil. I do this every year with no issue.
posted by fiercecupcake at 12:08 PM on April 15, 2020 [3 favorites]

Best answer: The more you can chop it up, the better.
Fun fact: all soil that plants grow in is full of root hairs. Also good soil is mostly made of old plants.
You're fine.
posted by SaltySalticid at 12:08 PM on April 15, 2020 [12 favorites]

Best answer: I've reused it, both in containers and in planting beds. I break it all up into small clumps, add some fresh potting soil (though I'm pretty sure I've never added as much as to make it 50-50), and add the little miracle grow fertilizer sticks as I usually would. Granted, this is just for random annual flowers, but they've always seemed content with it.
posted by stormyteal at 12:10 PM on April 15, 2020

Best answer: Yeah this is all good and what I'm doing. Just mix it up with some new soil compost- and try to take out the worst clumps, shred the roots fine. Also as mentioned above if you're doing food crops or any plant that's got a tendency to disease like roses, maybe don't put the same type of plant in the old soil as was planted previously.
posted by Homo neanderthalensis at 12:13 PM on April 15, 2020 [1 favorite]

Best answer: I don't even bother pulling it out or breaking it up. Just a few days with the whole pot submerged in compost tea, then another day to drain, and it's good to go.

New root hairs don't care whether what they're pushing their way through is a bunch of grains or a meshwork of cellulosic fibres, as long as they can find the nutrients they're after. The old roots will break down into humus before they cause new growth any grief. And even before they break down, they do good work as a moisture retaining sponge.

I do rotate plant types, though; no sense tempting fate.
posted by flabdablet at 12:16 PM on April 15, 2020 [2 favorites]

Best answer: To state what I think a number of folks were implying: the fibers aren't a problem but because the soil was used for a plant in a pot the soil it will be somewhat or even largely depleted of essential fertilizing minerals and molecules. So mix with new soil and add some fertilizer.
posted by tmdonahue at 12:36 PM on April 15, 2020

Best answer: The issue isn't so much the stringy roots, but rather that the soil is bereft of nutrients because of the previous occupants. This is why you want to mix it with new compost, to get it rich and nutritious again. There's other ways to do this too if you can't get hold of nutrient rich soil or compost to mix with. Basically you need a little plant food or fertilizer to liven it up again.
posted by iamkimiam at 1:56 PM on April 15, 2020 [1 favorite]

Best answer: With the right mix of nutrients and good aeration and moisture control you can grow just about anything in just about anything.

It really doesn't take long for an actively growing plant to use up the vast bulk of what's initially available from a pot of soil, and this is why some form of plant food is something pretty much every container gardener will be applying to their potted plants on a regular basis. Re-using old potting soil just makes that need to happen a bit earlier.

Compost tea is an excellent broad spectrum plant food that costs next to nothing, which is why I use it for the original soak on every pot I'm about to give another go around. I also add some to every watering.

This is much the same approach as used in hydroponics, which is all about growing lush healthy plants in "soils" with no nutritive content whatsoever; in a hydroponic garden, all the required nutrients are dissolved in the irrigation water. For potted plants I'll also add some compost on the top to make up for the bulk that the potting soil loses as its organic portions break down over time.

Making sure earthworms can get in and out of the pots is a good idea too.
posted by flabdablet at 2:55 PM on April 15, 2020 [1 favorite]

Best answer: I do this all the time, with way bigger root bits in it than you are describing. As long as you are fertilising the new plant somehow and the old one didn't have any terrible diseases, it will be fine.
posted by lollusc at 8:05 PM on April 15, 2020 [1 favorite]

Another excellent and super cheap plant food, especially for plants that like to make lots of leafy growth, is human urine. If you're watering with it, use 1 part urine to at least 20 parts water. If you can smell it, you're using it too concentrated and/or too often.
posted by flabdablet at 11:26 PM on April 15, 2020 [3 favorites]

I usually lay out a tarp, pull the whole dry root-ball up and bang it on the ground a few times until all the dirt comes out. Most of the root-ball will disintegrate, but the bigger roots left over you can crumble up in the bottom of the pot for drainage. (Then just pull up 2 sides of the tarp into a V and funnel the dirt right back into the pot.) I wouldn't worry too much about refertilizing...most potting soils are hella fertilized already. Top it off with some fresh stuff and you're good to go.
posted by sexyrobot at 2:22 AM on April 16, 2020