The Walden Effect: Farming, simple living, permaculture, and invention.

Reusing potting soil

What should I do with old potting soil?

-- Mark (by way of the army of pots outside the front door after his bout of housecleaning)

Old potting soilThose of you who love plants have probably jumped over a similar hurdle.  Maybe you tried to overwinter a tomato plant and it eventually gave up the ghost, or you have a house plant that was potted up into a larger containetr, leaving some soil behind.  Either way, you're left with dirt from which the nutrients are mostly gone, but with plenty of yummy organic matter left.

The best use of old potting soil depends in part on your house plant style, and on what kind of mixture you started with.  In the most typical scenario, you bought potting soil from the big box store and you use chemical fertilizers to feed your plants.  In this case, you might as well reuse the potting soil since you're basically growing your plant hydroponically, using the potting soil as a structural element only.  You might want to mix the old potting soil half and half with high quality compost or new potting soil, though, to add a few nutrients to the earth.

(As a side note, if your plant died from something soil related, you won't want to reuse the troubled dirt as is.  Some gardeners report good results with sterilizing the soil by pouring boiling water over it, or by baking the soil in the oven.  Finally, you could simply add the potting soil to the outdoors garden, where beneficial microorganisms will make short work of the bad guys.)

Adding soil to hugelkulturAt the other extreme, you might make your potting soil out of a mixture of compost, stump dirt, or other organic materials like I do.  In this case, your potting soil is providing the nutrients as well as the structural components, which is why you'll need to topdress your potted plants at least once a year and to cut away part of the roots of long-lived plants to replace some soil lower down.

In this homegrown scenario, depleted potting soil should head out to the garden, where it will act as a soil amendment.  I dumped mine between logs in a hugelkultur bed, but you could also apply the old potting soil to the top of a bed in the vegetable garden or alongside a tree mound that needs to be expanded.  The soil is probably low in nitrogen, but is unlikely to be low enough that it will grab nitrogen out of nearby earth.  Instead, it will help fluff up your garden soil, increase its water retention capacity, and make microorganisms happy.

(By the way, thanks for cleaning up the office, honey!)

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About us: Anna Hess and Mark Hamilton spent over a decade living self-sufficiently in the mountains of Virginia before moving north to start over from scratch in the foothills of Ohio. They've experimented with permaculture, no-till gardening, trailersteading, home-based microbusinesses and much more, writing about their adventures in both blogs and books.

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I use a mixture of homegrown compost and a purchased certified organic potting soil from a local company; I wish I had a source of stump dirt so that I didn't have to spend money on the potting soil . . . but at least it's not full of chemical fertilizers. When it's done being potting soil, I've always just dumped it in the garden, or mixed it in with some almost ready compost, because I figured "why waste dirt?" :D Heh, it's nice to know that I've been doing the right thing, more or less. :) For topdressing, do you just use the urine, or do you do something else? I've just been mixing in a little homemade compost with the top 1/4 - 1/2 inch of soil or so (I can't get my husband sold on the "collecting urine" idea, heh), but my success with that has been that has been rather sporadic - I've got a couple of geraniums that have been living happily and blooming in the same pots for the past ten years, but I can't seem to keep a rosemary or thyme plant going for more than six months (and usually less than that). :(
Comment by Ikwig Wed Feb 29 09:55:24 2012

Ikwig --- Gotta keep your eye out for stumps!

When I topdress my house plants, I just lay new compost on top of the old soil. Later, when I water, nutrients wash down out of the new compost and into the root zone (and the plant starts sending roots up into the compost too.)

My long-lived plants seem to sink down into the pot over time, so topdressing just raises them back up to pot level. If the crown is too low, I'll take the whole thing out and put the new compost underneath the plant instead of on top.

Worm tea is another good choice if you can't wrap your head around urine.

Comment by anna Wed Feb 29 15:38:37 2012

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