Rejuvenating Used Potting MIx?

Christiana, TN(Zone 6b)

I'm looking for a way to reuse/rejuvenate my used potting mix. It is composed of about 4 to 5 parts MirGro 2x, 1 part worm castings and 1 part perlite.
I grow a lot of herbs and from time to time I have pots of used soil filled with roots. I use some of it for top dressing outside plants but I would like to re-use it to save a little money. About the cheapest I've found WCs for is about a dollar a pound (including shipping). And a fifteen dollar bag of MG goes pretty quickly.
I'm sure this has been asked before but maybe not for my particular mix.
Thanks, wabbit

Bay City, MI(Zone 6a)

Because of its composition, your mix is extremely water-retentive - much more difficult to grow in than need be, and provides you with a very narrow margin for error when it comes to watering and fertilizing. The key element that allows be to make this assessment is the size of the particles that make up your soil. The MG is already extremely water-retentive, and the worm castings simply fill the pores between soil particles, making the pores even smaller and the soil even more water-retentive.

If I were you, I would look hard for pine bark in an appropriate size and use it generously. You'll find it to be much less expensive than your MG soil, and far superior in it's ability to provide a root environment that offers far greater opportunity for plants to grow to their genetic potential.

I suggest something like
5 parts of pine bark fines (like in the picture at 3, 6, or 9)
1 part of perlite (coarse grade if you can find it)
1 part of your old soil
2 teaspoons of dolomitic (garden) lime per gallon of new soil.


Thumbnail by tapla
Christiana, TN(Zone 6b)

I am going to try your suggestion, Al. If it works out I may go back to your mix. I wasn't really making it correctly. I used some gypsum but no lime. I couldn't grow many begonias with the mix. So, at someone's suggestion on the Begonias forum I switched to MG. And from there I starting using it for just about everything. I think the lime might be key? I would like to keep the castings though. I've started a little worm factory. Would you mind suggesting how much to use with the above mix you gave me? If not, no problem.
Thanks for your help.

I've been browsing different forums: organic, etc. hence the worm castings. Some people throw in everything imaginable. I'm holding off on guano for now. :-p

Westbrook, CT(Zone 6a)

After two or three year's use in an EB, I generally toss the used planting mix onto the mulch pile. After a year of decomposing the mulch gets reused in the garden or as a component of a new container mix.

Pittsford, NY(Zone 6a)

I'm with Don. I topdress every year to compensate for removal of old plants and soil that comes off with old roots.
I top dress with composted leafmold this year and generally top off with more mix. Pots tend to dry out here so I use Miracle gro moisture controle as I use lots of coleus.

Bay City, MI(Zone 6a)

K - Nutrition for container plantings is extremely easy, unless we MAKE it difficult, so I tend to give no consideration at all to nutrition other than my choice of fertilizer, which is Foliage-Pro 9-3-6 for virtually everything I grow in containers. Instead, I carefully guard against destruction of the soils structure by an over-abundance of fine particles or particles that break down quickly. I know you want to use your worm castings, but from the practical perspective I can tell you they don't do anything that you would notice or measure, insofar as nutrition goes; and because they are so fine they add to the soil's water retention while reducing aeration/drainage. The same holds true for guano and other manures and 'meals'. There is nothing in either of those soil components on the list of elements essential to plant growth that can't be found in Foliage-Pro ..... and FP has the added advantage of being able to furnish those elements in the same ratio at which plants actually use them, which allows you to keep fertility levels at the lowest level possible w/o nutritional deficiencies - a decided advantage to the plant.

You really can't rejuvenate fine soils. To see why, in your mind's eye picture a quart of pudding, then ask yourself how much perlite or compost would I need to add to this pudding to make it well-aerated and to get it to drain properly? No amount of compost would do it, and you can see it's only when the amount of perlite begins to become greater than 80-90% of the whole that you begin to see hope of drainage/aeration. Old, broken down soils, especially those originally based on already fine particles are the same. It's no accident that I suggested you use 6 parts of coarse material that breaks down very slowly or not at all (the pine bark + perlite), and only 1 part of your old soil. Unless you limit the % of fine ingredients to a small fraction of the whole (

Christiana, TN(Zone 6b)

Thanks, everyone. Seems unanimous....don't reuse. I'm going to just keep using it in my garden beds. :-)

SE Houston (Hobby), TX(Zone 9a)

No. It is NOT unanimous.

Because I repurposed about THIRTY 5-gallon buckets of used Miracle Grow potting mix into Al's formula above (modified to a 3:1:1 ratio of new pine bark fines:perlite:old MG potting mix). And, I used MG Water Soluble Plant Food for Veggies (24-8-16). I could've cried when I realized how happy those tomato plants were.

I grew the best harvest of tomatoes I ever had this past January...

Don't toss it just yet.

P.S. Starting with the higher proportion of Pine Bark Fines, as Al suggested, saved me a TON of $$$. The PBFs I get is $30 for a yard. I bought a half-yard for $18, and cut in approximately .75 yards of old MG potting soil. I used only 2 additional big bags of MG potting mix and 1.5 bags of Perlite (a 4 cu.ft/bag) for the blend.

I would have spent a fortune buying only Miracle Grow potting mix to fill up 38 buckets!

This past season plus the PBFs to fill up the 40 buckets.


This message was edited Jan 23, 2012 5:10 PM

Thumbnail by Gymgirl
Bay City, MI(Zone 6a)

PLUS, she could have had the same results if she had used peat instead of the MG potting soil she bought (less expensive). ;-)


Christiana, TN(Zone 6b)

I have decided to use all my used MG for outdoor begonia beds this year.

SE Houston (Hobby), TX(Zone 9a)

Yay! ^^_^^

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