Add more lime to re-used potting mix?

[Chucker]

I'm planning to re-use the potting mix in my tomato swcs from last year - probably just mix in some new to correct the volume.

Do I need to add more lime? Or is the lime from last year still in the mix? And if I do need to add more, do I add the same amount as if it was all new potting mix?

Thanks

[rnewste]

Chucker,

Yes, you will need to add in more Dolomite Lime. As there may be some residual left from the prior year, I would recommend decreasing it a bit. If for reference, you put in 3 Cups in the fresh mix last year, I would now put in 2 Cups this year.

Also, as last year's potting mix is "compacted" I would also add in both Bark Fines as well as Perlite to keep the Mix fluffy and aerated. I find a 3:2:1 ratio of potting mix, bark fines, and Perlite work well in my SWCs.

Raybo

[Chucker]

Thanks Raybo. I figured some of it was still there, but I wasn't sure.

[dustdevil]

The only way to tell if it needs more lime is to do a soil ph test. If you add lime willy-nilly it may make it too sweet.

[Gobig_or_Gohome_toms]

I cannot find dolimite lime anywhere I have found agricultural lime is that the same?

I have not been to the local Lowes yet so I am going to go there tomorrow.

Craig

[dice]

"Agricultural lime" and "Dolomite Lime" are usually different.
They have comparable effect on soil pH, but the Dolomite
lime contains magnesium that is absent from regular lime.

[Chucker]

Originally Posted by rnewste

Chucker,

Also, as last year's potting mix is "compacted" I would also add in both Bark Fines as well as Perlite to keep the Mix fluffy and aerated. I find a 3:2:1 ratio of potting mix, bark fines, and Perlite work well in my SWCs.

Raybo

I haven't been able to find pine bark fines yet, but will this pine bark mulch work? Or should I keep looking? Here's a couple of pics to show size.





Thanks
Scott

[rnewste]

Scott,

That looks like it will do it. It is a little finer than I would ideally recommend - but if you can't find the Decorative Groundcover Bark - then you are good to go.

Raybo

[Chucker]

Excellent - thanks!

[Chucker]

Theoretical questions I guess, but if I were to rejuvenate my potting mix every year with pine bark and perlite, would I eventually lose the wicking properties of the mix? As far as I can tell, pine bark doesn't have any wicking properties. Does it begin to start wicking once it starts to decompose in the mix?

Also, perlite doesn't really decompose, correct? So I'm thinking that once you add it, you'd never have to add it again, right?

I know many people recommend against reusing potting mix, but if you amend it every year, how many years could you get out of it before it becomes a big disaster?

Thanks

[dice]

There are three main threats to reusing container mix:
soil-borne diseases, salt buildup from fertilizers or salt
in the water supply, and loss of large pore air space from
organic matter breaking down to silt.

The first one you cannot do much about, although
a soil drench with a hydrogen peroxide solution
could help. The hydrogen peroxide does not hang
around long in the soil, so you do not have to worry
about it permanently polluting your container mix.
It will kill organisms like beneficial bacteria and
mycorrhizae as well as disease organisms in the soil,
so you want to do it a couple of weeks before adding
anything like that (maybe in fall, at the end of the season,
would be better than spring for a peroxide treatment,
although I used it to initially wet seed-starting mix this
year, no problems with the plants that sprouted).

Salt buildup you can deal with by putting the container mix
in a container that will drain and then leaving it out in the
rain for weeks. Flooding it with a garden hose a few times
might work, too, if your water is not salty at all. (Washing
salts out of used container mix is not an exact science if
you lack the necessary test instruments.)

Loss of large pore air space you can deal with by mixing in
more large chunky stuff each spring (big hunks of perlite,
bark fines, pumice, lava rock, gro rocks, compost, coir, etc
all work for this). Often standard potting mix has enough
chunky stuff in it to be usable. You can also sift used
container mix with a window screen and wheelbarrow
to let silt produced the previous season shake out of it.

Figure 3-5 years without doing anything except mixing
in some more container mix each year, if it does not
have any soil-borne diseases in the old container mix.
If you sift out silt each year and wash the salts out, too,
you can keep it going indefinitely.

[Chucker]

Thanks for all the info dice. I had not heard about or thought about the salt build-up issue. I am using SWCs and have them open on top (no mulch or plastic covering) - so I wonder if all of the rainwater that goes through during the growing season helps to flush the salt out?

[Talon1189]

Originally Posted by Chucker

Thanks for all the info dice. I had not heard about or thought about the salt build-up issue. I am using SWCs and have them open on top (no mulch or plastic covering) - so I wonder if all of the rainwater that goes through during the growing season helps to flush the salt out?

I am a newbie......pellet gypsum is known to help neutralize salty soil and and will help break down clay soils in lawns. I wonder if reusing the same soil in a SWC might benefit from a cup or 2 when redoing the same soil in the container from the season before?

[dice]

[pellet gypsum]

Probably would not hurt it. I mix gypsum into container mix,
just for the calcium in it.