Just realized my question wasn't specific enough. Can I reuse the soil from last year? If so, should I add anything to it?

I have used my previous seasons soil as a filler for my large containers, and then added the new soil, others will give you more advice, I'm glad to see I'm not the only one planning ahead for our next growing season :>)
Christine

You can use last years soil. Just mix in some new soil with it.

I agree with the above.
Depending on the condition of your previously used soil, you may need to refresh it a bit (20 -30%) to improve drainage. Also add some time release fertilizer, a bit epsom salt and dolomite lime( as source of calcium) and water its, let it sit few days before planting.

Soil (any potting mix) is just a MEDIUM. As long as it has the nutrienst an moisture the plant needs, does not matter how old or new it is. The issues in container gardening is to balance moisture retention and drainage.
JMO

The age of your soil mix is a limiting factor. The mix you are using is made up mostly of peat. Peat-based soils have a very small particle size, which causes drainage problems. Over time (months for peat) the soil structure will break down and there will be less room for air and more drainage problems. To quote from the long running thread on Container Soils -- Water Movement and Retention:

Container soils are all about structure, and particle size plays the primary role in determining whether a soil is suited or unsuited to the application. Soil fills only a few needs in container culture. Among them are: Anchorage - a place for roots to extend, securing the plant and preventing it from toppling. Nutrient Retention - it must retain a nutrient supply in available form sufficient to sustain plant systems. Gas Exchange - it must be amply porous to allow air to move through the root system and gasses that are the by-product of decomposition to escape. Water - it must retain water enough in liquid and/or vapor form to sustain plants between waterings. Air - it must contain a volume of air sufficient to ensure that root function/metabolism/growth is not impaired. This is extremely important and the primary reason that heavy, water-retentive soils are so limiting in their affect.

You can reuse your old potting mix if you mix it with fresh components that have better drainage characteristics, like pine bark fines and perlite. I would not make the old mix more than 20-30 percent of your new mix, and less would be better. If you want to build a superior mix, I suggest you try the 5-1-1 mix discussed in the link below.

Here is a link that might be useful: Container Soils -- Water Movement and Retention

I put the soil through a screen to remove the old peat and fine particles. The "left over" particles of acceptable size I then use to make my new soil. I usually use about 40% of the old to add to the new soil...:-)

...I just want to add that Dolomitic Lime provides both Calcium and Magnesium, and so the Epsom Salt wouldn't be necessary....in fact, adding it could disrupt the Cal / Mag balance.

Josh

I use the same soil in my containers for veggies - tomatoes, peppers, cucumbers.

In the fall, I pull out the old plant and its roots. That combined with what soil has decomposed usually leaves about 50% of my container full. Next spring I fill it with new soil and mix them together. I fertilizer throughout the season to add nutrients.

I have had no problem and have good crops. Again this is for my veggies and herbs.

Lots of talk on this forum about 5-1-1 mixes and some of it is pretty scientific. I'm sure it works great, but I have never bothered.

This post was edited by OregonEd on Mon, Dec 30, 13 at 11:40

"...I just want to add that Dolomitic Lime provides both Calcium and Magnesium, and so the Epsom Salt wouldn't be necessary....in fact, adding it could disrupt the Cal / Mag balance. "

Well even just putting lime in can do that. it has a ca/Mg ratio of 2:1. Far from ideal, way too much Mg. I myself use
calcitic lime which has a 6:1 ratio.

Thanks everybody! I have 2 Smart Pots from last season and ordered 3 more, so I'll distribute the old soil evenly between all five and then mix with new stuff. Now I just need to figure out what the new stuff should be. :-)

-Anne

While I mix my spent potting media into the perennial beds, a lot of the time the entire pot is so packed with roots that I just compost the whole thing and use new medium.