Concerns

• Old potting soil, especially peat-based, naturally decomposes and compacts into smaller particulates in time. While the container may contain a gallon of fresh potting mix, several years later that same volume may only be 2 or 3 quarts. Old potting mix also may have a build-up of fertilizer salts that modifies pH. Most problematic is that the mix likely contains fungal spores, bacteria, viruses and various insect pests in it. Reusing this nonsterile potting medium poses a threat to plants, especially germinating seeds and young seedlings. Don't use old potting soil when starting flower or vegetable seeds.
  
  

Solarization

• The best thing to do to old potting soil before reusing it and replenishing it with amendments is sterilizing it, killing any microbes, weed seeds and traces of disease that exist. This is more commonly referred to as solarization. It's a potentially aggravating but beneficial step. The key is heating the potting mix in sunlight under plastic until a temperature of 140 to 150 degrees Fahrenheit is met over several days. Old but sterilized potting mix has many more immediate uses in the garden.
  
  

Amendments

• Because peat-based potting mixes decompose and compact in time, amendments re-establish a crumbly texture. Amended potting soil containing fresh, sterile compost, fresh peat moss and perhaps some sand grit or perlite added ensures good porosity and drainage. Another option is mixing the old potting mix into fresh potting soil. While the fresh mix is best, diluting it at a ratio of 1 part old mix to 4 or 5 parts fresh mix is acceptable. You may even do a 1 to 1 ratio if you know the plant doesn't need a highly fertile soil or is tolerant of less-than-perfect soil conditions in a container.
  
  

Caveats

• Old potting soil that is slimy, smells of rot or is riddled with algae or fungus is not worthwhile for reuse, even if solarized. Discard it instead. If you are certain no fungus exists in the potting mix, throw it on the compost pile or scatter it broadly across the garden. Avoid using topsoil as an amendment or component to a potting mix. Topsoil also contains microbes, both beneficial and harmful, as well as weed seeds, worms or insects. Topsoil is much finer in texture and compacts and hardens when used in containers. The frequent watering and drying cycles occurring in plant containers exacerbates the hardening and compaction of topsoil.