When it comes to seeding medium its always a good policy to mix up a fresh batch using new ingredients to avoid pathogens and any other undesirables that will kill your precious seed before, during and after it germinates.  I like to customize my own mixes to suit the species being sowed.  A moisture retentive mix suitable for Gunnera is not what you want to use for Arbutus and Sassafras which needs a medium that is sharply draining.  But after growing out the seedlings after a month or one to two years and transplanting them out, I end up with lots of left over seeding medium.  I know that somewhere buried in this are ungerminated seeds that for some reason or another didn't break dormancy with its kin.  This is the nature of seed (and lots of other organisms) where some are left behind as a reserve population.  In the plant world its called a 'seed bank' where seed will lay dormant until conditions are right again.  So instead of throwing the used medium on the compost heap, I mix it into my potting medium for my larger plants.  I learned this lesson one year when I recycled the medium of what I thought was a dud pot of peony seeds and used it to pot up 2 gallon Rosa rugosa var albas.  That Autumn when checking the roses, peony seedlings were interspersed among them.

Now its common practice now to recycle my seeding medium and I never cease to be surprised.  To date I have Acer palmatum seedlings growing among the Cornus nuttalli, Iris forrestii seedlings among the Syringa, Trillium seedlings among the peonies, and hazelnut seedlings emerging with the walnuts.  Who knew hazelnuts take two years to germinate???  All these little treasures would of been lost if I had thrown out the seeding medium, but by recovering it and recycling it into my potting medium, I have more little treasures to grow!  Mind you I have to wait until the plants go dormant until I can separate them to pot them up, but considering what plants cost these days in the local nurseries, its well worth the effort.