Aha, I now have got the alchemy to work.  Thank you.  However there is a slight problem.  If you are carrying your seeds in a bag, there is no way to drop 1 seed -- you have to drop all of them.  Getting one potion for 8 seeds of one sort seems a bad trade....
But, on further experimentation, you can throw 3 seeds at an alchemy pot and get a potion.  So perhaps the message you get when you examine an alchemy pot should be changed from 'Drop some seeds here to cook a potion' to 'Throw 3 seeds here to cook a potion.' ?