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.' ?