This was recently discussed in advanced cult. Yeast nutrient is not yeast. It is a chemical nitrogen source to feed the nitrogen needs of yeast as it ferments sugars. Not at all the same thing. What you need is dried yeast, usually harvested by filtering it out of beer after it has fermented the beer. It looks like yellow, flakey powder.BTW, if you are looking for a substitute for agar in the recipe, try this thread: http://www.shroomery.org/ubbnoncgi/Forum2/HTML/007499.html
Arrowroot powder can be made into a media usable like agar but cheaper and easier to find. It is different though in that once made and cooked it will not remelt. It has some advantages and some disadvantages compared to agar depending on the application.
[This message has been edited by Doktor Dung (edited December 04, 1999).]