I used to be big on making my own sourdough bread yeasts, back when I lived in THAT part of Cali where L. sanfranciscensis ran wild. I remember simply starting with 1 part flour, and 1 part water, and letting it ferment in open air. Every week, I'd "feed" my "pet yeast" by mixing up the slurry, dumping out half of it, and replacing that with more flour and water. Super easy. This method was highly successful, for years, and it produced that San Francisco sourdough flavor in my breads that everyone loves. It was also extremely easy, as I can only surmise that the desired strain of yeast happened to have the most dominant airborne spores in that part of the state.
When I gave some of my yeast starter to my parents in Texas, they found that it worked for just two batches of bread, and wild yeasts in Texas took over sooner than expected, rendering the yeast useless for bread. They said it ended up smelling more like beer than sourdough. On that tangent, maybe I'll try my own Wild Texas Lambic when I get the chance to go home again!
Bottom line--if I want to reuse US-05 or S-04 or even Nottingham, how many uses can expect to get out of it, and would these yeasts benefit from regular feeding of simple sugars (sucrose or dextrose) and yeast nutrients (urea + ammonium phosphate)? Should I dump half of the yeast every week, only to replace that same volume with more water, sugars, and nutrient?
I'm extremely new to using yeast nutrients. I haven't tried adding it to my latest brews, even, for fear of fermenter explosions and what-not.