Oh, and no, it's not just a 'starter'. Unless you build the bales up like a house the 3 little pigs lived in LOL the straw will break down fairly quickly. If you have about a 12inch depth (or even a bit more), you will be surprised how well plants will root eventually into the soil below. I've grown in about a 10" depth and have had to put more straw around plants 3 months later but by then, if adding some nice nutriments in adding to the straw (that would depend on your soil but agricultural lime and so on are possible) over a few months and watering, the soil underneath will be much improved.
There's a product in Australia called "Charlie Carp" which is made from carp initially caught in our waterways (now more farmed I think). Carp was introduced here and carp unwittingly began to kill off lots of out native riverlife so finding a way to utilise this 'beast' was great. It's a liquid and is very beneficial to add to gardens and lawns every few months. This kind of product can be useful to add to straw gardens but again I'd do a google search and look for recommendations because what you add can also depend on what you're growing. Obviously if you are eating what you grow you're extra careful of what you add :-)