While we've all been used to the Unwither Crop Boosts that you receive from trading in the Peanut Collection that (as of this writing) still unwither every withered crop on your land without limit, these General Store Boosts aren't as useful. While they may look fairly similar visually, if you hover over the item, you'll see that they only "restore a section" of your crops back to life. As of right now, that appears to be either a single crop, or five crops per Boost (there are conflicting reports, leading us to believe that Zynga might have changed them recently).
One look at the FrontierVille forums confirms this, as users have been able to unwither just a single crop per a single Boost, or have been able to unwither five. Either way, we'll make sure to keep an eye on this situation, and will let you know if a definitive number comes out in the end. For now though, make sure to save your Horseshoes if you're expecting an entire field to magically be saved - you'll end up very disappointed if you don't.