Lime in itself is not poisonous and is most likely to make the grass taste unpleasant until washed off with a good shower of rain. though to some extent it does depend on what liming product you are able to use, I would also add that although liming grass fields periodically is a good idea it does not necessarily follow that buttercuos will miraculously disappear. in acidic soil buttercup easily outcompetes grass, which prefers a higher ph soil, but buttercup can still grow happily in most soil types, particularly if well established. I agree with those highlighting buttercups mild toxicity and regard it has having little in the way of value to a field beyond it looking pretty