A tells us that the first irrigated crop is not P, while B tells us that there is at least one irrigated crop before P. These do indeed look quite similar, however, there is one difference.
A simply tells us that the first irrigated crop is not P, which makes sense since If P is irrigated, it must be third.
B tells us that there must be an irrigated crop before P, but this is not necessarily true. It is possible that P is fertilized rather than irrigated- in which case P could actually be first.
