Enforce a plausibility threshold on fertilizer rates by redistributing the tonnage of every cluster above it over the remaining headroom of the clusters of the same region, crop group and year, or of the global pool for those crop group and year variables in which a region cannot compensate for its own excess. Receivers only receive a share proportional to their own headroom, so a single pass suffices and no one overshoots. Variables in which not even the global pool has enough headroom are left unchanged, so that the tonnage totals stay intact and toolCheckFertilizer reports the exceedance.