A sacrarium can indeed be used for disposing of water that has been used to dissolve a consecrated host. The sacrarium is a special sink takes the water directly into the ground instead of into the sewer system. As an alternative, the water can be poured directly into the earth. Keep in mind that once the Eucharist is dissolved that the Real Presence no longer remains. The water is treated carefully because it once held the Eucharist, but the Real Presence leaves once the consecrated host is no longer recognizable as bread.
What this means is that the priest was technically correct in how he handled the dropped Eucharist. But if the report given you is accurate, it was what used to be called “offensive to pious ears” for the priest to say that he intended to use the water to tend his plants.
