MikeVV,<br>
Yes, there are many off the shelf injector units like this available through greenhouse and hort suppliers, as well as some companies that sell animal medications which are put in the water. They range greatly in price and style, depending on the flow rates and accuracy you need. Most put a liquid or slurry into water, not many handle solids.<br>
Also, you can make these pretty easily if you are handy with things like PVC pipe.<br>
If you need specific leads, let me know. Dosematic and Dosatron are two that come to mind, but there are many others.<br>