It is possible. If I am not mistaken Altera DE2 has 8Mb SDRAM, so you are going to be able to store a database there. The interface might be a problem. When ordering the pH tester, you need to make sure that it supports the RS232. Otherwise, you will need to either switch to another board with pin headers out, or you will need to create the interface yourself (it will include some soldering).
Your other options for this board are infrared I/O and USB. The choice will depend on the external device interface.
If this is a project for a logic or electronics class, you will need some extensive knowledge of plants (and it is waaaayyy outside the scope of the course).
Good luck.