[benjaf]

This looks interesting, going to follow your progress.
May I suggest using the EEPROM in stead of the RTC memory? It serves the same purpose but is much easier to work with and has much larger capacity.

 

[benjaf]

Benjaf, I would use the EEPROM, however I will be reading/writing to it a lot over time, and EEPROM has a limited lifespan. The RAM on the DS1307 however, is unlimited read/writes

How often do you dose ferts? AFAIK only writing to the EEPROM cause wear, and even then the 100.000 cycle limit is said to be a very conservative estimate.