I don't keep a seed log but I keep my veggie garden records in one of those school "composition" notebooks because they are cheap and durable. Here's what I put in them:
-I record when and what I started from seed, what days I planted out, when I started getting different harvests and anything else noteworthy.
-Because I have raised beds, I draw a sketch of the beds each year so I can rotate where I place different crops.
-How I amended the beds each year. (For instance, this year was 15 bags of manure and 1 gallon of ground egg shell.)
-If I have bug problems I write down which plants and what I did to treat it. (Sometimes this is ripping up and discarding the plant.)
-First and last frosts, if I remember to write it down.
-any veggies that were particularly good or bad to grow. This includes specific varieties and overall plant notes. (For instance, I have given up on eggplants because I never get a harvest but sungold cherry tomatoes are always great.)
-Anything else that gets my attention.
I just keep the book by the back door through the growing season and add memos as I remember. It is a lot of stuff but I am used to taking notes for experiments, and tend to treat my vegetable garden as yet another science project. It takes several years to notice patterns but has been helpful a few times in working out problems.
