I think the actual system is okay, when you put a seed on the ground you get the +5/-2 modifier for the seed quality. But it sucks when you get 2 - 3 downgrade in a row and you don't have that much time to play to plant 4 times in one day and just pick the better one.
So, my suggestion, is to give another way to increase the seed quality beside just planting.
What if we "study" the seed?
You take the seed and put in your study report and, after some time, the seed that you selected increased in +1 quality.
The time to study should be something like: the time to the seed be growth on a neutral ground and make it reduce the time based on the quality of the seed and farming.
example:
A seed take 3 days to be growth ( 72 hours)
And it's quality is 10.
You have 10 farming skill.
So the time reduced would be ( quality ) /( farming skill ) = (10)/(10) = 1 ( which means 100%, or no reduction).
So the total time to increase from 10 to 11 is 72 hours.
But now you got 100 farming and the same seed quality ( 10 ).
So th time reduced would be ( quality ) /( farming skill ) = (10)/(100) = 0,1 ( which means 90% reduction )
So the total time to increase from 10 to 11 is 7,2 hours.
This way you can always plant a many of your seeds and just take some and put on the study report, so, if you had a downgrade, you have at least some seeds which a guaranteed +1 quality in the same time the crop would take to grow (if you have enough farming skill ).
Problem: If we just use the seed, it will take only 50 seeds for slot ( 1 for tubers ;/ ) which i think "it's fine". But, along with this, we should be able to create an item that use 2 - 3 slots of our study report to put more seeds ( maybe 500 ? )
What do you think?