Coal as a fertilizer

I know I'll get scorned for using coal as a fertilizer, but in doing so I noticed the following:
A
fter getting plenty down to 50%, you can't successfully add any more coal, but you can TRY to add coal. Each attempt to add coal seems to count as far as the reduction (or diminishing return), because I did the following after planting:
6 coal to reach 50% plenty, 9 attempts to add coal which failed, and then 1 dross. The dross added 13% speed, as if it was the 16th fertilizer, rather than 28% as if it was the 7th.
I presume the same is true for other fertilizers which can be unsuccessfully added.
A
fter getting plenty down to 50%, you can't successfully add any more coal, but you can TRY to add coal. Each attempt to add coal seems to count as far as the reduction (or diminishing return), because I did the following after planting:
6 coal to reach 50% plenty, 9 attempts to add coal which failed, and then 1 dross. The dross added 13% speed, as if it was the 16th fertilizer, rather than 28% as if it was the 7th.
I presume the same is true for other fertilizers which can be unsuccessfully added.