SOLVED. Historical romance heroine is a kind of scientist with fertilizer. [s]

[Aerulan]

Do you remember why she was developing the manure blend or what she meant to use it for?
Was she a gardener? Did she/her family have land or farms that needed it? Was there another reason?

[Lobstergirl, au gratin]

When did you read it?

[Kris]

Nothing To Commend Her by Jo Barrett seems to match your description.

Some Goodreads reviewers say, "Historical English poor spinster bluestocking that likes to study fertilizer who is kind and her kindness attracts the lonely and maimed in a fire, Earl of Pensby one night at a society ball when she defends him without knowing that he was present." | "Then Agatha starts experiencing terrifying accidents. The threats to her life give her even more determination to have a true marriage with her husband." | "There is a mildly entertaining secondary romance involving Magnus' best friend and Agatha's American fertilizer supplier..."

(site:www.goodreads.com "historical romance" scientist fertilizer american)

[Kris]

Great! Glad we could help, Hailee.

I limited my Google search to the Goodreads site. The detailed reviews on this site contain a wealth of keywords. (Sometimes I replace the "site:" with www.amazon.com or www.librarything.com.)

Google search - site:www.goodreads.com "historical romance" scientist fertilizer american