No to both, I suppose.
Is it a microorganism? – though I suppose those are physical objects. They can’t be directly seen by humans, though.
Are we looking for something that is a verb (or like one)?
No.
No. No gerund, either, which is the only thing I can think of that is “like a verb”. 
Could this puzzle be posed equally well in a randomly-selected other human language?
Is it a psychological phenomenon?
-
Yes.
-
No.
A bestselling author?
Again, no.
This line has made me a little bit uncertain. I’m trying to work out what this means and how significant it is. I don’t know how guessing the riddle is any different from answering the riddle. Are we trying to guess what/who this first person object/speaker is?
Again, no.
Huh.
Is the answer ‘nobody can guess what this riddle means in the standard sense of guessing a riddle, because the clues taken together make no sense’?
Yep.
The answer is “No”.
Sorry for the disappointment!
Groan!
– OK, maybe for a mostly non-agricultural audience this one’s a reasonable amount of difficulty.
The farmer planted the crop. The farmer is pleased with the yield they got from the crop.
But the crop never left the field it was planted in.
What was the crop?
Green manure aka cover crop?
Yup. Too easy. I don’t seem to be any good at coming up with these things.
Over to you.
(Not all cover crops are green manure / nitrogen fixers, but I won’t be picky about it.)
Damn, I knew it also.
A package is delivered for Aunt Fritzi. Nancy informs Aunt Fritzi her package of fish has arrived. Nancy is wrong. The package does not contain fish. So why does Nancy think the box contains fish?
Does it smell of fish?
Does it say fish on the packaging?
Is it because every other package that has been delivered for Aunt Fritzi
has contained fish ?
Is Aunt Fritzi a cat ? (sounds like a cat’s name !)
Is it to do with wordplay on the box? Either Nancy only saw part of the writing, or she misinterpreted the phrase written there?