On/at the farm? which one is appropriate?

Hi everybody,



Last week, I was checking my nephew's English homework and
found one sentence he wrote:

"Bob is going to be at the farm in October."



After checking my dictionaries and the internet, I found
"on" seemed to be more

appropriate and therefore changed it, which then earned a
red cross on the sentence

and made me so guilty.



But ....



1) Cambridge dictionary says:

# It was hard work on the farm but satisfying.

# The fertilizers and pesticides used on many farms are
polluting the water supply.

Source: Cambridge Advanced Learner's Dictionary - Third
Edition



2) Yahoo dictionary says:

# He works on a farm.

Source:



Besides, I googled "be on the farm" and "be
at the farm" with Google UK and limited

the search results in the UK.



Then I got 121,000 and 54,800 pages for the two searches
respectively, which means

"be on the farm" has been used at least twice more
than "be at the farm" in the UK.



I particularly restricted the searches in the UK because I
was afraid some usages

from non-native speakers would also be listed and bias the
comparison.



To my surprise, after a careful discussion between his
Taiwanese teacher and a white

teacher, they still decided to use "at" in this
sentence.



May I ask, does this have anything to do with AE/BE?



Or is it simply required to use "at" in this
sentence "Bob is going to be at the farm in October"?


Just one sentence and two confusing prepositions.

Many thanks for your comments.

Hector