Welcome to the forum!

I know you said the tractor would weigh about 5K. I would feel better if you designed the bridge to support double that. You could be carrying something with the tractor, pull a trailer with weight, etc.

Here's some reading material for ya. It should allow you to correctly calculate the wood needed. Just remember to pay attention to the species of wood - I believe they reference different species of wood in the links. You might have to call the company that makes the specific treated lumber that you are planning on using to see what species of wood they use for treated lumber.

http://www.awc.org/technical/spantables/tutorial.html

https://courses.cit.cornell.edu/arch264/calculators/example8.1/

www.hoosierpondpros.com

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).