I purchased a tractor for my business(sole proprietor) can i write off entire amount for 2018 or just payments etc

the amount is 25000$ is there a limit

Business & farm

Principal payments  on a loan  have no bearing on the asset depreciation allowed although the interest you pay is an expense ... here are the new 2018 rules ...

https://www.irs.gov/newsroom/new-rules-and-limitations-for-depreciation-and-expensing-under-the-tax-...