Whether taking a soil sample or applying fall fertilizer, mid September is generally considered to be too early for both.
Soil sampling. Closer to freeze up is the best time to capture soil test results that more closely resemble the situation next spring, but sampling soon after combining may satisfy a number of other issues: (1) That is the only time you can get the custom soil-sampler. (Maybe if you booked early, you can get a later more desirable time.) (2) You want test results in time to make fall fertilizer applications. (3) You want to see if harvest-time soil nutrient levels could explain lower- or higher-than-expected yields this year. Read more.
Applying fertilizer. Fall fertilizer is best applied as close to freeze up as possible to balance two objectives: (1) allow soil to seal over the band (frozen soil may not seal) and (2) reduce losses due to high microbial activity in warm soils. Consider using enhanced efficiency nitrogen sources to help protect your fall applied nitrogen from losses. Read more.