Help understanding tukey test difference of means.

Hi all,

I am using a Tukey test, posthoc analysis to look at the differences of means between 2 sample groups. My question is that I have one goofy comparison between 1 plot that received one treatment and 39 other plots that did not. I got a Standard Error for the difference of means. I believe this is correct, but can someone please confirm for me that this is accurate? Any insight as to why it is accurate would be much appreciated as well.

Thank you very much in advance. I really appreciate people like you all taking their time to help others interpret stats!