Id: Yellow soil

Hello. First time I have dug down deep to plant a tree, and notice the soil is very sticky clay. Anyone know why its yellow in sections?

Thanks in advance.

 

Most likely you have dug down into your subsoil it's nothing to worry about but when digging you should make sure to separate your subsoil from your topsoil and then re-bury it in the same order it came out and not mix it

 

Thanks :-)