Here's how crop raising will work. Initially, if you plant on a better spot, you'll hit that cap in a week or two. You should get an average quality increase of around 2.5 per generation. Once you hit that cap, you'll get an average quality increase of around 1.5 per generation. That quality cap just slows down progression, and getting that high node to start on gives you a couple weeks jump on others that would just plant on q10.
Is it the soil? Jorb said "crop nodes" so I'm guessing no. If I were to guess at this point, you could find it based on the WWW you find, but it might not be that, either.