I heard several times in school about the Wampanoag natives showing the Plymouth pilgrims how to grow crops in the land by laying fish in with the seeds. Is this a(n) myth/oversimplification or was fertilization relatively unknown to Europe at the time?

The story, for those unfamiliar with it, is that Tisquantum (or "Squanto"), who was the representative of the Wampanoag sachem Massassoit, helped the Plymouth colony survive by teaching the settlers to fertilize their soil for crops with fish. It seems to be a common story told in American grade schools (I recall it).

Charles Mann talks about this in 1491. As he describes it, the irony is that Tisquantum probably did teach such farming techniques to the Plymouth settlers (not all of whom were religious "Pilgrims" by the way, but that's another story), but he had personally learned them from his time in Europe (he had been captured by English slavers, taken to Spain and freed there, lived with Spanish monks, and made his way back home via England, Newfoundland and Maine). The fertilization technique was used apparently from Medieval times.

The reason Tisquantum had to teach a European farming technique to the Plymouth settlers is because they were not farmers. Their religious community had been residing for over a decade before their move to America in the Dutch city of Leiden, where they were mostly weavers in the city's textile industry. The city was a large urban society (Rembrandt was a contemporary resident of the city), and wholly different from the life that they had to adapt to in the new colony. They brought almost no livestock with them, and had planned to make a profit from fishing, but had brought unsuitable fishing gear. They survived the first winter mostly through grave-robbing nearby ruined Wampanoag villages whose populations had been wiped out from a decent epidemic. So when they finally managed to make an alliance with Massassoit, they were desperate need of Tisquantum, an English-speaker who could teach them European farming techniques.

Charles Mann's bits about Tisquantum from 1491 are condensed into an article, titled "Native Intelligence".

The Smithsonian Magazine has a short article titled "The Pilgrims Before Plymouth" about the community's Leiden years. It draws a lot on Jeremy Dupertuis Bangs' Strangers and Pilgrims, Travellers and Sojourners: Leiden and the Foundations of Plymouth Plantation.