A serf's job was not easy; in addition to being self-sufficient, a serf often needed to give up a large portion of their harvest to remain living on the land given to them by the manor. Despite the long hours of back-breaking work, a serf remained tied to the fief in which they worked because they needed the protection and food it provided them during the myriad of viking raids going on across Europe.
The main element that ushered the Middle Ages to the dawn of the Agricultural Revolution was a new way of farming. People had brought with them from the Mediterranean the two-field system to use as a means of farming. In this system, land was divided into two equal plots. Farmers would use one field to grow crops while leaving the other one vacant. The vacant land, or fallow, was left untouched for one growing season so that nutrients in the soil could be replenished. Each season, farmers would switch between the fields. The two-field system was very well suited for the climate of the Mediterranean. This was because in addition to crops being grown only half of the year, the flooding of the Tigris and Euphrates Rivers delivered many nutrients to the soil, so switching between two fields annually was enough to keep things fertile. However, the two-field system proved to not be nearly as effective as it was in the Mediterranean as people attempted to incorporate it into the climates of Europe further north. Since the climate of Europe was very temperate, serfs were able to grow their crops throughout the entire year instead of just half. Although more planting time implies greater harvest yields, the fact that crops were being grown and harvested all year-round meant that fields had only half of a year's time to replenish their nutrients instead of a full one. Soil was being over-used, and as a result, crop yields were suffering.
The Agricultural Revolution began with the introduction of the three-field system.