Farmers in Medieval times did not have tractors or farm machines as we do today. All that a farmer needed was a horse or ox, a wife and a plough.
The mould-board plough was invented after the fall of the Roman empire by Slavic tribes. This complicated device was made out of metal and wood. The design allowed six or more oxen to pull the plough which was used to break up ground, or heavy, clay burdened soils.
The second invention was the horse collar. The old horse tackle was useless because an animal could not use its full strength. The new horse collar did allow horses to use their full strength when pulling a plough, or heavy loads. This created a big increase in the horse population, as the horse was more versatile a beast of burden than the ox.
In later years people discovered the 3 field system from trial and error. The 3 field system was the system where a field was left fallow for a couple of years, depending on the quality of the soil, while two other fields were planted with crops. Farmers would then rotate fields leaving a different field fallow so that it could rebuild its fertility.