During the Middle Ages, the open field system had initially used a two-field crop rotation system where one field was left fallow or turned into pasture for a time to try to recover some of its plant nutrients. Later they employed a three-year, three field crop rotation routine, with a different crop in each of two fields, e.g. oats, rye, wheat, and barley with the second field growing a legume like peas or beans, and the third field fallow. Normally from 10% to 30% of the arable land in a three crop rotation system is fallow. Each field was rotated into a different crop nearly every year. Over the following two centuries, the regular planting of legumes such as peas and beans in the fields that were previously fallow slowly restored the fertility of some croplands. The planting of legumes helped to increase plant growth in the empty field due to the ability of the bacteria on legume roots to fix nitrogen (N2) from the air into the soil in a form that plants could use. Other crops that were occasionally grown were flax and members of the mustard family.