Ley farming is an agricultural system where the field is alternately seeded for grain and left fallow. Other name for the method is "alternate husbandry".
In ley farming, the field is alternately used for grain or other cash crops for a number of years and "laid down to ley" i.e. left fallow, used for growing hay or used for pasture for another number of years. After that period it is again ploughed and used for cash crops.
During the fallow/pasture period the soil is filled with roots of grasses and other plants. New ploughing mixes them with the soil and also increases the amount of nitrogen in the ground, removing need of artificial nitrogen fertilizers. It also disturbs the life cycle of pests.