Crop rotation : Fertility of soil is also maintained by crop rotation. It is the practice of growing different crops in a given area each season. For example, if a cereal crop is grown in one season, leguminous crops like peas, beans or grams are grown before the next crop of cereals. Leguminous plants have special bacteria in their root nodules which have the capacity to convert free nitrogen of the air to soluble nitrates which make the soil fertile.