Under continuous cultivation, fields are used year after year with only brief fallow periods. Weeds, pests and losses of soil nutrients are frequent problems. Thus, a higher level of cultural energy input is required. Frequently, such areas are dependent on nutrient inputs from seasonal flooding or alluvial sediments in paddy water. In paddy fields, nitrogen can also be added by N₂-fixing cyanobacteria. The net energy gain for continuous cultivation is less than in shifting cultivation. This system can be very sustainable; much of the traditional farming in India and Southeast Asia is based on continuous cultivation.