Leaching requirement when expressed as ratios of

The concept of 'Leaching requirement' was first developed by the US Salinity Laboratory, California. It has been defined as the fraction of the irrigation water that must be percolated out of the bottom of the root zone to prevent average soil salinity from rising above some level.
It can be expressed as EC_DW/EC_IW as well as Depth of DW/depth of IW.
Where, EC = salinity of the soil
             EC_IW = salinity of Irrigation water