What are the criteria used by the Salinity Lab to distinguish between plants that have high, medium, and low salt tolerance?

• According to pages 65-67 of USDA Handbook 60, the salt tolerance lists are arranged according to major crop divisions; and in each division, crops are listed in three groups.
• Within each group, crops are listed in the order of decreasing salt tolerance, but a difference of 2 to 3 places in the column may not be significant.
• ECe values given in the top of the column represent the salinity level at which a 50-percent decrease ion yield may be expected compared to yields on nonsaline soils under comparable growing conditions