224 THE FUTURE OF ARID LANDS 
the irrigation water that must be leached through the root zone to 
control salinity at any specified level. It depends on the salinity 
of the irrigation water and the salt tolerance of the crop. The 
leaching requirement may be readily estimated from salt balance 
considerations, and for steady state or long-time average condi- 
tions it is equal to the electrical conductivity of the irrigation 
water divided by the electrical conductivity of the soil solution at 
the bottom of the root zone. The latter would be 8 millimhos per 
centimeter for alfalfa, for example. If the conductivity of the 
water is 1 millimho per centimeter, then the leaching requirement 
would be one-eighth or 12.5% of the irrigation water. Hundreds 
of thousands of acres have been irrigated in western United States 
for more than ahalf century with water of this salinity and with no 
apparent decline in productivity. However, in areas of very fine- 
textured soil where water of this salinity was used, it was not 
feasible to provide adequate drainage under the existing economic 
conditions. Consequently, leaching was inadequate, the soil be- 
came saline, and the farms were abandoned. 
Sodium Hazard 
Some irrigation waters cause exchangeable sodium to accumu- 
late in the soil. This is usually due to a high proportion of sodium 
to the other cations in the water, but other processes are involved. 
The concentration of solutes that enter the soil in the irrigation 
water is increased in the soil solution by evapotranspiration. Some 
of the components such as the alkaline earth carbonates and cal- 
cium sulfate may precipitate during this process, thus decreasing 
the salinity hazard but increasing the sodium hazard. Research is 
yielding a more complete understanding of the chemical and 
physical processes taking place in the soil-water system. Recent 
progress has been made toward finding a general relation between 
the exchangeable sodium percentage and the relative activity of 
sodium in the surrounding liquid phase or in a water extract of 
soil. Classification schemes for water quality take these exchange- 
able sodium hazards into account, but the prediction of future 
soil amendment requirements from water analyses is somewhat 
complicated. Further experience will be required before the rate 
