282 THE FUTURE OF ARID LANDS 
followed by the replacement of calcium by sodium on the soil 
particles, the exchangeable sodium percentage of the soil increases, 
and the physical condition and permeability of the soil are likely 
to be impaired. In addition, the hydrogen ion concentration of the 
soil may decrease and organic matter may be dissolved, result- 
ing in the dark color which is characteristic of a so-called black 
alkali soil. 
The Salinity Laboratory has proposed a scheme of classification 
in which waters are divided into four classes based on salt con- 
centration and into four other classes with reference to the prob- 
able extent to which soil will adsorb sodium from the water and 
the length of time required to affect the soil adversely. These are 
designated as the salinity hazard and the sodium hazard. The 
salinity hazard is measured in terms of electrical conductivity 
expressed in micromhos per centimeter at 25°C. Class 1 water 
ranges up to 250 micromhos per centimeter; Class 2, from 250 to 
750; Class 3, from 750 to 2,250; and Class 4, in excess of 2,250. 
The relative proportion of sodium to other cations in an irriga- 
tion water has usually been expressed in terms of soluble sodium 
percentage, but it appears that the sodium adsorption ratio which 
is simply related to the adsorption of sodium by soil, has some 
advantage for use as an index of the sodium or alkali hazard of 
water. This ratio is defined by the equation 
SAR Ne 
- = (Cas ie Me**) /2 
where sodium, calcium, and magnesium represent the concentra- 
tions of these ions in milliequivalents per liter. The sodium 
hazard is largely determined by the proportion of sodium to cal- 
cium plus magnesium present, together with the total salt content 
as indicated by electrical conductivity. Thus, the curves are given 
a negative slope to take into account the relation of the sodium 
hazard to total concentration. For example, a water with an 
SAR value of g and a conductivity of less than 168 would be an 
Si water, from 168 to 2,250, an S2 water, and greater than 2,250, 
an S3 water. This system is somewhat arbitrary and tentative, 
but field and laboratory observations appear to support it. 
