ALKALI PROBLEM IN IRRIGATION — SCOFIELD. 217 



hind, forming a crust of crystals. As a matter of fact, under field con- 

 ditions, much of the water lost by evaporation from the soil is actu 

 ally vaporized within the soil mass, passing upward to the free air 

 through cracks and interstices of the soil. Where this takes place 

 the salts are, of course, left behind at the point where vaporization 

 takes place, which may be well below the actual surface of the soil. 

 When saturated soil dries rapidly, particularly if it is rich in clay, 

 it shrinks and cracks as is shown in plate 2, figure 2. In such a 

 soil the proportion of the dissolved salts finally deposited on the 

 surface may be very small. 



In some irrigated sections conditions are such that the subsoil be- 

 comes saturated with water and there exists what is known as a 

 ground-water table. This condition occurs naturally in some of the 

 alluvial valleys of desert streams such as the Nile in Egypt and the 

 Colorado and Rio Grande in this country. In other cases the ground- 

 water table comes into existence as the result of the downward per- 

 colation of some of the water applied in irrigation, together with the 

 seepage from canals. When the ground-water table is high — that is, 

 when the plane of saturation is within a few feet of the surface of 

 the soil — there may be established a capillary connection with the 

 surface so that some of the ground water is lost by evaporation. 

 Where this condition exists there is almost certain to be an accumula- 

 tion of salts at or near the surface and consequent injury to crop 

 plants or to the physical condition of the soil. 



As a matter of fact, in the great majority of cases in which alkali 

 salts cause trouble in irrigated lands, this trouble is associated with 

 a high ground-water table. It is obvious that this must be so, for 

 with a permeable soil and no ground-water table it Avould be very 

 easy to wash the excess of soluble salts out of the surface layer of the 

 soil and well below the root zone of crop plants by the simple ex- 

 pedient of a short period of heavy irrigation. 



From these considerations it may be concluded that the trouble- 

 some accumulation of alkali salts in irrigated land is due to one of 

 two conditions, either the ground-water table is too close to the sur- 

 face or the soil is not readily permeable to water. 



INJURIOUS EFFECTS ON PLANTS. 



When the concentration of the soil solution becomes excessive, 

 the alkali salts which occur in irrigated lands have a toxic or in- 

 jurious effect on crop plants. The limits of toleration for crop plants 

 is variable, depending on the nature of the salt, the kind of plant, 

 and the stage of growth of the plant when the high concentrations 

 occur. In some cases the injury caused by the alkali salts is prob- 

 ably due to the purely physical effect of interfering with the osmotic 

 action by which the plant roots absorb water from the soil solution. 



