276 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 5 



in both cases. There are limited areas of saline soil to be found in 

 regions where the climate is unquestionably humid and where soil 

 leaching is the normal condition. These limited saline areas occur 

 where local surface drainage is inadequate and where the texture of 

 the soil is such that no downward percolation of water occurs. Long- 

 continued evaporation of water from such an area results in the 

 gradual accumulation of soluble salts in the soil. On the other hand, 

 there are in arid regions very extensive areas of soil which are not 

 saline. This may be due in part to the fact that the rock material 

 from which the soil was formed contained little, if any, water-soluble 

 material and in part to conditions of soil texture or of surface 

 topography that favor soil leaching even by limited rainfall. 



In general, saline soils occur in topographic depressions where the 

 drainage from adjacent higher land is retained and evaporated. 

 There are, however, some saline soils of rugged topography, such as 

 the so-called " bad-land " formations, that constitute the remnants of 

 sedimentary deposition in saline or brackish waters. 



Within the boundaries of the United States the existence of natur- 

 ally saline soils is not an important agricultural problem. They 

 occur only to a limited extent in regions where the rainfall is ade- 

 quate for crop production without irrigation, and in the drier parts 

 of the country the extent of the nonsaline arable soil is generally 

 much in excess of the available supplies of irrigation water. This is 

 not to say that the pioneers of our arid regions have not attempted 

 to reclaim and to utilize areas of saline soils. Many such attempts 

 have been made, a few with some measure of success. With us, how- 

 ever, soil salinity as a major agricultural problem occurs not as a 

 consequence of natural preexisting conditions, but rather as the result 

 of the ill-advised use of saline irrigation water on soils that were 

 originally nonsaline and potentially productive. It is because the 

 problem of salinity in relation to crop production is almost wholly 

 consequent on the accumulation in the soil of dissolved salts trans- 

 ported by irrigation water that the subject is discussed from this 

 standpoint. 



THE SOURCES OF SALINITY 



The water-soluble salts that occur in our agricultural lands are 

 not, in any large part, the result of local soil weathering. They are 

 chiefly the result of water transport, either natural or artificial. 

 The original water is, of course, pure because it comes from rain or 

 melted snow, but it becomes contaminated with dissolved salts as it 

 passes over or through the soil. Most of the salt found dissolved 

 in natural water originates from the decomposition of rock mate- 

 rial, but there is a small part, at least, that comes from the interior 

 of the earth. Such salt constituents as carbon dioxide, chlorine. 



