284 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1935 



on the physical condition of the soil. Wlien sodium is the chief 

 constituent of irrigation water, and subsequently of the soil solu- 

 tion, the reactions between that solution and the soil are in the direc- 

 tion of increasing the quantity of sodium combined with the soil 

 and of displacing an equivalent quantity of calcium, or other ions, 

 from the soil to pass into solution. Reactions in that direction 

 change the characteristics of the soil in the direction of defloccula- 

 tion and impermeability. On the other hand, if a soil that is already 

 deflocculated by sodium is irrigated with water in which calcium 

 is the dominant ion, the exchange is in the direction of replacing 

 the sodium combined with the soil by calcium from the solution 

 with the result that the soil becomes flocculated and more permeable. 

 One of the chief reasons for applying gypsum or calcium sulphate 

 to irrigated soil is to improve its physical condition by this reaction. 

 In a soil that was puddled or impermeable and has been flocculated 

 by the application of gypsum, water may move freely either by 

 gravity or by capillarity. 



THE SYMPTOMS OF PLANT INJURY 



The more obvious or striking effects of high concentrations of 

 galinity in the soil solution are to be seen in irrigated fields contain- 

 ing areas of bare soil or of salt-tolerant weeds. In these areas the 

 growth of crop plants may be completely inhibited by excessive salt 

 concentrations. Surrounding these areas there is usually a zone in 

 which there are a few crop plants of subnormal size, while normal 

 and vigorous plants occur outside of the intermediate zone. This is 

 the characteristic manifestation of salt injury to field crops. One 

 seldom sees a whole field in which the intensity of injury is uniform. 

 This is because of the natural and universal variability of soil con- 

 ditions. Some species of crop plants exhibit characteristic symptoms 

 of injury in their leaves or stems that may be identified as the result 

 of high concentrations of one or another salt constituent. In gen- 

 eral, however, concentrations of salinity insufficient to kill plants 

 merely retard growth processes and reduce the size or yield of crop 

 plants. 



The absence of characteristic symptoms of salt injury in many 

 crop plants has made it difficult to distinguish that cause in its early 

 stages from other causes of poor growth or low yields. Such other 

 causes as adverse climatic condition, low fertility, or the depredations 

 of insect pests or plant diseases are often ascribed as the reason for 

 unsatisfactory crop growth whereas the dominant cause may be ex- 

 cessive salinity in the soil solution. The fact that the real cause of 

 trouble may be due to some one of the several constituents of that 

 salinity or to adverse soil conditions caused by one constituent in- 



