220 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



well and when dry is often very soft and easy to pulverize. But as 

 the excess salt is leached away by irrigation the symptoms described 

 above are likely to appear. 



The direct consequence of the puddled condition, or the formation 

 of a hardpan in the soil, is that the normal movement of water is 

 impeded and the crop plants are unable to obtain the supply needed 

 for their growth. Thus it is found that the plants in an irrigated 

 field may be suffering for water during critical periods though the 

 ground has been irrigated frequently. Instances have been observed 

 where after a season of normal irrigation the water has not pene- 

 trated into the soil more than a few inches beyond the depth to which 

 it had been broken by the plow. Under such conditions it is not pos- 

 sible for crop plants to do well. 



The way in which the alkali salts cause the soil to puddle and 

 bake is through a chemical reaction with the clay or finely divided 

 portion of the soil. This clay is essentially a compound of alumina 

 and silica. Its chemical composition varies within rather wide 

 limits both as to the proportions of the two substances named and 

 as to the kinds and quantities of other elements, chiefly bases, com- 

 bined with them. Clay is not soluble in pure water, but when acted 

 upon by solutions of the salts of sodium or potassium some chemical 

 changes appear to take place which influence profoundly its physical 

 reactions. 



When the clay of the soil has been acted upon by solutions of 

 sodium salts, and particularly by sodium carbonate, some of the 

 sodium combines with some of the silica of the clay and forms a 

 so-called colloidal substance very similar in character to water glass. 

 This substance remains nearly or quite insoluble and inert as long 

 as the soil solution contains appreciable quantities of sulphates or 

 chlorides. If these are leached away the colloidal silicates combine 

 with the soil water and become gelatinous. When this takes place it 

 works a profound change in the physical character of the soil, par- 

 ticularly in relation to its absorption of water and its condition on 

 drying. 



The salts of calcium and magnesium do not form soluble or col- 

 loidal compounds with the silica of the soil, and consequently they 

 take no part in injuring the physical condition of the soil. In fact, 

 there is reason for believing that these latter salts tend to retard 

 the deleterious action of the sodium salts. 



THE QUALITY OP IRRIGATION WATER. 



In most of the places where irrigation is undertaken there is more 

 irrigable land than there is water to irrigate it, so that it is possible 

 to use the water on the better land. Where this condition exists 

 the alkali problem can be avoided at first by leaving the alkali spots 



