122 RELATION TO PHYSICAL CONDITIONS 



The acute form of deflocculation manifests itself in the 

 crust at the surface of the soil resulting from sodium car- 

 bonate or black alkali. Only slightly less troublesome is 

 the brown crust found where large quantities of sodium 

 nitrate are present. Where these crusts are found it be- 

 comes almost impossible to raise crops successfully. Not 

 only is the land difficult to till but the crust that may 

 form after a tender plant come's up is so hard that the 

 plant cannot make a normal growth. There is an actual 

 physical impediment in addition to any chemical corroding 

 which the salt may exert on the plant. 



Effect of Colloids. All agricultural soils contain some 

 particles called colloids so small that they have properties 

 entirely different from the larger particles. The colloidal 

 material acts somewhat like dissolved salts and yet it 

 obeys some of the laws that apply to the larger particles. 

 During recent years it is being recognized that many of 

 the effects of alkalies on the physical conditions of soils 

 come about through this colloidal material. 



Kellerman (12) showed that the impermeable condition 

 of an alkali soil at Fallon, Nevada, was due largely to the 

 condition of the colloidal matter in the soil. 



Gedroits (9), as a result of extensive experiments on the 

 relation of salts to soil colloids, found that many of the 

 physical changes ordinarily brought about in soils by 

 salts come from their effect on colloids. 



Important as are the investigations already made on 

 the relation between alkali and soil colloids, they may be 

 considered as only pioneer work in view of what the future 

 promises. 



Hardpan. Under the surface of many of the soils in 

 arid regions, particularly in sections of abundant alkali, a 

 hard layer is found which obstructs the .penetration of 



