EFFECT ON MOISTURE MOVEMENTS 129 



that the profound physical changes brought about in the 

 soil by alkali are due largely to forces other than surface 

 tension. This is illustrated by the fact that while in the 

 experiments of Briggs and Lapham (4) sodium carbonate 

 increased the capillary rise of water, it is a well-known fact 

 in field practice that the presence of large quantities of 

 sodium carbonate, or black alkali, interfere with the pas- 

 sage of water through the soil. In an experiment con- 

 ducted by the author, there was added to a fertile loam 

 soil 5 per cent of sodium carbonate. The soil was then 

 placed loosely in percolators so that the total depth of 

 soil was four feet. The same soil containing no sodium 

 carbonate was arranged in similar manner. Water was 

 then added to each soil and kept six inches deep over the 

 surface. In the normal soil the water percolated through 

 the four-foot column in two hours, whereas it failed to 

 penetrate the four feet containing carbonate in a year. 

 The organic matter was dissolved from the upper layer 

 and washed to a lower level where it made the soil im- 

 penetrable. 



Excessive nitrates in the soil act in much the same way 

 as the carbonates except that the crust they form has a 

 brown, instead of a black, color and it is not so im- 

 penetrable. The nitrates also interfere much less with 

 the passage of water. Alkali spots are often found where 

 the soil remains permanently dry several inches below the 

 surface even though irrigation water is run over them 

 every week for several months. It is very evident there- 

 fore that though the salts may not exert a strong influence 

 of direct capillary action they do very materially affect 

 the absorption of irrigation and rain water in practice. 



Where gypsum is present in large quantities in an ir- 

 rigated soil, it is gradually washed out, causing the soil to 



