SOILS AND FERTILIZERS 91 



Second, when underdrainage has been provided for, 

 either naturally or artificially, irrigation may be useful 

 in removing from the soil the excess of mineral salts 

 (usually termed alkalies), when such are present. Such 

 excess is often found in the arid regions and in lands 

 which were once covered by the sea, such as reclaimed 

 sea marshes. 



Soils that are not apparently alkaline' naturally have 

 often become so after they have been irrigated for 

 some time without good underdrainage. How this is 

 brought about may be shown by using in the last experi- 

 ment a can which will not leak at the bottom, and 

 supplying water from the top, being careful not to add 

 too much water at one time. 



Note. The best water for irrigation purposes is that which carries 

 a large amount of plant food, either in solution or suspension, and is 

 of a temperature sufficiently high 'not to check the growth of vegeta- 

 tion. Rivers whose waters are muddy are often the best for irrigation 

 supply, because the muddiness indicates a large amount of fine silt, 

 which will enrich the soil it is spread over, especially if that soil is a 

 sandy one. 



Suggestive Experiments. (/?) To show how inland soils may 

 become alkaline, mix a little soda with some wet, fine sandy loam or 

 clay. Place it in a box and let the top dry, but keep the bottom 

 moist. Observe the gradual accumulation of soda on the surface. 



The soda, corresponding to the plant foods scattered throughout the 

 soil, is taken into solution by the water as it rises through the soil 

 by capillarity and is left on the top as this water evaporates at the 

 surface. 



(/) That irrigation in connection with underdrainage may be 

 useful in removing this excess of salts from the soil may be shown as 

 follows : Prepare an alkali soil by mixing potash or soda with dry, 

 sandy soil. Test it with blue litmus paper to see if it is sufficiently 

 alkaline to change the color. Place this soil in a tin can having holes 



