ALKALI LANDS AND THEIR RECLAMATION 287 



while the latter is very beneficial to the soil in its puddled condi- 

 tion. The reaction is as follows: 



CaS0 4 = Xa 2 S0 4 + CaC0 3 . 



The amount to be applied depends upon the amount of alkali 

 present. Twice as much gypsum as black alkali is needed, but it is 

 best to apply 200 to 400 pounds per acre annually. Moisture is 

 necessary for the reaction to take place. The change in the phy- 

 sical condition of the soil is as important as the chemical effect. The 

 impervious soil begins to swell up, becomes porous and soon the de- 

 pressed spot is brought to the general level. 



5. Removing the Salts from the Soil. The removal of the 

 salts is the only permanent remedy for reclaiming alkali lands. 

 This is accomplished in several ways. 



(a) By tfcrapiny. When excessive evaporation has brought 

 large quantities of alkali to the surface it may be scraped off with 

 two or three inches of soil and thrown into drainage systems that 

 will carry them off the land. Large amounts of alkali may be re- 

 moved in this way, but this applies to small areas only. 



(b) Flooding. The alkali may be leached doirnirard into the 

 soil to a depth of three or four feet by Hooding so that the crop may 

 be temporarily relieved from any danger of injury. Attempts have 

 been made to wash the salts off the land, but since they soak into 

 the soil as soon as dissolved this is impossible. 



(c) By Cropping. This method is to produce crops that take 

 up large amounts of alkali in their growth which will be removed 

 with the crop. The Australian salt bush when mature contains 20 

 per cent of ash and yields as much as five tons per acre. A single 

 crop will remove approximately a ton of alkali. 



(d) Underdrawn ge. Leaching out the salts through under- 

 drainage is the most practical a'nd permanent remedy that has been 

 devised. This, of course, requires a thorough drainage system as 

 complete as for draining the swamps of humid regions. After the 

 drainage system is installed, the soil must be flooded to leach out 

 a large per cent of the salts, so that there will be little danger from 

 alkali later. Tin's requires a large amount of water, as the flooding 

 must continue for several months ( Fi<r. i:?1). With every irriga- 

 tion system a corresponding underdrainasre svstem should be in- 

 stalled to carry off the water from excessive irrigation and seepage. 

 which is largely responsible for the rise of alkali. 



To give an idea of the way reclamation is accomplished by 



