REDUCING EVAPORATION 157 



cumulation. If the soil is very low in soluble salts no 

 harm may be done, but arid soils usually contain sufficient 

 salt to render high evaporation dangerous. 



If virgin soil contained 3000 parts per million of alkali, 

 the growth of most crops would not be greatly hindered; 

 but if through a constant movement of salt to the surface 

 the salt of the top four feet were concentrated in the upper 

 six inches, it would contain 24,000 parts per million, which 

 would make it entirely unsuited to crop production with- 

 out reclamation. If evaporation is reduced to a mini- 

 mum, an accumulation of this kind is checked. In the 

 reclamation of alkali land by any method, it is desirable 

 to prevent evaporation as nearly as possible, because 

 evaporation causes the salt to accumulate where it will 

 do most harm. 



In practice, many devices to reduce evaporation are 

 employed. These usually consist of cultivating the soil, 

 shading it, or the establishing of a good mulch by adding 

 manure, straw, leaves, or sand. Of the various materials 

 to be added, manure is usually to be recommended since 

 it has sufficient beneficial effect in addition to the mulch- 

 ing to pay for its use, while others are of questionable 

 economic importance. 



The most practical means of preventing evaporation is 

 through cultivation. An unstirred soil, particularly if it 

 is heavy - - as many alkali soils are - - forms a crust 

 which acts as an excellent conductor of moisture. Break- 

 ing up this crust by cultivation leaves the soil loose and 

 with but few points of connection with the lower layers 

 of soil. As a result evaporation is slight even though the 

 subsoil remains moist. It is particularly important that 

 the land be cultivated soon after irrigation since evapora- 

 tion at that time is especially high. 



