ALKALI PROBLEM IN IRRIGATION — SCOFIELD. 221 



untouched. Were the alkali problem in irrigation confined to the 

 reclamation of those lands in which an excess of salts had accumu- 

 lated under natural conditions it would be much less serious and 

 would cause much less apprehension as to the future of irrigation 

 farming. 



The really serious aspect of the alkali problem lies in the fact 

 that in a majority of instances it develops after a period of success- 

 ful irrigation, after the land has been leveled and improved with 

 roads, fences, and farm homes. When this happens the real cause 

 of the trouble is to be sought in the quality of the irrigation water 

 rather than in the quality of the land. 



The water used for irrigation is taken from streams or from wells. 

 All such water contains at least a small quantity of salt in solution. 

 In general the stream and underground waters of arid regions con- 

 tain more dissolved salts than the waters of humid regions. The 

 river waters of typical humid regions usually carry less than 200 

 parts of dissolved salts in a million parts of water, while many 

 streams of the arid regions carry more than 1,000 parts of salt per 

 million. Ocean water contains about 35,000 parts of salt per mil- 

 lion. For purposes of comparison it may be said that an acre-foot 

 of water, that is, the quantity required to cover 1 acre 1 foot in 

 depth, weighs about 2,750,000 pounds. In ordinary irrigation prac- 

 tice, it is not uncommon to use as much as 3 or 4 acre-feet of water 

 per acre per year. From these figures it will be seen that if all the 

 salt carried in the irrigation water were retained in the soil the 

 annual increment of salt might be as much as 4 or 5 tons per acre. 



These figures serve to illustrate the importance of considering the 

 quality of irrigation water in relation to its long continued use 

 on the same land. If conditions are such that a fair proportion of 

 the water applied percolates downward through the soil and eventu- 

 ally finds its way out in the country drainage, the excess of salts may 

 be carried awaj^ with it. But if, on the other hand, conditions are 

 such that most of the water applied is held in or near the surface 

 soil and evaporates there, its burden of salts will be left behind and 

 in time the accumulation must reach and pass the danger limit. 



Another feature of the quality of irrigation water that is quite as 

 important as its salt content is the relative proportions of the dif- 

 ferent salts. Where the dissolved salts are largely compounds of 

 calcium and magnesium the injurious effect, if any, is confined to 

 plant growth and may be remedied, when it becomes apparent, by 

 suitable measures of artificial drainage. But if the salts are largely 

 compounds of sodium or potassium there is always serious danger 

 that the physical condition of the soil may be injured to such an ex- 

 tent that it will remain unproductive even after the excess of salt 

 has been removed by drainage. 



