218 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



In other cases there are doubtless internal derangements of the 

 plant's nutrition which interfere with its normal growth. In still 

 other cases, particularly when sodium carbonate is an important con- 

 stituent of the salt complex, it has been observed that actual corrosion 

 of the plant tissues takes place. 



It is sometimes difficult to determine the extent to which the alkali 

 salts are responsible for certain nonparasitic diseases which are often 

 serious, particularly with orchard fruits, on irrigated lands. Ordi- 

 narily the symptoms induced by these salts are easy to identify. In 

 any field where salts are troublesome the concentration is so much 

 greater in some spots than in others that the growth of the crop is 

 very uneven. There may be some spots in which there is no growth 

 at all because the salt concentration was so great as to prevent the 

 germination of the seed or to kill the young plants before growth 

 started. Other spots may be found where the plants remain small, 

 as though suffering from lack of water. 



These irregularities of growth of plants in fields apparently uni- 

 form as to soil conditions, together with the well-known fact that 

 some kinds of plants are much more tolerant to alkali salts than 

 others, have led to many attempts to overcome the alkali problem by 

 selecting varieties or species of plants for use on alkali lands. The 

 success of such work has been hampered by the fact that when soil 

 or ground-water conditions are such as to favor the accumulation 

 of alkali salts the concentration may soon be carried to a point which 

 exceeds the limits of tolerance of the most promising species. On 

 the other hand, some modification of the conditions may take place 

 with the result that the salt concentrations will fall well below the 

 limits of a wide range of crop plants. 



It appears to have been the general experience that the better way 

 to deal with the alkali problem is to remove the salts or prevent their 

 accumulation rather than to attempt to meet the difficulty by the use 

 of special crops or varieties. It is true that there are certain kinds of 

 crop plants that are so sensitive to alkali salts that their production 

 is quite out of the question on ordinary irrigated land. But there 

 remains a very large number of crops that do well if the concentra- 

 tion of the soil solution can be kept within reasonable limits. 



In actual field experience it has been found that when the salt 

 content of the soil is much above one-half of 1 per cent it requires 

 special care to use it for general crop production. This quantity of 

 salt is really very large when expressed in terms of tons per acre. 

 It is usually estimated that an acre of soil 1 foot deep weighs about 

 4,000,000 pounds, so that when a tract of land is said to contain one- 

 half of 1 per cent of salt it means that it contains 10 tons of salt per 

 acre for each foot in depth to which that percentage of salt extends. 



