ALKALI SOILS. 



445 



tive ingredients potash, phosphoric acid and nitrates present 

 in the land in the water-soluble form. 



As has been shown in the preceding discussion, the analysis 

 at the surface foot alone, which has frequently been alone 

 made, gives no definite clew whatever to the total amounts of 

 salts to be controlled. A full estimate is of special importance 

 in enabling us to forecast what culture plants are likely to suc- 

 ceed on a given tract, by reference to the table of " tolerances " 

 given below (chapter 23, page 467). 



Composition of Alkali Soils as a Whole. As may be im- 

 agined, the presence of the alkali salts finds expression in the 

 analytical statement of their composition, although not to the 

 extent usually anticipated from their superficial aspect. The 

 table annexed gives the composition of fourteen alkali soils, 

 taken to the depth of one foot, at times when there was no visi- 

 ble accumulation of salts on the surface. The averages of the 

 several ingredients determined are given in the fifteenth col- 

 umn, and a comparison of its figures with those of the 

 general table on page 377 of chapter 20 will show some 

 marked characteristics. We find the average potash-content 

 to be but little less than t\vice as great as in the general average 

 for the state of California; in the case of lime the ratio is nearly 

 as one to three, in the case of magnesia nearly one to two; in 

 that of phosphoric acid, one to two and a half, of which in the 

 presence of carbonate of soda an unusually large proportion is 

 in a readily soluble, often in the water-soluble, condition (see 

 preceding table). 



The usual proportion of soda, of one-fourth to one-half of 

 the amount of potash, is changed to one-half or three-fourths; 

 in the case of the strongest alkali lands soda may equal or even 

 exceed the potash content. As the latter, however, is in- 

 variably high to very high, it does not happen as frequently as 

 might be supposed that the soda content exceeds that of potash 

 as shown by the usual method of soil-extraction with water. 



That the potash percentage should always be high in alkali lands, is 

 hardly surprising when it is considered that the continued presence of 

 the salts resulting from rock decomposition affords opportunity for the 

 full exercise of the preference with which potash is known to be retained 



