444 SOILS. 



ash, phosphate of soda, and nitrate of soda, representing the 

 three elements potassium, phosphorus, and nitrogen upon 

 the presence of which in the soil in available form, the welfare 

 of our crops so essentially depends, and which we aim to supply 

 in fertilizers. The potash salt is usually present to the extent 

 of from 5 to 20 per cent of the total salts ; phosphate, from a 

 fraction of one to as much as 4 percent; the nitrate from a 

 fraction of one to as much as 20 percent. In black alkali the 

 nitrate is usually low, the phosphate high ; in the white, the re- 

 verse is true. Both relations are readily intelligible from a 

 chemical and bacteriological point of view. 



Estimation of Total Alkali in Land. The investigations 

 detailed above having shown that in California at least, out- 

 side of the axes of valleys no practically important amount of 

 alkali salts is usually found at a depth exceeding four feet, it 

 became possible to determine approximately the amounts of 

 salts that would have to be dealt with when irrigation and 

 evaporation should bring the entire amount to or near the sur- 

 face; a necessary prerequisite to the determination of possible 

 cultures. While, as already shown, the salts occur lower down 

 in very sandy lands, yet the diagram on p. 435 shows that even 

 then, an estimate on this basis would not be very wide of the 

 truth. It is at least probable that the same is measurably true 

 of level alkali lands elsewhere, when not underlaid by geologi- 

 cal deposits impregnated with salts. 



The total amount of these salts ordinarily found in alkali 

 lands (i. e. in such as in the dry season show saline efflores- 

 cences on the surface) is from about one tenth of one per cent 

 to as much as three per cent of the weight of the soil, taken to 

 the depth of four feet. The percentage of salts having been 

 determined in samples representing a tract, it becomes easy to 

 calculate, approximately, the total amounts of each salt present 

 per acre, on the basis of the weight of the soil per acre foot. 

 For the soils of the arid region, such weight will usually range 

 from three million five hundred thousand to four million 

 pounds per acre-foot; the latter being the most usual figure, of 

 which it may be conveniently remembered, that forty thousand 

 pounds represent i per cent. We are thus enabled to esti- 

 mate e. g. the amount of gypsum required to neutralize the 

 carbonate of soda in the salts, or the amounts of valuable nutri- 



