VARIATION IN THE CHEMICAL COMPOSITION OF SOILS. 15 



tively large quantities of easily .soluble compounds have to be washed 

 from a relatively small quantity of a nearly insoluble compound, 

 an appreciable quantity of the latter may be lost before all of the 

 former have been removed. As an example, in the fusion analysis 

 of a soil after removal of the silica there remains an acid solution of 

 all the bases with a large excess of sodium chloride. In the pre- 

 cipitation of calcium as calcium oxalate, it is necessary, of course, 

 to remove this excess of chlorides, and calcium oxalate being slightly 

 soluble, some is lost and may or may not be recovered later in the 

 analysis. The personal factor of judgment comes in at this point 

 and will explain many discrepancies. 



The limits of error stated apply to analyses of soils of average 

 composition by the fusion method. It is not meant, for instance, 

 that the limit of error in all CaO determination is 0.10, for in the 

 case of a dilute acid extract of a soil where the only excess of salts 

 introduced is ammonium salts, the error allowed should be much 

 smaller. 



One of the reasons for drawing attention to this matter here is 

 the practice in some quarters of publishing soil analyses in terms of 

 pounds per acre. Such results are presumably based on an average 

 of several analyses, but the variations of such analyses usually do not 

 appear. 



Assuming that an acre of soil 6 inches deep weighs 1,750,000 

 pounds, the limits of error stated when calculated to pounds per acre 

 6 inches deep would be as follows : 



CaO limit 0.10 per cent or 1,750 pounds. 

 K 2 limit 0.05 per cent or 875. pounds. 

 P 2 5 limit 0.04 per cent or 700 pounds. 



Stated in another way it would appear as follows: Suppose a 

 soil on analysis gave K 2 0, 0.50; CaO, 0.30; P 2 5 , 0.08, allowing the 

 above limits of error, the composition 'might be: 



Per cent. 



K 2 c 0. 525 or 0. 475 



GaO 0. 35 or 0. 25 



P 2 5 0. 10 or 0. 06 



Or in pounds per acre: 



K 2 .' 9, 187. 5 or 8, 312. 5 



CaO 6, 125 or 4, 375 



P 2 5 . 1, 750 or 1, 050 



It is quite plain then that too much significance should not be 

 attached to differences of a few hundred, pounds of any ingredient 

 when the composition of a soil is expressed hi pounds per acre, not 

 because the actual presence or absence of such an amount might not 

 have .an effect, but because the statement based on chemical analysis 

 may depart that much from the actual fact. 



