102 THE ANALYSIS OF SOILS. V. 



further treatment, placed in a flask or bottle, and treated with 

 2000 c.c. of water containing 20 grammes of pure crystallised 

 citric acid. The two are left in contact, with frequent stirring 

 or shaking, for seven days, at the ordinary temperature of 

 the laboratory. The liquid is then filtered, best through a 

 Buchner's " nutschen" or porcelain funnel with parallel sides 

 and perforated base, by the aid of a filter pump. 500 c.c. of 

 the filtrate are taken for the determination of the available 

 potash and a like quantity for the available phosphoric acid. 



These two portions are evaporated to dryness and gently 

 ignited to destroy organic matters. The potash and phos- 

 phoric acid are then determined exactly as already described. 



That this method of determining the available potash and 

 phosphoric acid affords valuable help in judging of probable 

 fertility is shown by its application to the two soils whose 

 analyses have been given. 



The results were as follows : 



Soil A. Soil B. 



Available potash ... -0062% -0060% 



Available phosphoric acid. . . -0049 % '0205 % 



It is thus evident that so far as potash is concerned the two 

 soils are almost alike, but that soil A, though containing more 

 total phosphoric acid than B, contains less than one-fourth as 

 much in an available form. The beneficial effects of basic slag 

 on soil A is thus easily understood, soil B containing an 

 abundance of available phosphates. 



Dyer points out that less than "01 % of available phosphoric 

 acid in a soil indicates that it stands in immediate need of 

 phosphatic manures. With reference to the limit for the avail- 

 able potash, Dyer thinks, though he expresses more doubt in 

 this case than with the phosphoric acid, that soils containing 

 much less than 005% available potash require application of 

 potash manures/'' 



A few other constituents may require determination for 

 special purposes : 



Determination of sulphuric acid. Sulphur is found in a soil 



* In a subsequent paper (Proc. Boy. Soc. 1901, 11) Dyer states that for cereals the 

 limit denoting deficiency in phosphoric acid is between '01 and 0'03 % soluble in 1% 

 citric acid solution. In root crops, especially turnips, the limit would probably be 

 higher. He also states that soils containing OtH% of potassium soluble in 1 % citric- 

 acid probably require no further application of potash manures. 



