100 THE ANALYSIS OF SOILS. V. 



they cannot be accurately determined in the three or four 

 grammes of soil taken for the previous determinations, espe- 

 cially as at least half of the solution will have been used for 

 determinations of iron and alumina and total potash. 



It is usually advisable, therefore, to take 6 or 8 grammes of 

 soil, treat and digest with hydrochloric acid as before, remove 

 the silica, ferric oxide, alumina, and phosphoric acid as before 

 from the whole solution, then to the nitrate from ferric oxide, 

 &c., to add ammonium oxalate, allow to stand twelve hours, 

 filter, wash, dry, ignite in platinum crucible to constant weight, 

 and weigh as calcium oxide. 



The filtrate from the calcium oxalate is freed from ammo- 

 nium salts by evaporation with nitric acid in the usual manner 

 and the magnesium precipitated as ammonium magnesium 

 phosphate and weighed as magnesium pyrophosphate. 



Determination of the amount of calcium carbonate. A direct 

 determination of the amount of carbon dioxide evolved on 

 treatment with dilute hydrochloric acid is sometimes advisable, 

 since for many purposes the lime existing as carbonate is of 

 more importance (as regards nitrification, for example) than 

 the total lime, some of which may be as silicate. This can 

 be done by any of the usual methods, either receiving the 

 evolved gas in weighed potash bulbs ; or, if its amount be 

 large, by determining the loss in weight of an apparatus in 

 which it is generated and from which it can be wholly re- 

 moved by a current of air. Details of these methods will be 

 found in any manual of quantitative analysis. 



A complete chemical analysis, though of service as giving 

 the limits of the plant food which a soil can provide, is often 

 of disappointingly little use and frequently affords no informa- 

 tion of value as to fertility or manurial requirements. 



A good example of such failure is seen in the following 

 analyses of two soils from pasture land at the Experimental 

 Farm at Garforth made by the author in June, 1900 : 



" Fine soil " contains 



Soil A. Soil 13. 



Moisture ... ... ... ... 3-13 1-70 



Loss on ignition ... ... ... 10-85 7*79 



(Nitrogen ... ... ... -274 -247) 



