90 PHOSPHOKIC ACID LIME MAGNESIA 



chloric acid, is probably unavailable to plants for many years to come, 

 so that its determination is not often of importance. 



Determination of " total " phosphoric acid. This can conveniently 

 be done in the portion of the soil taken for determination of loss on 

 ignition. The residue in the platinum dish is treated with strong 

 hydrochloric acid, digested for some time, evaporated to dryness, and 

 heated to render the silica insoluble, moistened with strong hydro- 

 chloric acid, taken up with water, filtered and evaporated with strong 

 nitric acid. It is again diluted and mixed with excess of ammonium 

 molybdate solution and allowed to stand in a warm place for eighteen or 

 twenty-four hours. It is then filtered, washed by decantation with dilute 

 nitric acid and once with water, then dissolved in dilute ammonia 

 (filtered- if necessary), mixed with " magnesia mixture," 1 and allowed to 

 stand twelve hours. The ammonium magnesium phosphate, NH 4 MgP0 4 , 

 is then filtered off, washed with ammonia, dried, ignited slowly and 

 carefully, and weighed as Mg 2 P 2 7 . Phosphoric acid may also be 

 determined in a portion of the hydrochloric extract of the soil. 



Determination of the lime and magnesia. Except in some few 

 cases, the amount of these constituents is so small that they cannot be 

 accurately determined in the 3 or 4 grammes of soil taken for the previ- 

 ous determinations, especially 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 filtrate from ferric oxide, etc., 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 ammonium salts- 

 by evaporation with nitric acid in the usual manner and the magnesium 

 precipitated as ammonium magnesium phosphate and weighed as mag- 

 nesium 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 by 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 removed 

 by a current of air. Details of these methods will be found in any 

 manual of quantitative analysis. 



Limitations of Chemical Analysis. A complete chemical anal- 

 ysis, 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 information of value as to fertility or manurial requirements. 



1 A solution containing magnesium chloride, ammonium chloride and ammonia. 



