DETECTION OF PLANT NUTRIENTS 63 



The liquid is filtered into a porcelain dish, the contents 

 evaporated to dryness, and the residue ignited. When 

 cold, the residue is extracted with cold water and again 

 filtered. The clear filtrate is tested for potassium — 



(i.) By adding acetic acid and then some sodium 

 cobaltinitrite solution. A reddish precipitate indicates 

 potash. 



(ii.) By addition of platinic chloride solution (see 

 p. 10). 



{c) Nitrogen. — In normal soils nitrogen will be present 

 chiefly in the organic form, but a small proportion 

 should also exist as nitrates (see 41, p. 65). 



Organic Nitrogen. — A rough idea of the amount 

 and nature of the organic nitrogen of a soil may be 

 obtained by heating about 2 grams of the sample 

 with an equal weight of soda-lime^ in a test-tube. 

 The amount of ammonia liberated should be compared 

 with that from a soil of known composition, and should 

 be confirmed by measuring the loss in weight on 

 ignition. The latter figure is generally roughly pro- 

 portional to the organic nitrogenous matter present 



Nitrates. — Twenty to 30 grams of the soil are ex- 

 tracted by shaking for five minutes with 100 c.c. of 

 distilled water. The soil is allowed to settle, the 

 clear liquid filtered off and evaporated to a small bulk. 

 The residue is tested for nitrates by adding pure strong 

 sulphuric acid, and then a drop or two of diphenylamine 

 dissolved in sulphuric acid. A blue colour indicates 

 the presence of nitrates. 



The sulphuric acid used must be perfectly pure, 

 since the ordinary acid generally contains sufficient 

 nitric acid rapidly to produce a blue colour with 

 diphenylamine. 



1 Soda-lime is lime slaked with caustic soda solution instead 

 of water. 



