350 SOIL CONDITIONS AND PLANT GROWTH 



per litre are added, together with a few drops of paraffin lamp-oil 

 to stop foaming. The upper stopper is replaced and a current of 

 purified air is sucked through the apparatus until the whole of the 

 ammonia has been carried over into the absorber. With the 

 amounts given a total of 1000 litres of air is sufficient ; a good filter 

 pump should aspirate this amount in from 3-^ to 4 hours. The 

 current should not fall below 250 litres per hour. The acid is then 

 poured into a hard glass flask, titrated with N/5o caustic soda till 

 nearly neutral, boiled to expel carbon dioxide, cooled, and titrated 

 to a clear yellow 1 (Fig. 32). 



40 



50 



Scale of CenMmetres. 

 FIG. 32. 



Carbonates are determined by treating a weighed quantity of the 

 soil with dilute sulphuric acid and estimating the carbon dioxide 

 evolved. Collins' calcimeter is a satisfactory instrument. 2 



Lime Requirement. Hutchinson and McLennan's method for 

 the determination of the lime requirement of the soil is as follows : 



1 See D. J. Matthews, Journ. Agric. Sci., 1920, 10, 72. 

 2 y0'/ra. Chem. Soc. Ind. t 1906, 25, 518. The apparatus is made by Messrs. 

 Brady and Martin, Newcastle-on-Tyne. 



