150 SOIL CONDITIONS AND PLANT GROWTH 



Total Nitrogen. Kjeldahl's method is almost invariably adopted. About 

 25-30 grams of soil are ground up finely in an iron mortar; 10-15 grams are 

 then heated in a Kjeldahl flask with 20-25 c - c - f strong sulphuric acid for 

 \ hour ; then 5 grams of potassium sulphate are added, and shortly after a crystal 

 of copper sulphate. The heating is continued till all the black colour has 

 gone. Then cool and dilute the mixture, transfer the fluid part to a distilla- 

 tion flask, but leave as much as possible of the sand behind, wash well to 

 remove all the adhering liquid. Add saturated soda solution till the liquid is 

 strongly alkaline, distil, and collect the ammonia in standard acid. 



Nitrates must be determined in a sample taken direct from the field and 

 dried without any delay at 55 C. ; 200-500 grams of the dried soil are pressed 

 firmly on to a Buchner funnel fitted to a filter flask, and distilled water is poured 

 on. The first 300 c.c. of water passing contains practically all the nitrates, but 

 it is safer to wash more fully. The solution is concentrated in presence of 

 a little magnesia, acidified with acetic acid, and reduced by a zinc-copper couple 

 at 25 C. for 30 hours. The ammonia formed is estimated in the usual way. 



Ammonia is estimated by distilling with magnesia and water under reduced 

 pressure (241). 



Carbonates are determined by treating a weighed quantity of the soil with 

 dilute sulphuric acid and estimating the carbon dioxide evolved. Large 

 quantities can be determined rapidly and with sufficient accuracy by the 

 Scheibler apparatus, but much better results are obtained by absorbing the 

 CO 2 in potash and determining the amount by titration. The details have 

 been worked out by Amos. 1 Great care is needed when only small quantities 

 are present ; the treatment with acid must be effected at as low a temperature 

 as possible to avoid decomposition of organic matter. Small quantities can 

 also be determined by Hall and Russell's method. 2 



Mineral Substances. Complete analysis of a soil after the silicates have 

 been decomposed and the silica volatilised by treatment with hydrofluoric 

 acid is only rarely attempted. The British method, adopted by the Agricul- 

 tural Education Association, is thus described by Hall : "20 grams of the 

 powdered soil are placed in a flask of Jena glass, covered with about 70 c.c. 

 of strong hydrochloric acid, and boiled for a short time over a naked flame 

 to bring it to constant strength. The acid will now contain about 20-2 per 

 cent, of pure hydrogen chloride. The flask is loosely stoppered, placed on the 

 water bath, and the contents allowed to digest for about forty -eight hours. The 

 solution is then cooled, diluted, and filtered. The washed residue is dried and 

 weighed as the material insoluble in acids. The solution is made up to 250 

 c.c., and aliquot portions are taken for the various determinations. The 

 analytical operations are carried out in the usual manner, but special care must 

 be taken to free the solution from silica or organic matter " (The Soil). As a 

 rule only potash and phosphoric acid are determined, but where other bases 

 are wanted they are estimated in the usual way. 



l journ. Agric. Sci., 1905, i., 322-26. ^Journ. Chem. Soc., 1902, Ixxxi., 81-85. 



