vi.] ANALYSIS OF TYPICAL SOILS 169 



If the liquid shows any tendency to come through 

 the filter turbid, this can be obviated by adding a few 

 drops of sulphuric acid to the water. In the filtered 

 liquid the nitrates may be determined by reducing with 

 the zinc copper couple, distilling off the ammonia and de- 

 termining it either by Nesslerising or by titration, accord- 

 ing to its amount. The couple is prepared by dipping 

 half a dozen strips of thin sheet zinc, 6 inches long by 

 1 J broad, successively into dilute caustic soda, very dilute 

 sulphuric acid, and then into a 3 per cent, solution of 

 copper sulphate, in which they are allowed to remain 

 until they have acquired a good black deposit of copper. 

 They are washed by immersion in water, and finally in 

 ammonia-free distilled water, and placed in a bottle with 

 200 c.c. of the soil extract and a crystal of oxalic acid. 

 The bottle is kept in a warm place or an incubator at 25 

 for twenty-four hours before distilling off the ammonia. 



The table (Appendix I.) shows the analyses by the 

 method above described of a few typical soils. 



It will be seen, as a rule, that the water retained by 

 the soil when air dry, the loss on ignition, and the 

 nitrogen, rise and fall together, because the humus 

 which contains the nitrogen is the most hygroscopic 

 constituent of soils. Clay soils which tend to conserve 

 humus also contain the most constitutional water ; this 

 further tends to increase the loss on ignition in their case. 



The proportion of nitrogen found ranges from 0-5 

 per cent, in very rich pasture soils down to below o-i 

 per cent, on light arable soils, it is rarely up to 0-2 per 

 cent, in arable soils, and the warmer, the more open, 

 and more worked the soil is, the less will be the pro- 

 portion of nitrogen. In the fertile hop-gardens of 

 East Kent the percentage of nitrogen is rarely as much 

 as 02 per cent., despite the great dressings of nitrog- 

 enous manure that are annually applied. 



