C 530 1 



( CO NH 2 ) anc * amm0nium cyanate (NH 4 CNO) which are 

 metameric and have the same composition (but not constitu- 

 tion) as also Ammonium Sulpho or Thio-Cyanate (NH 4 CNS) 



and Sulph-urea CS,, 2 , have totally different values as 



plant-foods. Ammonium Cyanate and Am -Sulpho Cyanate 

 are poisonous to pjants, while urea and sulph-urea are valuable 

 plant- foods. 



850. Ammonia. The soil under ordinary circumstances 

 absorb and condense minute quantities of NH :i from the atmos- 

 phere, but the NH 3 of the soil is also being continually diffused 

 into the air. If a soil contains a good deal of NH 3 and is 

 in a moist state, it is rich in plant-food, but on drying such 

 soil readily parts with its NH 3 . On moistening this soil 

 again and drying it, more NH 3 is given out and so on. The 

 constituents of soil which have the greatest attraction for 

 NH 3 are clay, ferric hydrate and aluminum hydrate. With 

 acids of the humus group and with compound silicates, NH 3 

 forms compounds which are very sparingly soluble. NH 3 

 escapes in the air probably as (NH 4 ) 2 Co 3 . If CaSo 4 and 

 (NH 4 ) 2 Co 3 are mixed together the mixture smells strongly 

 of NH 3 , but if the mixture be thoroughly moistened with 

 water the odour of NH 3 is no longer perceived, Am -sulphate 

 and CaCo 3 being formed. If the mixture is dried (NH^a- 

 Co 3 is again given off and CaSO 4 is formed. CaSO 4 is 

 therefore called a fixer of ammonia but it is only in the 

 damp state that it is a fixer. KC1, Kainit, clay and peat 

 are also fixers of Ammonia. Of these, gypsum may be 

 used in dung-heaps and stables, but Kainit is the best 

 substance to use, then peat, then clay, and then gypsum. It 

 is difficult to estimate the amount of NH 3 in soil and 

 manure. It is being continually changed into HN0 3 and part 

 of it also is constantly being dispersed into air. If a quan- 

 tity of soil is boiled with caustic alkali, not only is the NH 3 

 actually present obtained in the distillate, but also an addj- 



