58 FERTILISERS CONTAINING NITROGEN [CHAP 



is that nitrate of soda requires to be used with phos- 

 phates and potash in order to make up a complete 

 fertiliser, and that only in this way can the fertility of 

 the soil be maintained. As has already been seen, 

 potash can generally be omitted in practice, but 

 phosphates must be added except in the special cases 

 when only a nitrogenous manure is needed, as in top 

 dressings for wheat. The prejudice against nitrate of 

 soda is probably really due to its injurious effect upon 

 the tilth ; if it is used in large quantities or repeatedly, 

 not only does the humus content run down but the land 

 begins to work badly, so that poor crops result, although 

 the land has not been particularly exhausted of plant food. 



As a fertiliser the special value of nitrate of soda 

 lies in its immediate availability ; no change has to take 

 place before it passes into the plant ; in consequence, it 

 has a very immediate effect in early spring, when the 

 land is still so cold that the production of nitrates by 

 bacterial processes is almost suspended however rich 

 the soil. As an aid to the rapid production of spring 

 vegetables, or to give a start to a field of spring corn 

 dwindling in the cold east winds, or to push a crop 

 through an insect attack, nitrate of soda is without a 

 rival. 



The great rival of nitrate of soda is at present 

 sulphate of ammonia, of which over 200,000 tons are 

 annually produced in this country. The source of 

 origin is coal, which contains about 1-5 to 2 per cent, of 

 nitrogen derived from the original vegetable matter 

 giving rise to the coal. When coal is subjected to any 

 destructive distillation by heat, as in the process of 

 gas-making or even when it is burnt, about 15 per 

 cent, of its nitrogen is given off as ammonia, which may 

 be recovered from the gases by simply washing them 

 with water. The ammoniacal gas liquor thus produced 



