SCIENTIFIC AGRICULTURE. 169 



absorbs water with such avidity as to contain three fourths of 

 its own weight without being moist 



Weak acids have little effect on humus, except to dissolve 

 the alkaline and metallic or earthy matters which are usually 

 mixed with it Potash and soda dissolve humus entirely, with 

 the evolution of ammonia: from this solution, acids cause a 

 precipitate of a brown inflammable powder resembling ulmine. 

 Humus contains more carbon and nitrogen than the vegetables 

 from which it is derived : the nitrogen may be partly formed 

 from the excrements of insects which live in the humus. 



Humus contains, besides some mineral elements, carbon, 

 oxygen, hydrogen and nitrogen, phosphoric, sulphuric and 

 humic acids. Humus is dissipated when exposed to the air by 

 a slow combustion, with the disengagement of carbonic acid. 

 This, and all vegetable earths, are entirely destructible. Salts 

 are formed during the decomposition of humus, by the union 

 of bases with the humic acid, these are called humates. 



Besides the above elements, humus contains, according to 

 Berzelius, humic, crenic and apocrenic acids, and traces of 

 glairin. Humus is an indispensable ingredient in all fertile 

 soils, hence the necessity of replacing it in the soil as fast as it 

 is exhausted. 



Agriculturists who think to supply the place of manure by 

 frequent and deep ploughing, have been disappointed, and 

 their fields have been gradually impoverished by crops, until 

 they became barren. When humus is put on a clay soil, it is 

 retained with such tenacity by the clay, that the free contact 

 of air is prevented, and it decomposes more slowly, for this 

 reason clay requires a larger quantity, other things being 

 equal, to produce the same effects, than other soils. 



Sand allows free access of air to the humus, which is incor- 

 porated with it, and thereby favors its decomposition and 

 consequent fertilizing power. Lime and potash destroy the 

 acidity of sour humus, and favor its decomposition: sour 



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