44 THE MECHANICAL ANALYSIS OF SOILS [chap. 



Substances akin to humus can be formed from the 

 carbohydrates (such as sugar, starch, and cellulose), by 

 heating them for some time with water under pressure, 

 the action being more rapid if a trace of mineral acid be 

 present ; the resulting substances are weak acids and 

 form salts, so are generally termed humic acid : — 



As a rule, the active humus of the soil is 

 there present in the form of salts of calcium, which 

 on treatment of the soil with dilute hydrochloric acid 

 are decomposed, a little of the humic acids going into 

 solution but the greater part remaining undissolved. 

 By filtering off the acid and then treating the soil with 

 a weak (4 per cent, by volume) solution of ammonia or 

 other alkali, the liberated humic acids are dissolved and 

 may be reprecipitated either as free acids by the addition 

 of hydrochloric acid, or as calcium salts by the addition 

 of a solution of calcium chloride. The humic acids thus 

 going into solution are sometimes estimated as " soluble 

 humus," they do not include the whole of either the 

 organic matter or the nitrogen in the soil. The brown 



