THE PROPERTIES OF SOILS 203 



little chalk and mix it with the peat before applying it to the 

 litmus paper ; the red colour of the litmus will be changed again 

 back to blue. Soils derived from chalk or limestone often 

 contain large proportions of carbonate of lime, and the presence 

 of a little is necessary to the building up of a fertile or even of a 

 healthy soil, though sands and clays are to be found in which it 

 can hardly be detected. 



One other constituent which plays an important part in 

 the soil, the organic matter or humus, the debris of vegetable 

 origin, has already been noticed as giving the soil proper rather 

 a darker colour than the subsoil. Weigh out into porcelain 

 basins about ten grams each of the clay soil and subsoil, the 

 alluvial soil and subsoil, and the peat soil, each of which had 

 been put into the oven for a few hours previously to get 

 thoroughly dry, then char them for some hours over a Bunsen 

 burner, or best of all in a muffle furnace. At first the colour 

 of the soil will darken, but gradually it will change to a 

 bright red as the whole of the carbonaceous matter burns away. 

 After cooling, re-weigh the dishes ; the loss of weight represents 

 the organic matter, though it includes also a certain amount of 

 water that was previously in chemical combination with the clay 

 and kindred constituents of the soil. However, by far the larger 

 part of the loss is due to the organic matter, and from the ex- 

 periment it will be safe to conclude that the peaty soil contains 

 an exceptional amount of humus, and that clay soils generally 

 contain more than sandy ones. Soils also are always much richer in 

 humus than the subsoils, except in the case of alluvial soils, which 

 do not differ much in composition from their subsoils, because 

 both have been alike washed from other land, carried down by the 

 stream, and redeposited. The smell given off in the early stages 

 of charring by any of these soils suggests the presence of nitro- 

 genous compounds, but this can be better demonstrated by 

 mixing with some of the soil in a separate dish before heating a 

 little soda-lime or even lime itself; the smell of ammonia will 

 soon be palpable, and its presence may be confirmed by holding a 

 reddened litmus paper in the escaping gases. Without pushing 

 the matter further, all soils can be shown to possess a store of 

 organic matter containing nitrogen the humus as it is some- 



