38 An Account of the Soil [pt. i 



accumulate on'soils like old garden soils to which much 

 plant matter is added, and they are probably partly 

 responsible for the curious difficulty in wetting these 

 soils when once dry ; drops of water tend to stand on the 

 surface and not to soak in. 



The mixture known as humus plays a specially im- 

 portant part in soil productiveness. In days gone by 

 humus was regarded as a distinct chemical group and 

 was subdivided into humic acid, ulmic acid, crenic acid, 

 aprocrenic acid, etc., but it is now known that these 

 substances are only imaginary; we shall therefore not 

 concern ourselves with them. Humus is a mixture which 

 is not yet satisfactorily resolved into its component 

 parts. 



Some of it can be extracted from the soil by means 

 of dilute alkahs, by the following method. Shake 100 

 grams of soil with 500 c.c, of 5 per cent, hydrochloric 

 acid, allow to settle, pom' off through a filter, and wash 

 with water. Then transfer the soil to a bottle, add 

 500 c.c. of 5 per cent, caustic soda solution, shake, and 

 leave for some hours lying on its side so that as large 

 a surface as possible is exposed to the alkah: shake 

 periodically. Before long the alkah becomes dark 

 coloured. Again allow to settle and syphon off, or, if 

 you can, filter on the Buchner funnel by aid of the 

 pump : this is rather a slow process. To the clear dark 

 coloured filtrate add some strong hydrochloric acid drop 

 by drop till the hquid is just acid. A dark brown pre- 

 cipitate is thrown down containing much of the organic 

 matter. Some, however, still remains in solution : a part 

 of this can be brought down by exactly neutrahsing 

 with caustic soda, but the rest is only recovered by 

 evaporation. It is sufficient, however, to examine the 



