28 PRACTICAL AGRICULTURAL CHEMISTRY [67-69 



* 67. The Nature of Humus. — The organic matter 

 or humus in a soil consists principally of certain acids, 

 which, like all other acids, combine with caustic potash to 

 form salts. The soil acids themselves are not soluble in 

 water, but the compounds which they form with caustic potash 

 are soluble. To show this, place a small quantity of peat soil 

 at the bottom of a test-tube and fill the tube about one-third 

 full of caustic potash solution. Warm for a few minutes, 

 then fill up the test-tube with water, shake well, and 

 filter. The liquid which comes through will be coloured 

 brown by the potash compounds in solution. Take a 

 little of the solution in a test-tube and add an excess 

 of dilute hydrochloric acid. The liquid will become cloudy 

 because the soil acids will be again set free from the 

 potash, and, as they are not soluble in water, they will form 

 a precipitate. 



EXPERIMENTS ON MANURES 



68. The most important constituents of manures arc 

 lime (CaO), potash (K 2 0), phosphoric acid (P 2 5 ), and 

 nitrogen (N). Most natural manures, such as farmyard 

 manure and guano, contain all these substances. Artificial 

 manures are generally made by mixing substances which 

 contain one or more of the above constituents in a concen- 

 trated form. The experiments described in paragraphs 69, 

 70 are to show the properties of the various simple manures 

 before mixing. 



Lime Manures 



69. Slaking Lime.— Place a lump of freshly prepared 

 lime, about the size of a small nut, on a tile and pour a few 

 drops of water on to it. When this water is soaked up, pour 

 a few more drops, and continue this process as long as the 

 lime will continue to absorb it and still remain dry. The 

 lime will shortly become hot and fall into a fine powder, 



