352 CHEMISTRY. 



mus. It is of a dark color, and hence fertile earth has a 

 darker color than sand. The substances forming humus 

 are chiefly those that are composed of carbon, oxygen, 

 and hydrogen, vegetable fibre as contained in wood, bark, 

 leaves, etc., being the principal. There is some nitrogen, 

 of course, in humus, from the juices of plants and the seeds, 

 and also from the decomposition of animal substances. 

 The immediate products of the decomposition of humus 

 are humic acid, so called, humic acid salts, carbonic acid, 

 water, ammonia, etc. The decomposition produces a good 

 effect mechanically upon the whole body of the soil, loosen- 

 ing it, and so making it mellow, as it is commonly termed. 

 Humus is also a great absorbent of water, swelling up as a 

 sponge, and this helps the mechanical effect produced by 

 the generation of gases by decay. Heat also is developed 

 by the chemical changes, which is often of very material 

 benefit when the soil is naturally a cold one. 



491. How Soil was Originally Made. All the soil, with 

 the exception of that portion of the carbon which has been 

 supplied from the air, and also the water which is diffused 

 in it, came originally from the rocks. There was a time 

 when there was nothing but rocks and water and air. 

 Some of the rocks became at length broken and ground up 

 by processes which the geologist describes, and thus was 

 furnished the soil on which the plants first grew. Soil be- 

 ing thus prepared, seeds were supplied by the Creator, the 

 plants from which, sending down their roots into the pow- 

 dered rock, took up there the soluble matters, and sending 

 up branches and leaves into the air, collected carbon there 

 with their outspread nets. And now the plants, decaying, 

 added humus to the soil, which, increasing year after year, 

 at length made the soil a fertile one. Besides those agita- 

 tions which break up and scatter fragments of rocks and 

 grind them to powder, there is another process, called 



