GEOLOGY OF AGRICULTURE. 75 



vegetable matter in process of decay, but not yet 

 wholly decomposed. It always consists of carbon, 

 hydrogen, oxygen, and nitrogen (CHON). In a poor, 

 worn-out soil, there is very little organic matter. In 

 a new and rich soil, such as we have been considering, 

 there is a large amount, sometimes as high as 20 per 

 cent., and very often as high as ten. So much, how- 

 ever, is not necessary, even to the highest fertility. 

 Some of the most productive soils contain not 

 more than two per cent. Organic matter in soils 

 passes through successive changes before it is wholly 

 decomposed into its original elements. At first you 

 will find it in the form of decaying grass, weeds, stub- 

 ble, leaves, roots, &c. In this state you may sift it 

 out with a coarse sieve. As the process of decay goes 

 on, it takes in oxygen and becomes an acid, as we 

 have seen that sulphur, carbon, and other substances 

 become acids by combining with oxygen. As the 

 process proceeds, it takes more oxygen, and becomes 

 another and different acid. These are called organic 

 acids. Chemists have distinguished no less than five 

 of them — humic, ulmtc, geic^ crenic^ and apocrenic acids. 

 Others have chosen to call the decaying matter in the 

 soil geine. They make this distinction, however, down 

 to that point in the process of decay at which it dis- 

 solves in water, they call it insoluble geine^ and beyond 

 that, soluble geine. But as vegetable matter, in pro- 

 cess of decay, becomes sour, and then changes its cha- 

 racter, becoming a somewhat different substance at 

 each stage in the process, there may be a propriety in 

 calling it an acid, and in giving it a separate name for 



