ORGx^NIC MATTER 109 



in some districts may contain organic matter which was carried 

 in with the soil at the time of formation. For example, the 

 Greenville (Utah) Experimental Farm which was deposited by 

 water contains .53 per cent of humus in the first foot, nearly twice 

 as much in the second, and .57 per cent in the tenth. However, 

 the active organic matter of the surface is considerably higher 

 than it is in the subsoil. 



Composition. — The active organic material of the soil is com- 

 posed of plant tissue which contains carbohydrates, fats, and pro- 

 teins. The carbohydrates consist of the various sugars, starches, 

 and celluloses or woody fiber. The protein may consist of any 

 of the complex nitrogen-carrying compounds, whereas in the 

 fats we find waxes, solid fats, and oils. Intimately mixed 

 with all and often forming an integral part of the material are 

 the essential plant-foods — ^phosphorus, potassium, and nitrogen — 

 in the form of organic and inorganic compounds. 



The humus consists of a great variety of ill-defined chemical 

 compounds. Among these we would expect to find, and do find, 

 the building blocks of the proteins — the amino-acid. These, how- 

 ever, lose their ammonia 5 hence, there is a tendency for acid sub- 

 stances to accumulate, which if not neutralized by some base pro- 

 duce a sour soil. Not long ago it was believed that humus con- 

 sisted mainly of ulmic and humic acids. We now know, how- 

 ever, that it is much more complex. 



Function. — Organic matter has a very far-reaching influence 

 upon the soil. It often determines whether a soil will be produc- 

 tive or barren. Some of the more pronounced influences are the 

 following: 



I. Organic matter in various stages of decay gives to a soil its 

 dark-brown or black color. The absorption of heat and conse- 

 quently the temperature depends to a large extent upon the color 

 of the soil. It is common knowledge that dark-colored soils are 

 earlier than light-colored soils. A very interesting demonstration 

 of this can be made in the spring by filling a tray with light-^ 

 colored soil. Seeds are planted throughout and then one-half is 

 covered with soot. In the soot-covered part plants come up in 



