124 THE PRINCIPLES OF SOIL MANAGEMENT 



on black soils, other things equal, and, as has been 

 stated, this dark color is generally due to humus. 



54. Amounts present. The amount of organic matter 

 present varies greatly with different soils. Peat and 

 muck deposits are very largely organic material, the 

 per cent depending on the state of decomposition. 

 Some porous, well-drained soils are almost lacking in 

 this constituent. But nearly all soils have a moderate 

 per cent. The accumulation is larger in the soil than in 

 the subsoil, and generally decreases with depth. In 

 237 types of soil, representing thousands of samples 

 from all parts of the United States, the soil was found 

 to contain 2.06 per cent, and the subsoil .83 per cent. 

 This latter refers to the upper subsoil, and at greater 

 depths the organic content is very much less. But in 

 those soils recently formed by stream action the organic 

 content in the third, fourth and fifth foot may be very 

 considerable, as is indicated by the color. 



In general, arid soils contain less organic matter 

 than soils of humid regions ; those of cold climates 

 more than those of warm climates. The soils of the 

 northern states and Canada are very generally quite 

 dark colored, while those of the southern states under 

 similar treatment are much lighter colored, due to dif- 

 erence in organic content. Wet soils contain more than 

 dry soils, and clay soils contain more than sandy soils. 



These facts are illustrated by the following figures, 

 showing the amount of organic matter in different soils, 

 which in the first six lines are the average of ten samples 

 representing several soil types of approximately the 

 same natural drainage. 



