152 SOIL PHYSICS AND MANAGEMENT 



certain regions almost all of the surface soil and its organic matter 

 have been removed, and yellow " clay points " are quite common. 

 These are nothing more than the outcropping of a stratum, either 

 of subsurface or subsoil, which contains little or no organic matter. 

 Even on brown silt loam areas much loss of organic matter takes 

 place through erosion, and this becomes more serious the longer 

 cropping*continues. 



(c) By Leaching. In the partial decomposition of vegetable 

 matter soluble organic acids are formed. These may be removed in 

 part by the water which percolates through the soil during heavy 

 rains. This is especially true of acid soils. It is not uncommon to 

 see the drainage water of peat bogs of a brownish color, due to the 

 dissolved organic matter. The presence of small amounts of certain 

 alkalies, as ammonia and sodium carbonate, increases the solvent 

 power of water for humus. 



(d) By Fires. Fires of even moderate intensity destroy large 

 amounts of organic matter from the immediate surface, and even in 

 the burning of straw, stubble, or corn -stalks considerable organic 

 matter is lost from the soil. Snyder 7 gives the following : " The 

 soil from Hinckley, Minnesota, before the great forest fire of 1893 

 showed 1.69 per cent humus and 0.12 per cent nitrogen. After the 

 fire there were present 0.41 per cent humus and 0.03 per cent nitro- 

 gen. The forest fire had caused a loss of 2500 pounds of nitrogen 

 per acre, or thirteen tons of organic matter." Much organic matter 

 that should be plowed back into the soil is burned. 



(e) By Oxidation or Nitrification. The process of oxidation 

 is carried on through the influence of bacteria which are always 

 present in fertile soils. Under favorable conditions of moisture, 

 temperature, and aeration these organisms are very active agents in 

 destroying organic matter. They are especially active in cultivated 

 and well-aerated soils, and while their work means destruction to 

 organic matter, they are at the same time performing a function 

 absolutely necessary for the growth of plants. In the destruction 

 of organic matter they are producing plant food essential for crops. 

 In the growing of crops, one and one-half pounds of nitrogen are 

 required for a bushel of corn, one for oats, and two for a bushel of 

 wheat, and this must be obtained from organic matter through the 

 agency of these bacteria. The greatest loss occurs when no crop is 

 growing, and these soluble plant foods are lost by leaching, although 

 some loss of nitrates is nroincr on whenever drainage takes place. 



(f) By Use of Quicklime. A very serious objection to 



