ORGANIC MATTER 53 



(4) It improves drainage by reason of the properties 

 stated under (1) and (2). 



(5) It increases the extent of root growth for the same rea- 

 sons. 



(6) By making the soil darker, it facilitates heat absorp- 

 tion. 



(7) It is a source of plant-food material. 



(8) It furnishes energy for the growth of bacteria. 



(9) Its decomposition produces carbonic acid gas and 

 other acids that help to render plant-food materials soluble. 



52. Porosity of organic matter. — The way in which or- 

 ganic matter promotes a granular structure in soils has 

 already been described, as has also the relation of soil struc- 

 ture to tilth. In addition to this effect on soils, organic 

 matter also serves to make soil more porous by reason of 

 its own porosity. It may be compared to a sponge in its 

 ability to hold air or water. A peat soil, for instance, will 

 hold more water than its own weight of dry matter. Or- 

 ganic matter extracted from a peat soil was found to carry 

 twelve times its own weight of water. It may readily be 

 seen that the porous nature of this organic matter may 

 greatly increase the water-holding capacity of a soil. At 

 the same time it may increase the capacity of the soil for air. 



53. Organic matter and drainage. — By reason of the 

 greater porosity due to the presence of organic matter, the 

 movement of water through soils is facilitated and thus the 

 soil is better drained. The advantages of good drainage 

 will be discussed more fully later, but an important one 

 of these is a greater growth of roots, which increases their 

 opportunity for securing food and thus increases the size of 

 crop. 



54. Organic matter and soil color. — Partly decomposed 

 organic matter generally gives a dark color to a soil. A 

 dark soil absorbs heat more readily than does a light-colored 



