436 PEACTICAL BOTANY 



401 . Composition of soils : organic matter. A brief study 

 of soils under the microscope would show that in addition 

 to rock in different stages of decomposition, there is usually 

 present considerable other material. Leaves, twigs, wood, her- 

 baceous plants, and the bodies of animals decompose, and the 

 products make up a most important part of the soil (Fig. 342). 

 In almost all soils some organic matter may be found. When 

 a large quantity of plant material lies upon the surface of 

 the earth and partially decomposes, it is usually spoken of as 

 humus. In undrained or poorly drained swamps into which 

 little soil washes from adjacent hills, the deposit at the bottom 

 of the standing water is almost pure organic matter, which is 

 the peaty material often found in regions which once were 

 swampy. 



402. Composition of the soil : water and air. Around and 

 between the particles of the more or less decomposed rock, 

 and absorbed by organic matter, there is always some water, 

 though it may be present in amounts so small as to be difficult 

 of detection. Some water adheres closely about solid particles 

 of the soil. Water also may fill the spaces between the par- 

 ticles of solid material, and such water is known as the free 

 water of the soil. Water may take into solution some portions 

 of the soil. 



The amount of water in the soil varies largely and depends 

 upon many factors. If there is little rainfall and the supply 

 is not replenished from below, the coarse soils (gravel and 

 coarse sand) will soonest become dry. But the amount and 

 nature of the organic matter in the soil has much to do with 

 ability to hold- water. A good supply is usually held by fine 

 sandy and clayey soils in which there is an abundance of 

 organic matter. At times of continued heavy rainfall all kinds 

 of soil may become filled with water, and the prevention of 

 dangers from this surplus is discussed in the later sections of 

 this chapter. 



In the spaces between the solid particles of the soil air is 

 also found. Even in soils that are below ponds and streams 



