20 THE PRINCIPLES OF AGRICULTURE 



or wave, and become floating islands. Such 

 islands are often several acres in extent. In 

 high latitudes, where the summer's growth does 

 not decay quickly, one season's growth is some- 

 times added above another until a deep organic 

 soil is formed. This is especially noticeable in 

 the gradual increase in height of sphagnum 

 swamps. Peat bogs are organic lands, and they 

 fill the beds of former lakes or swamps. Of 

 course, all these organic soils contain mineral 

 matter, but it is mostly such as comes from the 

 decay of the plants themselves. It was origi- 

 nally obtained from the earth, but is used over 

 and over again ; and each year a little new 

 material may be added by such plants as reach 

 into the hard land below, and by that which 

 blows into the area in dust. 



33. Decaying organic matter forms mold or 

 humus. The mineral elements may be said to 

 give "body" to the soil, but the humus is w r hat 

 gives it "life" or "heart." Humus makes soils 

 dark- colored and mellow. Humus not only adds 

 plant -food to the soil, but improves the physical 

 condition of the soil and makes it congenial for 

 plants. It augments the water -holding capacity 

 of the soil, modifies the extremes of temperature, 

 facilitates the entrance of air, and accelerates 

 many chemical activities. It is the chief agent 

 in the formation of loam : a sandy loam is a 



