NATURE OF SOIL 227 



soil. Surface soil differs from subsoil chiefly in the organic 

 matter it contains. 



2. Organic Matter in Soil 



Examine carefully a lump of common field soil. Pul- 

 verize it and spread it out on a paper. In addition to the 

 grains of sand, silt and clay which represent the rock por- 

 tions, note all the different organic particles, such as pieces 

 of roots, fibers of plants, and parts of insects. Place it 

 under a magnifier, and see whether you can make still fur- 

 ther discoveries. 



Need of organic matter. Although the rock particles 

 make up far the greater part of the mass of the soil, or- 

 ganic matter is of the highest importance to plants. Indeed, 

 it is practically impossible to raise crops on soil lacking 

 organic constituents. All the upper layers of ordinary soil 

 contain from two to five per cent, of organic material, com- 

 ing chiefly from the roots and stems of plants. 



Humus. When vegetation decays on top of the 

 ground it is really burned up as effectually as if put into a 

 stove, only more slowly. The gases pass off into the air 

 and only a slight ash remains on the soil. From this proc- 

 ess the soil receives comparatively little benefit. If, how- 

 ever, decay takes place under the surface, where but little 

 oxygen is present, a substance is produced which is called 

 humus. All soil on which plants are grown is therefore 

 constantly producing humus from the roots, and from the 

 stubble and stems if these are turned under. Dig up a piece 

 of timothy sod and note the mass of roots about two tons 

 of roots to the acre on a good field. Blue-grass yields some 

 six tons of roots to the acre. Soils that are cropped contin- 

 uously with the common cereals and the crops removed with- 

 out returning manure to the field become deficient in humus, 

 and the yield is decreased. 



