ORGANIC MATTER AND IRON COMPOUNDS 4! 



of coal. 17 In well-drained and well-manured land, the 

 additional heat is an important factor for stimulating 

 crop growth, particularly in a cold, backward spring. 

 The production of heat from manure is utilized in 

 the case of hotbeds where well-rotted manure is covered 

 with soil ; this results in raising the temperature of the 

 soil. When soils are well manured, heat is retained 

 more effectually. In case of early frosts, crops on 

 well-manured land will often escape. 



4. Relation of Heat to Crop Growth. All plant 

 life is directly dependent upon solar heat as the source 

 of energy for the production of plant tissue. The 

 heat of the sun is the main force at the plant's dis- 

 posal for decomposing water and carbon dioxide and 

 for producing starch, cellulose, and other compounds- 

 The growth of crops is the result of the transformation 

 of solar heat into chemical energy which is stored up 

 in the plant. When the plant is used for fuel or for 

 food the quantity of heat produced by complete oxida- 

 tion is equal to the amount of heat required for the 

 formation of the plant's tissues. 



COLOR OF SOILS 



42. Organic Matter and Iron Compounds. The 



principal materials which impart color to soils are or- 

 ganic matter and iron compounds. Soils containing 

 large amounts of organic matter are dark-colored. 

 A union of the decaying organic matter with the 

 mineral matter of the soil produces compounds brown 

 or black in color. When moist, many soils are darker 

 than when dry, and soils in which the organic matter 



