44 SOILS AND FERTILIZERS 



if it were burned. Manured land is usually i° or 2° 

 warmer in the spring than unmanured land ; this is due to 

 the oxidation of the manure. In an acre of rich prairie 

 soil it has been estimated that the amount of organic 

 matter which undergoes oxidation produces as much 

 heat annually as would be produced from a ton of 

 coal. 16 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 illustrated 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 the case of early frosts, crops on 

 well-manured land will often escape. 



41. 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 forma- 

 tion. 



