5O SOILS AND FERTILIZERS 



47. 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 disposal for 

 decomposing water and carbon dioxide and for produc- 

 ing 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 oxidation is equal to the 

 amount required for the formation of the plant's tissue. 



48. Color of Soils. The principal materials which 

 impart color to soils are organic and iron compounds. 

 A union of the decaying organic matter (humus) with 

 the minerals of the soil produces compounds brown or 

 black in color, and consequently soils containing large 

 amounts of humus are dark-colored. When moist, soils 

 are darker than when dry, and soils in which the organic 

 matter has been kept up by the use of manures are 

 darker than unmanured soils. 18 When rich, black, prai- 

 rie soils lose their organic matter through injudicious 

 methods of cultivation, or when in chemical analysis it 

 is extracted, the soils become light-colored. 



The red color of soils is imparted by ferric oxide ; the 

 yellow, by smaller amounts of the same material. A 

 greenish tinge is supposed to be due to the presence 

 of ferrous compounds, such soils being so close in tex- 

 ture as to prevent the oxidizing action of the air. Color 



