36 THE PRINCIPLES OF AGRICULTURE 



pupil may now be able to explain why the windows get dirty after 

 a rain ; and he will be interested in the streaks on the cornices 

 of buildings and on exposed statuary. He may have heard that 

 even sailing ships get dusty when at sea. 



42a. See Eoberts' "Fertility of the Land," p. 16. Read all 

 of Chapter i. The food which is not available, or not in condi- 

 tion to be used by the plant, but which may become available 

 through good tillage or otherwise, is called potential plant-food. 



43a. The soil is not a simple reservoir of plant- food in the 

 condition of salt or sugar, ready to be dissolved in water and 

 immediately taken up by roots. The soil is plant -food ; but 

 most of it must be changed in composition before it is available 

 to plants ; and the elements are not present in the proportions 

 which plants require, so that much of the soil is in excess of the 

 needs of plants and can never be used as food. 



48a. For supplementary reading on the formation of soils, 

 Chapter i. of King's "Soil" should be consulted. Most text-books 

 of geology also treat the subject to some extent. Shaler's article 

 on soil, in 12th Annual Report of the U. S. Geological Survey 

 (pp. 319-345), is excellent. A discussion of weathering may be 

 found in Chapter vi. of Tarr's "Elementary Geology;" and other 

 references are contaimed in Chapters xiii.and xxi.of his "Elemen- 

 tary Physical Geography." Stockbridge's "Rocks and Soils" 

 (1895) has special reference to agriculture. A readable account 

 of the formation of soil may be found in Chapters iii., iv. and v., 

 Gaye's "Great Word's Farm." Merrill's "Rocks, Rock- Weather- 

 ing and Soils" (1897) is a full scientific discussion of the subject. 

 Consult Hilgard's "Soils," and the text by Lyon and Fippin; also 

 the part on soils in Vol. I, Cyclopedia of American Agriculture. 



