VI CONTENTS 



transported soils ; Rocks and minerals from which soils are derived 

 as quartz, feldspar, mica, hornblende, zeolites, granite, apatite, 

 kaolin ; Disintegration of rocks and minerals ; Value of geological 

 study of soils. Pages 45-56. 



CHAPTER III 



Chemical Composition of Soils. Elements present in soils ; Clas- 

 sification of elements ; Combination of elements ; Forms in which 

 elements are present in soils ; Acid-forming elements, silicon, 

 double silicates, carbon, sulphur, chlorine, phosphorus, nitrogen, 

 oxygen, hydrogen; Base-forming elements, aluminum, potassium, 

 calcium, magnesium, sodium, iron ; Forms of plant food ; Amount 

 of plant food in different forms in various types of soils ; How a 

 soil analysis is made ; Value of soil analysis ; Interpretation of the 

 results of soil analysis ; Use of dilute acids as solvents in soil anal- 

 ysis ; Distribution of plant food in the soil ; Composition of typical 

 soils ; "Alkali " soils and their improvement ; Acid soils ; Organic 

 compounds of soil ; Sources; Classification; Humus; Hutuates ; 

 Humification ; Humates produced by different kinds of organic 

 matter; Value of humates as plant food, amount of plant food in 

 humic forms ; Physical properties of soils influenced by humus ; 

 Loss of humus by forest fires, by prairie fires, by cultivation ; 

 Humic acid ; Soils in need of humus ; Soils not in need of humus ; 

 Composition of humus from old and new soils ; Influence of differ- 

 ent methods of farming upon humus. Pages 57-96. 



CHAPTER IV 



Nitrogen of the Soil and Air, Nitrification and Nitrogenous 

 Manures. Importance of nitrogen as plant food ; Atmospheric 

 nitrogen as a source of plant food. Experiments of Boussingault, 

 Ville, and Lawes and Gilbert; Result of field trials ; Experiments 

 of Hellriegel and Wilfarth and recent investigators ; Composition 

 of root nodules ; Amount of nitrogen returned to soil by legumi- 

 nous crops and importance to agriculture ; Nitrogenous compounds 

 of the soil ; Origin ; Organic nitrogen ; Amount of nitrogen in 

 soils ; Removed in crops ; Nitrates and nitrites ; Ammonium com- 

 pounds ; Ammonia in rain and drain waters ; Ratio of nitrogen to 

 carbon in the soil ; Losses of nitrogen from soils ; Gains of nitrogen 

 to soils; Nitrification; Former views regarding ; Workings of an 

 organism ; Conditions necessary for nitrification ; Influence of cul- 



