CONTENTS IX 



PAGES 



various types of soils ; Acid soluble matter of soils ; Acid 

 insoluble matter ; Action of organic acids upon soils ; How 

 a soil analysis is made ; Value of soil analysis ; Interpre- 

 tation of the results of soil analysis ; Use of dilute acids 

 as solvents in soil analysis ; Use of dilute mineral acids in 

 soil analysis ; Available and unavailable plant food ; Vola- 

 tile matter of soils ; Distribution of plant food in the soil ; 

 Composition of typical soils ; i Alkali ' soils and their 

 improvement ; Acid soils ; Organic compounds of soil ; 

 Sources ; Classification ; Humus ; Humates ; Humifica- 

 tion; Humates produced by different kinds of organic 

 matter ; Mineral matter combined with humus ; 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 cultiva- 

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

 need of humus ; Composition of humus from old and new 

 soils; Influence of different methods of farming upon 

 humus. 



CHAPTER IV 



NITROGEN OF THE SOIL AND Am, NITRIFICATION, AND NI- 

 TROGENOUS MANURES 116-157 



Importance of nitrogen as plant food ; Atmospheric ni- 

 trogen as a source of plant food. Experiments of Bous- 

 singault, Ville, Lawes and Gilbert, and Atwater; Result 

 of field trials ; Experiments of Hellriegel and Wilfarth 

 and recent investigators ; Composition of root nodules ; 

 Amount of nitrogen returned to soil by leguminous crops 

 and importance to agriculture; Nitrogenous compounds 

 of the soil ; Origin ; Organic nitrogen ; Amount of nitro- 

 gen in soils ; Removed in crops ; Nitrates and nitrites ; 

 Ammonium compounds ; Ammonia in rain and drain 



