UNIVERSITY OF CALIFORNIA. 



Correlative with Sodium. Karely deficient. 

 Fluorine. Very generally present in small quantities. 



Most largely in bones. 

 Iodine. Common, in traces. 



Carbon. Ultimately derived from air: directly in part from 

 soil. Formation of humus by decay of vegetable matter. 

 (Physical effects of humus on soils see above). 

 Chemical effects on soil ingredients. 



Action of Crenic and Apocrenic acids. 



Oxidation of humus. Production of carbonic acid, the 



universal solvent. 



Direct absorption of soluble vegetable matter. 

 Hydrogen. Absorbed by plants in shape of water. 

 Nitrogen. Highly important nutritive and constituent ingredi- 

 ent. Flesh-former. 

 Free nitrogen of air not assimilable. 

 Ammonia and nitric acid of atmosphere absorbed by 



soil. Inadequate for crops. 

 Nitrogen in soil. How contained? Small percentage 



and largely unavailable. 

 Connection with humus Nitrification accompanying 



oxidations, evaporation etc. 



Ozonization. Formation of Hydrogen Peroxide. 

 Possible agency of microscopic plants in soil. 

 Necessity for artificial supply of Nitrogen for crops. Ni- 

 trogen Theory versus "Mineral" Theory. 

 General effects of nitrogenous manures on plants. 



EXHAUSTION OF SOILS. 

 Eecapitulation : 



' 'All plant ingredients must be simultaneously present in 

 sufficient quantities. Absence of one renders all inert. " 

 "Supplying that one deficient ingredient enormously pro- 

 fitable." 



"Excess of any lies inert in the soil as dead capital." 

 Hence the necessity for 



Eolation of Crops. 

 Causes of necessity for rotation. Abstraction of different 



ingredients in unequal degrees by different crops. 

 Different depth of roots fibrous and tap roots. 

 Examples. 



Eotation utilizes soil resources best. Makes interest ac- 

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