SOME GENERAL CONSIDERATIONS 5 



partially or wholly decayed organic matter present therein. 

 It is utilized by the plant ordinarily in the form of nitrate. 

 The atmosphere, composed of four-fifths nitrogen by 

 volume, has been the original source of this element; 

 and through natural processes which are continually 

 at work the nitrogen has been transferred to the soil. 

 The encouragement of this natural fixation, thus draw- 

 ing upon the great body of gas surrounding the earth, 

 has become of great practical importance in agricultural 

 operations. 



Phosphorus has its origin in the mineral apatite and 

 exists in most soils largely as a tricalcium phosphate 

 (Ca3(P04)2). In case of a lack of lime or of the presence 

 of considerable quantities of humus, phosphorus may be 

 present as ferric or aluminium phosphates or as organic 

 phosphoric acid. Phosphorus is probably taken up by 

 the plant as the mono- or di-calcic phosphate (CaH^PO^ 

 or Ca 2 H 2 (P0 4 ) 2 ). 



The potassium of the soil exists largely in feldspar 

 (K 2 . A1 2 C>3 . 6 Si0 2 ), in mica, or in hydrated aluminium 

 silicates, which, while rather insoluble, supply potash 

 to the soil solution in the bicarbonate, chloride, nitrate, 

 or sulfate forms. It is from such compounds that the 

 plant draws upon the soil for this element. 



4. Abundance of plant-food elements. — Having con- 

 sidered the plant-food elements, especially those of primary 

 importance, it is of interest to note their distribution in 

 the earth's crust. Clarke 1 estimates the composition 

 of the lithosphere, which makes up 93 per cent of the 

 known terrestrial matter, as follows : — 



1 Clarke, F. W. Data of Geochemistry. U. S. Geol. Survey, 

 Bui. 491, p. 33. 1911. 



