xxvi INTRODUCTION. 



soils in the form of titanic dioxid, is not known as performing 

 any important function in soils, and is not, so far as known 

 at present, ever taken up by plants. Aluminum, in the form 

 of its compounds with oxygen and silicon, is a very prominent 

 and physically very important soil ingredient, but does not, 

 apparently, perform any direct function in plant nutrition, and 

 is absent from their ash, except in the case of some of the 

 lower plants (horsetails and ferns). 



lodin appears to be normally present in all seaweeds, and 

 occurs in traces in some land plants. Fluorin is a normal in- 

 gredient of animal bones, and its presence in plant ashes is 

 often easily shown. The remaining fourteen, however, are 

 always present in plants ; carbon, hydrogen, oxygen and nitro- 

 gen forming the volatile or combustible part, while the rest 

 occur in the ashes. 



It is true that other elements, or rather their compounds, are 

 sometimes found in plants, being taken up by them from solu- 

 tions existing in the soil. Thus the alkalies caesium and rubi- 

 dium, also barium, strontium, zinc, copper, boron and some 

 others, may be absorbed when present in soluble form. But 

 they are neither necessary nor beneficial to plant economy, and 

 when in considerable amounts are harmful. Thus fifteen ele- 

 ments, ommiting iodin and titanium, alone require discussion. 



The J'olatile Part of Plants, as already stated, consists 

 of carbon, hydrogen, oxygen and nitrogen. Of these, car- 

 bon is obtained by the plant exclusively from the carbonic 

 (dioxid) gas of the air; hydrogen and oxygen, from the soil 

 in the form of water; nitrogen, directly from the soil but in- 

 directly also from the air, through the agency of certain 

 bacteria. The asli ingredients of course are all derived from 

 the soil through the roots, and must all be present in the lat- 

 ter in an available form, to a sufficient extent to supply the 

 demands of vegetation. 



The Agencies of Soil Formation. With respect to their 

 mode of formation, soils may be defined as the residual product 

 of the physical disintegration and chemical decomposition of 

 rocks; with, ordinarily, a small proportion of the remnants of 

 organic life. The agencies producing these changes are those 

 classed under the general term " atmospheric " or " meteoro- 



