86 



PHYSIOLOGY or NUTRITION 



§6. The Plant and the Soil.* — Plants obtain all their essential ash-constitu- 

 ents from the soil. The following table gives an idea of the compositions of 

 several different kinds of soil, the numbers representing the amounts of usual 

 soil bases, calculated as oxides and expressed as percentages of the total dry 

 weight of soil. 



Every soil covered with vegetation contains organic as well as mineral sub- 

 stances. Bog soils are particularly rich in organic materials, as is evident from 

 the following table, which again presents percentages on the basis of the dry 

 weight of the soil. 



The chemical analysis of a soilcan give no definite idea of its properties, 

 however.^ In order to predict a good crop from a given soil it is not enough to 

 know that it contains potassium, phosphorus and the other essential elements; 

 it must also be known whether these elements occur in compounds that plants 

 can assimilate. Nile silt, famous for its fertility, contains only 0.5 per cent, of 

 potassium and needs no further addition of this element, but mica-schist soil 

 contains 3 per cent, of potassium and remains unproductive unless a potassium 

 fertOizer is added. 



To obtain a better idea of the productiveness of a soil, the analysis of its 

 water or hydrochloric acid extract is carried out, in addition to determining the 

 essential minerals present. The necessary elements for plant growth are con- 

 tained in very small quantities in the extract, but it must be borne in mind that 



^ An excellent treatise on the soil is: Mitscherlich, E. A., Bodenkunde fur Land- und 

 Forstwirte. 2 Aufl. 317P. Berlin, 1913. A less scientific treatise is: Hilgard, E.W., Soils, 

 their formation, properties, composition, and relations to climate and plant growth in the 

 humid and arid regions. 593 p. New York, 191 2. Best of aU presentations of the soil, from 

 the standpoint of plant physiology, is that of Russell (1915). [See note *, p. 69.] — Ed. 



* Cameron, F. K., The soil solution. 136 p. Easton, Pa. ig^x.—fid. 



