THE SOIL AS A SOURCE OF MINERAL ELEMENTS 403 



better understood, it is doubtful if any even partially adequate explanation 

 of this fundamentally important fact can be formulated. 



TABLE 39 PERCENTAGE OF CALCIUM, POTASSIUM, MAGNESIUM, NITROGEN, AND PHOSPHORUS 



IN THE TOPS OF SEVERAL SPECIES OF PLANTS GROWN IN A GREENHOUSE IN AN ALBERTA 

 "black, belt" loam SOIL (data OF NEWTON, I928) 



The composition and other properties of the soil in which a plant is 

 rooted will also have an effect on the proportion of each of the various ele- 

 ments absorbed by that plant. Innumerable examples of this fact can be 

 cited from the practice of fertilizing. Addition to the soil of a compound 

 which can be absorbed by plants usually results in an increased intake of 

 that substance by the plants although the increase in the amount of the 

 element within the plant tissues is usually not proportionate to the increase 

 in the amount of that element in the soil. Plants often absorb from the 

 soil mineral salts far in excess of their actual metabolic requirements. Po- 

 tassium, phosphate, sulfate and other ions often accumulate in plant cells in 

 excess of the quantities actually utilized by the cells. 



The Soil as a Source of Mineral Elements. — With only negligible ex- 

 ceptions, all of the mineral elements which enter into the composition of 

 terrestrial plants come from the soil. In discussions of the absorption of 

 both water and mineral salts by plants attention is generally focussed on 

 the soil solution (Chap. XVI). Recent advances in soil science have made 

 it increasingly clear, however, that the mineral salts dissolved in the soil 

 solution are not the only ones which must be considered in any evaluation 

 of the mineral salt relations of soils as they influence the intake of solutes 

 by plants. 



The fundamental physico-chemical properties of most soils are due 

 mostly to the clay fraction, except in soils relatively rich in organic com- 

 pounds in which they also play an important part in determining soil prop- 

 erties. The clay fraction of the soil consists entirely of particles of colloidal 

 dimensions. The clay particles of the soil are apparently composed largely 



