ABSORPTION OF NUTRITIVE SALTS 417 



are potassium, sodium, calcium, magnesium, iron, man- 

 ganese, aluminium, phosphorus, sulfur, silicon, and chlo- 

 rine. In addition to these, nitrogen is absorbed from the 

 soil in the form of soluble salts. Of these the substances 

 known to be absolutely essential to the normal growth of 

 plants to maturity, are potassium, calcium, magnesium, 

 iron, phosphorus, sulfur, and nitrogen, while the others 

 are probably beneficial to the plant in some way not yet 

 discovered. 



Of the substances acting as plant nutrients, each must 

 be present in an amount sufficient to make possible the 

 maximum growth consistent with other conditions, or 

 the yield of the crop will be curtailed by its deficiency. 

 To some extent certain essential substances may be re- 

 placed by others, as, for instance, potassium by sodium; 

 but such substitution is probably possible only in some 

 physiological role other than that of an elemental con- 

 stituent of an organic compound. The substances that are 

 likely to be so deficient in an available form in any soil 

 as to curtail the yield of crops, are potassium, phosphorus, 

 nitrogen, and possibly sulfur; while the addition of cer- 

 tain forms of calcium is likely to be beneficial because of 

 its relation to other constituents and properties of the soil. 

 It is for the purpose of supplying these substances, and 

 to some extent to improve the mechanical condition of 

 the soil, that mineral manures are used. 



338. Relation of plant growth to concentration of nu- 

 trient solution. It has already been stated that the 

 addition of soluble salts to a soil has been found by some 

 experimenters to apparently increase the concentration 

 of the soil solution (par. 250). It has also been found 

 that plant growth, as measured by weight of plants, in- 

 creases with the concentration of the nutrient solution in 

 2 E 



