Law of the Minimum 281 



shells; other elements such as aluminum, fluorine, and bromine are 

 taken up by certain plants but seem to have no nutritional significance. 

 Although sodium and chlorine are commonly present in plant tissues, 

 these substances are probably not necessary for most species. Cer- 

 tain elements found in plants and also in animals have been concen- 

 trated hundreds or even thousands of times more than they are in 

 the environment. This condition is strikingly true of trace elements 

 and is easily apparent in the sea where the chemical composition of 

 the surrounding water is highly uniform. Silicon and iron are greatly 

 concentrated in the bodies of diatoms, and titanium, known to occur 

 in certain marine organisms, has not yet been detected in free sea 

 water (Sverdrup et al., 1942, Ch. 7). Vanadium may constitute al- 

 most 0.2 per cent of the dry weight of Ascidia mentula. Since vana- 

 dium is present in sea water in a concentration of only 0.3 to 0.6 mg 

 per cu m, the element has been concentrated roughly four million 

 times in the body of this animal. 



Several other elements, that may or may not be essential to the 

 plants, are nevertheless significant for the animals feeding upon the 

 plant material as being either beneficial, e.g., cobalt, iodine, and 

 nickel, or harmful, e.g., selenium and molybdenum. Few if any of 

 these minor nutrients exist as elements in soil or in the water; they 

 mostly occur and must be absorbed as salts or ions in chemical com- 

 bination with other elements. Above-normal amounts of some of the 

 trace elements are definitely injurious to plants— often with only a 

 narrow range between minimal, optimal, and harmful concentrations. 

 For example, an increase of the boron concentration in the soil to 

 1 ppm will kill some plants, whereas for other species 1 ppm is optimal 

 and 5 ppm are toxic. 



Law of the Minimum. If any necessary nutritive element is com- 

 pletely lacking, the growth and eventually the maintenance of a plant 

 will obviously be prevented. In some habitats all the essential sub- 

 stances may be present, but one or more of them may exist in con- 

 centrations so low that certain species cannot absorb them rapidly 

 enough to satisfy their nutritional needs. Under these conditions 

 the growth of the plants will be limited in conformity to Liebig's law 

 of the niinirnum. This law states that growth is limited by the sub- 

 stance that is present in minimal quantity in respect to the needs of 

 the organism. Liebig's law was originally delineated, and is best 

 applied, in relation to limitation by nutrients, but it is sometimes 

 used in a broader sense to include limitation by other factors of the 

 environment. 



The reader should understand that the limiting substance is the 



