282 Nutrients 



nutrient that is minimal relative to requirements and not necessarily 

 the one that occurs in the smallest absolute amount. In this sense 

 the law of the minimum in ecology is analogous to the law of com- 

 bining weights in chemistry. The following equation gives the pre- 

 cise amounts of Na and CI that will combine to form NaCl: 



23g Na + 35g CI -^ 58g NaCl 



If lOOOg of Na were added to 35g of 01, no more than 58g of NaCl 

 would be formed. Under these circumstances the 35g of CI limits 

 the total amount of NaCl produced. If, however, 23g of Na were 

 allowed to combine with 34g of CI, somewhat less than 58g of NaCl 

 would be obtained. The 34g of CI is now the limiting substance, 

 although, in this instance, it is not the smaller quantity of the two 

 components. 



The same sort of limitations occur in the growth of plants, but the 

 amounts of nutrients absorbed are not nearly as rigidly fixed as the 

 combining weights in simple inorganic reactions. Much has been 

 learned from water culture methods regarding the variations in the 

 quantities of nutrients used by higher plants in normal growth and 

 also regarding minimal requirements (Hewitt, 1952). Sometimes 

 the amounts or the ratios of nutrient materials used are completely 

 altered at different seasons. For example, a certain group of roses 

 tested in July, took up 450 ppm N and 250 ppm K, whereas in Decem- 

 ber they absorbed 150 ppm N and 750 ppm K (Turner and Henry, 

 1948). It has also been learned that a surplus of some nutrients 

 can in certain instances make up for the lack of others— another illus- 

 tration of the principle of partial equivalence. Experiments have 

 shown that Na is necessary for the growth of the sugar beet, regard- 

 less of the amount of K that may be present, although Na is not 

 essential for most plants. Furthermore, if sugar beets are grown 

 with insufficient K, the symptoms of K deficiency are practically elim- 

 inated by supplying extra Na (Lehr, 1942). 



Similar information on the variation in nutrient requirements has 

 been obtained from cultures of algae. The amount of one nutrient 

 taken in is sometimes found to depend upon the amounts of other 

 nutrients present, and the ratio may change radically according to 

 circumstances. Ketchum (1947) found that the rate of assimilation 

 of phosphorus by the diatom Nitzschia closteriutn was related to the 

 concentration of both phosphate and nitrate in the medium, whereas 

 the uptake of nitrate was independent of the concentration of phos- 

 phate and related only to the concentration of nitrate. Various indi- 

 vidual cultures grown in the same medium varied by as much as 50 



