THE ESSENTIAL ELEMENTS 413 



suggests that such substitution is possible (Sect. 22), and Wolff's 1 experiments 

 afford a direct answer to this question. Wolff grew oats in nutrient solutions 

 containing 42-83 per cent, of potash and 7-03 per cent, soda, or 11-65 P er cent, 

 potash and 33-61 per cent, soda, but otherwise of similar composition, and found 

 that in the first case the ash of the harvested plants contained 50-28 per cent, 

 potash and 7-03 soda, in the second case 30-69 potash and 22-04 soda. Experi- 

 ments in which calcium was partly replaced by magnesium gave similar results. 

 A limited substitution of strontium for calcium seems also to be possible 2 . The 

 nature of the neutralizing base is not always immaterial, for when calcium is 

 present an insoluble oxalate is precipitated. Combination with an alkali pro- 

 duces a more highly osmotic salt than does combination with an alkaline earth 

 (Sect. 24). That the fixation of organic acids is often the sole or most important 

 function of such non-essential bases can hardly be doubted, and in the bacteria 

 which excite lactic and butyric fermentation we have organisms which are rapidly 

 injuriously affected as the acid which they produce accumulates (Sect. 103). It is, 

 however, hardly the sole task of the alkalies and alkaline earths to neutralize organic 

 acids, as Liebig supposed, being led thereby to the conclusion that they might 

 replace one another to an unlimited extent. The diametrically opposed view of 

 Sprengel, that all such substitution is totally impossible 3 , is also inaccurate. 



In the absence of any one of the essential elements development is 

 impossible, just as a watch ceases to go as soon as any of the wheels 

 which form part of its mechanism are removed. This applies as well 

 to iron, of which but a trace is required, as to potassium or phosphorus, 

 of which large amounts are necessary, though not nearly as much as of 

 carbon 4 . When the deficient element is presented to the plant in gradually 

 increasing amount the rapidity of growth also increases, though by no 

 means proportionately 5 , as has already been shown in the case of 

 nitrogen (Sect. 68), while any increase above the optimum amount required 

 must ultimately lead to a depression of the vital activity. This only 

 occurs with a potassium salt at a high degree of concentration, whereas 

 a poisonous action is exercised by a very dilute solution of a salt of iron. 

 All the physiological results which may be produced by progressive 

 changes in any of the external conditions may as a general rule be 

 represented by a curve exhibiting minimum, optimum, and maximum 

 points. The so-called law of minimums 6 expresses the first portion of 

 this curve, which may be constructed not only for the ash constituents, 



1 O. Wolff, Versuchsst, 1868, Bd. x, p. 370; Pellet, Ann. d. chim. et d. phys., 1879, v. sen, 

 T. xvii, pp. 145, &c. 



2 Hasselhof, Landw. Jahrb., 1893, Bd. xxn, p. 851 ; Molisch, Bot. Centralbl., 1896, Bd. LXVIII, 

 p. 146 (Algae). 



3 Liebig, Die Chemie in ihrer Anwend. auf Agric., 1840, p. 87; also Mulder, Physiol. Chem., 

 1844-51, p. 78 ; C. Sprengel, Die Lehre vom Danger, 1839, P- 53- 



* Even Saussure (Rech. chim., 1804, p. 261) recognized that the amount was not all-important. 



5 For examples, see Wolff, Versuchsst., 1874, Bd. xvii, p. 138 ; Ville, Bot. Jahresb., 1890, p. 47. 



6 Cf. Ad. Mayer, Agr.-Chem., 1895, 4. AuH., p. 306. 



