July 17, 1916 Influence of Calcium and Magnesium on Plant Growth 591 



the study of chlorophyll and finds it to be a magnesium compound with 

 generally three times as much green pigment as yellow pigment. He 

 found that the magnesium content of chlorophyll was constant in both 

 land and sea plants; therefore, it must function other than as a catalyzer. 

 Pfeffer (27, p. 425), Macdougall (17, p. 219), Peirce (26, p. 100) and 

 others believe magnesium and calcium play an important and necessary 

 function in plant synthesis and cell formation, but are unable to assign 

 any specific role to either of these elements. 



There has been considerable contention as to whether calcium could 

 be replaced by other members of the group. Haselhoff (7) grew beans 

 and maize in solutions containing varying quantities of calcium and stron- 

 tium and concluded that strontium seemed to take the place of calcium, 

 replacing it only when the supply of calcium was inadequate. But it 

 must be remembered that he first used calcium and strontium together 

 in the solution and later reduced the calcium. However, Loew (16, p. 48) 

 was unable to substantiate these results when he used species of Trades- 

 cantia. 



Loew explains the toxicity between calcium and magnesium as being 

 due to the formation of an insoluble condition of the phosphoric acid 

 being fixed by the calcium, and that the framework (15) of the nucleus 

 and plastids is a double organic salt of calcium and magnesium. How- 

 ever, Meurer (19) and Nathansohn (23) offer another explanation: Cells 

 being selective in their absorption of ions can check osmosis before a 

 balance is reached between the solution within and without the cell, and 

 the absorption of salts does not increase proportionally with the increase 

 of concentration of the outside solution. Osterhout (24) using calcium 

 nitrate and magnesium nitrate was unable to substantiate Loew's 

 assertion. 



Considerable work has been done upon the antagonism of respective 

 salts for each other in solution. Kearney (11, p. 20) shows that calcium 

 salts are most beneficial in reducing toxicity. Lipman (14) reports 

 toxicity between magnesium and sodium but not between magnesium 

 and calcium. 



Numerous investigators have sought answers to the proposed theory 

 of a lime-magnesia ratio with just as numerous and conflicting results. 

 Solutions, pot cultures of soil and sand, and field soils have all been em- 

 ployed in attempts to settle the controversy. Ulbricht (34) showed that 

 yellow lupines, barley, and vetch were injured by applications of lime, 

 especially when it contained high percentages of magnesia. Magnesia 

 apparently increased the proportional yield of grain in the case of barley 

 and lupines. Dojarenko (2), however, concluded that the theory of a 

 definite calcium-magnesium ratio was not tenable, as many Russian soils 

 containing great excesses of calcium over magnesium were benefited by 

 liming. 



