186 Plant Physiology 



The lime-magnesia relation in the soil is, moreover, o! 

 much practical importance, and it is certain that when 

 magnesia is relatively abundant in soils there is usually 

 need of liming. Under such circumstances it is obvious 

 that the application of a dolomitic limestone (rich in mag- 

 nesium) should be avoided. The evidence upon this 

 point is also extensive. There are practical difficulties, 

 however, in determining the proper ratio of CaO : MgO in 

 the soil, for the question of availability must be considered. 

 Water culture and pot experiments have suggested such 

 differences in the requirement of plants as shown by the 

 following favorable ratios : 



Buckwheat CaO to MgO 3 to 1 



Cabbage CaO to MgO 2 to 1 



Oats CaO to MgO 1 to 1 



Loew believes that the greater the leaf surface produced in 

 a given time, the greater the necessity for lime; that is, 

 the higher the ratio. In this connection it is shown that 

 with respect to the lime requirement the need of tobacco, 

 clover, grass, sugar-beet, and wheat form a decreasing 

 series. The cereals never show a ratio higher than 2:1. 

 In solution cultures in the laboratory the effect of the omis- 

 sion of calcium from a solution containing magnesium, 

 or the inappropriate ratio of calcium, results in a markedly 

 decreased growth. In the following chart, giving the 

 effect of the calcium-magnesium ratio upon the growth 

 of the Canada field pea, this fact is made clear, 1 the 



1 These data correspond to the series of cultures in Figure 51 , and are 

 from careful laboratory experiments by graduate students. 



